Beat Frequency Oscilla·tor Type 1014

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A beat frequency oscillator covering the range

20 to 20ooo c/ s. The apparatus is designed to
meet the numerous requirements of a signal
source for audio frequency work. It is excellently
suited both for electrical and electroacoustical
measurements, as well as for acoustic research .

BHUEL&KJJEH
Nrerum, Denmark . Phone 80 05 00 . Telegrams: BRUKJA, Copenhagen

BB 1014
Beat Frequency Oscillator
Type 1014

Reprint January 1962

 Contents
Description 5
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Description of the Oscillator and Mixer-Section . . . . . . . . . . . . . . . . . . . . 6
Frequency Marking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Partial Blocking of Frequency Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Description of the Output Amplifier Section ...................... 11
Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
General ......................................................... 14
Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Operation Using the Output Terminals Marked "Load" .............. 15
Operation Using the Built-in Attenuator ............................ 16
Frequency Modulation ............................................ 16
Automatic Recording ............................................ 16
Marking Adjustment ............................................ 20
Partial Blocking of Frequency Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Automatic Regulation of the Output Power ........................ 22
Remote Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Trouble Shooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Accessories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Group A. Electronic Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Measurement of Frequency Response of Four-Terminal Networks .. 25
A.C. Bridge Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Measurements of Temperatures and Temperature Differences ...... 27
Measurement of Gain in A.F. Amplifiers .......................... 30
Group B. Acoustical Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Recording of the Frequency Response of Loudspeakers ............ 30
Recording of the Frequency Response of Microphones . . . . . . . . . . . . . 32
Recording the Frequency Characteristic of Hearing Aids and Ear-
phones ...................................................... 33
Checking of Hearing Aids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Measurements on Air Filters, Carburettor Inlets etc. . . . . . . . . . . . . . . . . 36
Testing the Qualities of Airborne Sound Insulation . . . . . . . . . . . . . . . . 38
Measurement of Reverberation Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Absorption Qualities on Sound Insulation Material . . . . . . . . . . . . . . . . 44
 Group C. Mechanical Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Strain Measurement on Vibrated Objects .......................... 45
The A.F. Response and Spectrum Recorder Type 3326 .................. 47
Description ...................................................... 47
The Audio Frequency Spectrometer .............................. 48
The Level Recorder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
Recording Paper ................................................ 50
Copying of Recorded Information ........ . .... .... ............... 51
Operation ...................................................... 52
General ......................................................... 52
Synchronization ................................................. 52
Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
Automatically Recording Harmonics ............................. . 54
Vibration Measurements ... .................................... . .. 57
Noise Measurements ............................................. 57
Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
 Description
General.
The Beat Frequency Oscillator Type 1014 is primarily designed for electrical
and electro-acoustical measurements and consists of an oscillator- and mixer-
section , and an amplifier section.
It works on the heterodyne principle using two high frequency oscillators,
one uf which operates on a fixed frequency , while the frequency of the other
can be altered by a variable capacitor. The required audio frequency is then
obtained as the difference between the two high frequencies and can be read
off a large illuminated scale, the pointer of which is connected to the variable
eapacitor. The scale is logarithmic and graduated from 20 to 20ooo c/s. An
"Incremental Scale" is also provided , allowing exact frequency selection in
the range - 50 to +50 c/ s around any setting on the main scale.
The zero-adjustment is carried out by obtaining a beat between the ' frequency
of the mains voltage, and that of the oscillator voltage occurring'' when the
oscillator is tuned to the frequency of the mains and the pressbutton marked
" Power Frequency Beat" on the front panel of the oscillator is press~d.
The variable capacitor has two control knobs , one of which is in a fixed
position on the capacitor spindle and is used for quick setting to the de-
sired frequ en cy. The other will , when depressed , rotate the spindle with a
ratio of 1 to 5 giving greater accuracy and final selection of the frequency.
A worm gear permits the capacitor to be tuned automatically, for example,
with the aid of the motor of the Level Recorder Type 2305. The mechanical
connection to the Level Recorder is effected by means of a flexible shaft
which can be screwed onto the bushing on the side of the Oscillator's cabinet
during which the motor should be kept running. The worm gear can be
engaged and released with the aid of a built-in electromagnetic clutch,
operated from a switch on the front panel, or by a remote control arrange-
ment. The electromagnetic clutch is a friction-device allowing manual tuning
of the variable capacitor even when the clutch is engaged.
Being also designed for use in room-acoustical measurements, the Beat Fre-
quency Oscillator is equipped with frequency modulation, for which a re-
actance tube controlled by saw-tooth oscillations from a built-in oscillator is
switched into tbe circuit of the fixed oscillator. Both the frequency and the
amplitude of the saw-tooth oscillation are adjustable and may b~ read off
two printed dials. Provision is also inade for external modulation, whereby
very wide limits of frequency modulation can be obtained.
By means of a compressor circuit, which can be controlled from an externn.l

5
voltage, it is possible to keep the voltage, current, or sound pressure constant
during measurements when using the oscillator as a power source.

Description of the Oscillator and Mixer-Section.

Fig. 1 shows a block diagram of the complete Oscillator.
The fixed oscillator is tuned to 120 kc/s and can be frequency modulated. The

Impedance
Fixed Variable )J Low Pass Output Matching
Oscillator Amplifier Mixer Filter Amplifier Circuit

Load
Output

Regulating Variable
Amplifier Oscillator

100
°C(~ emote
L.~_Jfontrol
Ref. Sig.
Attenuator
Output

Magnetic : Output 1605+6

Clutch 1 Attenuator
I

Fig. 1. Block Diagram of the Beat Frequency Oscillator 1014.

reactance tube circuit acts as a variable inductance and the modulation swing
can be continuously varied from 0 to ± 200 c/ s by means of a potentiometer
on the front panel of the apparatus, marked "Modulation Swing".
By means of the switch marked "Modulation Frequency" the frequency of
the built-in saw-tooth oscillator may be chosen. Frequencies of 1- 2- 4- 8- 16
and 32 c/ s are available. The oscillator is a blocking type, tuned to approx-
imately 7 Mc/s, and the frequency of the saw-tooth oscillations is set by
changing the grid resistor.
Provision is made for external modulation. The external generator should then
be connected to two terminals of the socket on the front plate marked
"Remote Control". For external modulation a voltage of approximately 7 volts
is necessary when a modulation swing of ± 200 c/s is required. The im-
pedance of the external generator must be low (approx. 1 kfl).
When external modulation is employed the switch marked "Modulation
Frequency" must not be in position " Mod. Off.", as in this position of the
switch the reactance tube is cut off.
A variable capacitor of 60 pF, inserted in the tuned circuit of the fixed
oscillator, and operated by the knob marked "Frequency Increment", permits
exact frequency selection in the range ± 50 c/ s for any setting on the main
scale.

6
By means of a noiseless switch on the front panel, marked "Oscillator Stop",
the voltage on the anode of the 120 kc/s oscillator can be disconnected. This

---------------------~ ' , DISTORTED

'~SIGNAL
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' MAXIMUM METER

DEFLECTION
-20

-301t---------
"OUTPUT VOLTAGE"
CONTROL

-1.0

- 501t--------

-60

-70

1
: -80
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1-
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0

-
100
o .1 .2 .3
----!0~~~~0
t. .5 .6 .7 B 9 1.0 1.1 1.2 1.3
VOLTAGE ON "COMPRESSOR" INPUT _ _ _ __.
1.1. 1.5 VOLTS
1605+7
("COMPRESSOR VOLTAGE" ON MAXIMUM)

Fig. 2. Regulation Characteristics for different positions of the potentiometer

marked ((Output Voltage".

7
innovation is specially provided for reverberation measurements. The same
method is used for remote control, the appropriate wiring of which can be
seen by referring to the circuit diagram of the Oscillator.
The output voltage from the fixed oscillator is fed to the grid circuit of a
pentode, tpe grid bias of which, is controlled, by means of a regulating
amplifier. To obtain a higher degree of control the working-point of the
pentode is chosen on the non-linear portion of the Ia-Eg characteristic, near
cut-off.
The purpose of this circuit is to control automatically the output power of
the Beat-Frequency Oscillator by an A.F. control voltage. For example, the
voltage from a standard microphone placed in the sound field of a loud-
speaker which is fed from the Oscillator. In this case the output power of
the Oscillator will be so controlled that a constant sound pressure is main-
tained on the standard microphone.
The A.F. control voltage should be fed onto the screened socket marked
" Compressor Input" on the front panel of the Oscillator. A variable potentio- ·
meter, marked " Compresso'r Voltage" is inserted in the input circuit of the
regulating amplifier and can be used as volume control for the output power
from the Oscillator when automatic regulation is employed. The regulating
amplifier has a linear frequency characteristic from 20 to 20ooo c/s and
should have an input signal of approximately 1 volt on the grid of the A.F.
amplifier tube for full regulation. The input impedance, measured across the
terminals of the socket marked " Compressor Input" is approximately 100
k.Q, and the maximum obtainable range of regulation is 45 db.
The amplified A.F. control voltage is rectified in a full-wave double-diode
rectifier, designed to give a DC output voltage proportional to the average
value of the A.F.-control voltage. ·
By means of the switch marked " Compressor Speed" on the front panel of
the oscillator the regulation speed can be varied. Regulation speeds of 30-
100-300 or 1ooo db/ sec. may be chosen by changing the value of the
capacitor in the A-C filtering network for the rectified control voltage. When
the switch "Compressor Speed" is in position "Comp. Off." the output from
the rectifier is short-circuited thereby disconnecting the automatic regulation
circuit.
To make regulation of the output level possible, even when maximum output
power is required from the Oscillator, the level of the high frequency voltage
from the 120 kc/ s fixed Oscillator is raised 10- 15 db when the automatic
compression ' is switched in.
The anode-circuit of the pentode in the variable-It amplifier is tuned to
120 kc/s, forming a band-pass filter, the output of which is fed to the mixer.
In the mixer tube, which is one half of a twin triode, the 120 kc/s voltage is
mixed with the output voltage from the variable oscillator. The frequency
of the variable oscillator can be altered continuously from 120 to 100 kc/s
by means of a specially designed variable capacitor. This capacitor is made

8
with a high degree of accuracy and a maximum deviation of o. 7 degrees
from a logarithmic frequency curve is obtained. A worm gear, connected to
the capacitor spindle, permits automatic tuning with the aid of an external
motor, for example the motor in the Level Recorder Type 2305, and the
worm gear can be set and released by means of a magnetic clutch. This is
operated from a switch on the front panel of the oscillator, or it can be
operated from an external switch or relay. Connection must then be made
to the appropriate terminals of the socket marked "Remote Control" on the
front panel, and the control switch for the magnetic clutch must be in
position " Clutch On".
By means of a pushbutton marked "1ooo c/s Ref. Signal" an extra capacitor
is introduced in the tuning circuit of the variable oscillator. This changes
the frequency of the oscillator so as to increase the output frequency an
amount of 980 c/s. With the scale pointer set to 20 c/s the output frequency
is thus 1ooo c/ s and may be used for adjustment purposes as explained on
page 19, item 17.
As ,previously mentioned, the frequency scale is logarithmic and calibrated
20-20ooo c/ s. When the capacitor is set to frequencies above 20ooo cis
or below 20 c/ s the fixed Oscillator is blocked, and consequently no output
voltage will be obtained. For automatic recording of frequency characteristics,
i.e. when using the Level Recorder Type 2305, this is a great advantage as no
unwanted curves will then appear on the corresponding section of the fre-
quency calibrated paper.
The voltage developed across the grid circuit of the variable capacitor is
fed onto the mixer tube and mixed with the voltage from the variable-.u
amplifier. The mixer tube is of the triode type, whereby a low hum level
is obtained in spite of the AC-heating of the filament.
A low-pass filter having a cut-off frequency of 50 kc/ s is inserted in the
anode circuit of the mixer tube, thus passing only the lower frequency
obtained by the frequency conversion, onto the grid circuit of the first tube
in the output amplifier section.

Frequency Marking.
An arrangement, which can be operated by the control switch for the
magnetic clutch, is the marking of a certain frequency. At this frequency
the fixed Oscillator is short-circuited, while at all other frequencies the
normal output voltage is available. Basically, the blocking is effected by
closing a pair of relay contacts. These contacts are operated by a cam disc
which is mounted concentrically on the tuning capacitor spindle. Normally
the cam rotates with the spindle, being held in position by a spring-loaded
friction clutch. However, by means of a pawl the cam disc can be located at
the point of its cycle where the short circuiting relay is closed.
This is carried out by setting the control switch for the magnetic clutch to
position "Marking Adjustment", and turning the capacitor spindle until a

9
small "clickl' is heard. Because of the friction clutch, it is now possible to
turn the capacitor and the scale pointer relative to, and independent of the
cam. Release of the pawl, by means of the control switch, permits the cam
to rotate once again with the capacitor.

Partial Blocking of Frequency Range.

When employing the special cam disc OD 0065 delivered with the B.F.O.,
a part of th~ frequency range can be blocked, i.e. the output signal is inter-
rupted. The cam disc operates the same relay arrangement as used for the
frequency marking. The range length which can be blocked corresponds to
approximately two decades. This means that any range from 20 c/s up to
approximately 2ooo c/s, an1 from 20ooo c/ s down to approximately 200 c/ s
can be blocked in addition to the normal blocking outside the scale gradu-
ation. In Fig. 3 is illustrated the ranges obtainable. The two hatched ranges
represent the blocking ranges. The range indicated by "Fixed" is the normal
blocking arrangement which functions outside the scale graduation. The
other range can be added to the normal, as shown in the figure, or it can
function discontinued. A complete instruction of the mounting of the cam
disc will be found under part Operation.
In applications where the B.F.O. is employed in conjunction with the B & K
Level Recorder, and where automatic recording is employed, the blocking
arrangement can also be used for remote lifting of the Level Recorder's

Frequency Marking

Fig. 3. The two blocking arrangements : Frequency Marking and Partial

Blocking of Frequency Range.

10
writing pen. This is a great asset in for example measurements where the
compressor circuit of the B.F.O. is used. In this instance the pen-lifting
arrangement of the Level Recorder can be con~rolled from the frequency
blocking circuit by making the appropriate connections to the "Remote
Control" jack of the B.F.O. In cases where the entire frequency range
(20-20ooo c/s) of the B.F.O. is utilized, the normal frequency blocking,
which functions outside the scale graduation, should be set out of pperation.
The writing pen of the Level Recorder can now be lifted from the paper
outside the frequency range of interest and a proper working of the som-
pressor also at the initial frequency (20 c/s) is ensured during the automatic
scan. If the described method is not utilized, the following would take place:
No signal will be present in the range 20ooo c/s to 20 c/s (outside the scale
graduation), i.e. the compressor of the B.F.O. will be in such a condition to
give full output signal of the B.F.O. Consequently, when the scale pointer
goes inside the scale graduation (20 c/ s) full output level will be transmitted
at 20 c/s, and fi~st after the chosen time delay (Compressor Speed) the signal
level will be compressed to the proper (preset) value. A deflection on the
recording paper which is not a response of the measured object would thus
be recorded.

Description of the Output Amplifier Section.

The voltage from the low-pass filter is fed to the control grid of the first
tube in the two-stage audio frequency output amplifier via a variable
potentiometer. This potentiometer is operated by the knob marked "Output
Voltage" on the front panel of the Oscillator and is used for continuous
adjustment of the output power.
The gain of the amplifier is stabilized by means of negative voltage feedback,
and the anode circuit of the output tube is coupled to an auto-transformer
for impedance matching.
Four different output impedances are available and can be chosen with the
switch on the front panel marked "Matching Impedance". The different
positions of the switch are indicated by 6, 60, 600 and 6ooo ohms respective-
ly, and the output voltage is taken from the terminals marked "Load". It
should be noted that the output impedance of the Oscillator is only approx-
imately 10-20% of the indicated values, but with correct loading a max-
imum output power is obtained with a minimum harmonic content. Further-
more, correct loading ensures the output voltage to be practically independent
of the frequency.
A fifth position of the switch "Matching Impedance" is marked "Att." and
connects the output transformer to an attenuator, variable in steps of 10 db
from 120 ,uvolts to 12 volts, and is operated by the switch marked "Attenu·
ator" on the front panel. With the impedance switch in this position the
output circuit is connected to the screened socket on top of the front panel,

11
 10
Ofb

0.5

c 0.2

~
:0 0.1

i
D

z
0.05 - - - = Typical Distortion Curves
------ • Guaranteed Upper Limit

' 0.02
~ 1 ~1o------------~1o~o-------------1+oo-o----------~1o~o~oo~ f C/s
---Frequency ~6~072

Fig. 4. Distortion curves for different loads. The curve marked "Att. 10 volts"
is obtained from measurements taken on the "Attenuator" output terminals:
open circuit.

the output impedance being constant and approximately 50 ohms. The overall
accuracy of the attenuator is approximately 2 %.
The voltage on the output terminals is indicated by a vacuum-tube voltmeter
which measures the average value of the A.F. voltage. It is calibrated in
r.m.s. values of a sinusoidal voltage, and the accuracy in the frequency
range 20-20ooo c/s is 1.5 % of full scale deflection.
The sensitivity of the voltmeter is automatically changed when the position
of the switch marked "Matching Impedance" is altered. Full deflection of
the meter is indicated on the switch. When the "Matching Impedance" switch
is in position "Att." the output voltage available from the Oscillator will
depend on the position of the "Attenuator" switch, in this case full
deflection of the meter corresponds to the value indicated by the switch
position.
The signal-to-noise ratio of the Oscillator is greater than 70 db for maximum
output voltage. It is independent of the position of the attenuator, but some-
what dependent on the position of the potentiometer marked "Output
Voltage". The best result is obtained when this is positioned on or around
the point marked 8.
Harmonic distortion is dependent on the setting of the "Output Voltage
Potentiometer". The distortion increases as the output voltage is increased,
but as long as the output is kept within the meter range, the distortion will
be of the order indicated in Fig. 4.

12
Power Supply.
The Oscillator can be operated from a 240, 220, 150, 127, 115 or 100 volts AC
power line, the power consumption being about 70 watts .
The proper voltage is selectable by a switch-fuse combination situated at
the rear of the instrument. To select the voltage it is necessary to firstly
remove the fuse by turning the hexagonal disc head in the centre of the
switch anti-clockwise . Then with the aid of a screwdriver turn the head of
the voltage adjuster until the white mark is aligned with the required voltage.
The fuse is then replaced.
It should be noted that if the apparatus is to be operated from a DC power
line, or from an accumulator, a vibrator unit or a rotary converter is
required.

13
 Operation
General.' ·
Firstly ascertain that the Beat Frequency Oscillator is set to the appropriate
power supply voltage by means of the selector at the rear of the instrument
and that the Remote Control plug on the front panel is firmly in its place.
After connection to the power supply, the instrument can be switched on
by the toggle switch marked " Power" on the front panel. The dial lights in
the meter and in the frequency scale should immediately come on.

Attenuator
Output

Magnetic Load
a~ch ~--------~~~...
Corrtrol

Compressor
Input

Output

Modulation
Frequency
1 000 r;s Ref. Signal
160590
Modulation Compressor Frequency
Control Swing Speed Scale
Adjustment

Fig. 5. Drawing of the Oscillator 1014.

14
A. Calibration.
1. Turn the toggle switch marked "Power" to " On" and allow two minutes
warm up.
2. Set "Modulation Frequency" and "Compressor Speed" to their "Off"
position.
3. Set "Clutch, Marking" to "Marking Adjustment" position. Rotate the main
scale until a small click is heard and extra weight is felt on the drive.
Continue to rotate until the pointer is set within uncalibrated portion of
scale, i.e. between 20ooo c/s and 20 c/s. Then return clutch switch to
"Off".
4. Now turn main scale pointer until it is on the frequency of the line
voltage (e.g. 50 or 60 c/s), checking that the frequency incremental
scale is on zero. If not, set by "Frequency Increment" knob to this
point.
5. Set suitable deflection on the meter by tuning the knob marked "Output
Voltage" to higher than center scale reading.
6. Press "Pow~r Frequency Beat" button and hold to "in" position and at
the same time rotate "Frequency Scale Adjustment Fine" slowly, until a
large fluctuation registers, slows up, and practically ceases on the meter
dial. Two points may be found where this occurs, only one of which is
correct and therefore a check as outlined in the following paragraph
should be carried out, firstly releasing the "Power Frequency Beat"
button.
7. Turn the main scale pointer to 20 c/ s and with the "Frequency Increment"
knob reduce scale reading to -20 c/s or -30 cis depending upon supply
frequency in use. If the frequency is correct the meter needle will drop to
zero indicating that the B.F.O. is properly tuned. If it does not reach zero,
repeat procedure from item 4.
8. Finally return "Frequency Increment" to zero. The B.F.O. is ready
for use.
N.B. If a zero point cannot be found and is outside the range of the '"Fre-
quency Scale Adjustment Fine", re-align the variable capacitor marked
)<Coarse" with a screwdriver to give a suitable ~etting, which should occur at
some point between 4 and 6 on "Frequency Scale Adjustment Fine".

B. Operation Using the Output Terminals Marked "Load".

Apply the folowing procedure:-
1. Set-up and calibrate the oscillator as described in A.
2. Place the "Matching Impedance" switch in a suitable position for the
application.
N.B. Full deflection of the instrument meter corresponds to the voltage
indicated by the switch position.
3. Connect the load to the output terminals marked "Load".
N.B. Right terminal is grounded.

15
 4. Turn the pointer on the main frequen 1y dial to the desired frequency,
finely adjusting the Frequency Incren1 ent if necessary. (For automatic
frequency sweep, see under E).
5. Select a suitable output voltage by h rning the knob marked "Output
Voltage".

C. Operation Using the Built-in Attenuatof .

Apply the following procedure:-
1. Set-up and calibrate the oscillator as iescribed in A.
2. Set the " Matching Impedance" switcl• in the position "Att.".
3. Select the appropriate voltage range IJy means of "Attenuator" .
N.B. Full deflection of .the instrumPut meter corresponds to the voltage
indicated by the switch position.
4. Connect the load to the screened oulput socket on the top of the instru-
ment marked "Attenuator".
5. Proceed as in B. 4 and 5.

D. Frequency Modulation.
When a frequency modulated output ~ :gnal is required, the following proce-
dure should be adopted:-
1. Turn the knob marked " Modula :ion Frequency" to the required fre-
quency.
2. Turn the knob marked "Modulati .m Swing" to zero.
3. Re-calibrate the Oscillator as described in A.
4. Set the "Modulation Swing" knob to the required frequency swing
(bandwidth).
5. Proceed as described in B items 2 to 5, or C items 2 to 5, dependent on
the requirement.

E. Automatic Recording.
By combining B.F.O. Type 1014 and Level Recorder Type 2305, or using
Automatic Frequency Response Recorder Type 3304, it is possible to auto-
matically record the frequency response of four terminal networks. When
using B.F.O. Type 1014 and Level Recorder 2305, it is necessary to connect
the two instruments mecha:Qically by a Flexible Shaft UB 0040 as in Fig. 6
and to make the electrical connec tions also shown. Fig. 7 depicts the use
of the Automatic Frequency Response Recorder Type 3304 with the required
external connections.
For setting-up, calibrating and synchronising the combination shown in
Fig. 6 the following procedure should be adopted : -
1. Ensure power supplies are correct and switch powe~ toggles to the
" On" position.

16
 1014

Object
under !60519
test

Fig. 6. Example of Mechanically Connecting the BFO 1014 to Level

Recorder Type 2305.

3304

Object
under
!60SSO
test

Fig. 7. External Electrical Connections when using Automatic Frequency

Response Recorder Type 3304.
17
 2. Calibrate the B.F.O. as described in A.
3. Connect the instruments as shown in Fig. 6. This is done by connecting
a flexible driving cable (UB 0040 ) to the upper driving shaft of the
Recorder "Drive Shaft I" located at the right-hand side and to the front
of the Level Recorder. Taking the other end of the cable, insert and
screw in drive on left-hand side of B.F.O. (Check engagement by
switching the Level Recorder "Start/ Stop" switch to "Start" and the
B.F.O. magnetic clutch to "On" and note if scale pointer rotates).
4. Switching "Paper Drive" to "Stop", continue with the following proce -
dure referring Fig. 8.

Single Chart- Cont. Record

Evant Marking

Potentiometer Range
db

Range Potentiometer

Drive Shaft n
Drive Shaft I

Fig. 8. Level Recorder Type 2305 viewed from above.

5. Load the Level Recorder with the desired recording paper. (Follow
instructions in Level Recorder Manual).
6. Select and insert required Range Potentiometer. (N.B. Place "Potentio-
meter Range db" switch to "Standby" when changing potentiometers).
7. Switch " Potentiometer Range db" until figure corresponds to the Range
Potentiometer being used, i.e. " 10", "25", "50" or 80".
8. By means of the switch " Rectifier Response", select R.M.S. or if
specially required one of the other three positions Average, Peak, or D.C.
9. Turn the "Lower Limiting Frequency" switch to the cut-off value (10,
20, 50 ox: 200 c/s) .
10. Set "Writing Speed" to required position.
(Full ex]>lanations of items 8, 9 and 10 can be obtained from the Level
Recorder · Manual).

18
11. Place "Reverse/Forward" switch to "Forward".
12. Select "Paper Speed" to a suitable speed, e.g. 10 mm/sec.
13. Pull gear-lever marked "x" to the outer position. (See Fig. 36).
The actual paper drive speed now corresponds to the small numQers
marked around the "Paper Speed" knob.
14. Two types of recording can be made:-
(a) Single chart recording (automatic recording over a length of 250 mm
paper only),
(b) Continuous recording over any length of paper.

(a) Single Chart Recording:

Set the "Paper Drive" toggle switch to "Start" commencing the
paper to run, which will continue until the built-in automatic
stop switch declutches the drive mechanism (less than one chart
length).
Reset recording paper by finger wheel "Z" (Fig. 36) until the
stylus rests on, for instance, the 10 c/ s line.
A chart of 250 mm length will now run off when the "S ingle Chart
- Continuous Recording" pushbutton is pressed and released
again immediately afterwards. (It is possible to stop the' recording
at any time by setting the "Paper Drive" toggle switch to "Stop").

(b) Continuous Recording:

The operator should follow the instructions outlined under (a),
i.e. "Single Chart Recording", except that to start the recording
it is necessary to press the "Single Chart - Continuous Re-
cording" push-button and turn it clockwise. Recording will now
automatically take place until the push-button is released again
and the "Paper Drive, Start-Stop" toggle switch is set to "Stop".
Note: Whenever the "Paper Drive, Start-Stop" toggle switch is
in the "Stop" position the paper drive is completely controlled
by the "Single Chart- Continuous Recording" push-button.
15. In order to synchroni·s e the units , stop the paper so that the stylus rests
on the 20 c/ s line.
16. Adjust the commencing of the reference line on the paper to a suitable
level, any necessary fine adjustment being made with the Input Potentio-
meter.
17. Set the pointer of the B.F.O. on 20 c/ s and engage the magnetic clutch
by use of the clutch switch. The units should then be synchronised.
18. Push the "1ooo c/s Ref. Signal" button. The B.F.O. then generates a
signal of looo c/s enabling the opera~or to select a reference signal
which is in the middle of the range. (This makes certain that when

19
 taking a recording of frequency characteristics, where the lowest
attenuation is around 1ooo c/s, that the deflection of the stylus lies
within the scalar limits of the paper during the recording).

Continuous Recording with ten Times Enlarged Paper Speed.

The following method is adopted: Set the "1 : 10 Synchronizing Gear Lever"
in its inner position (released). The actual paper drive speed then corresponds
to the large numbers marked around the "Paper Speed" knob. Recording on
frequency calibrated paper is not possible in this position.
The start and stop of the recording will in this case be completely controlled
by means of the "Paper Drive, Start-Stop" toggle switch.

F. Marking Adjustment.
Should an identity mark at some particular frequency be desired the follow -
ing procedure should be used:-
1. Set the magnetic clutch control switch to the position "Marking Adjust-
ment".
2. Turn the pointer of the main frequency dial until a small "click" is
heard, the click being associated with an increased effort in turning the
pointer.
3. Set the pointer to the frequency at which marking is desired.
4. Release the magnetic clutch control from the Marking Adjustment posi-
tion, reverting to clutch "Off" or "On" as required.
The instrument is now operable as in B, C, D or E the output voltage
~isappearing at the "marked" frequency.

G. Partial Blocking of Frequency Range.

When replacing the cam disc E (refer Fig. 9), which provides the frequency
marking, :with the special cam disc (OD 0065) delivered with the B.F.O. a
partial blocking of the frequency range can be obtained. (Refer also part
Description).
I. The replacement procedure is as follows (refer Fig. 9):
1. Disconnect the instrument from the power supply line.
2. Remove the perforated back plate of the cabinet.
3. Carefully remove the steel ball clutch contact from behind the clutch
contact spring at "A" by pulling the contact spring gently outwards
for a few millimeters (1/8") to release the contact pressure on the ...
ball which should be placed, after removal, in a small receptacle
for safe retention.
4. Set the clutch switch on the front panel to the "Clutch On" position.
5. Unscrew the three screws marked "B" in Fig. 9, taking the pre-
caution that the small spiral springs are retained as with the ball.
Remove the now free metal retaining plate "C". When removing this

20
 plate, pass it carefully between the contact spring at "A" and the
insulated bushing assembly "D".
6. Likewise remove the outer insulated cam ring "E", which will now
be loose, in the same manner as the metal retaining plate. The re-
maining insulated cam ring should not be removed.
7. Install the cam ring to be fitted, making sure that the white spot
on the surface near the slot on the perifery faces outwards towards
the operator.

Fig. 9. Cam disc arrangement. For explanation of letters, refer to text.

8. Replace the metal retaining plate, and the three screws together with
the three spiral springs, making sure that the screws are properly
tightened .
9. Replace the steel ball.
10. Replace the perforated back plate to the cabinet.

II. Adjustment of blocking range.

1. Set "Clutch" control to position "Marking Adjustment".
2. Turn main frequency scale pointer until a small "click" is heard, the
click is associated with an increased effort in turning the pointer.

21
 3. Set pointer to:
a. The frequency at which the blocking should be cancelled, if
blocking of the initial scale part is wanted .
b. Two decades higher, approximately, (will be outside the scale
graduation), at which the blocking should commence, if blocking
of the last part of the scale is wanted.
Check for proper commence of blocking. If necessary readjust
as in item b.
4. Release "Clutch" control from position "Marking Adjustment", re -
verting to position "Off" or "On" as required.
The B.F.O. is now operable as in B, C, D or E.

H. Automatic Regulation of the Output Power.

By means of the compressor circuit it is possible to regulate the output from
the oscillator. When a constant output voltage is required, the output voltage
from the Oscillator is used as a control voltage. A constant current is obtain-
able if the voltage drop across a resistor connected in series with the load,
is used as the control voltage, and a constant sound pressure is maintainable
with the aid of a regulating microphone. The microphone is then placed in
the sound field from a loudspeaker which is driven by the Oscillator, and
the microphone output voltage used as control voltage. (It is essential that
the frequency characteristic of the microphone is linear).
Proceed as follows:-
1. Calibrate the Oscillator as described under Calibration, see under A.
2. Set the "Matching Impedance" switch in the desired position.
3. Connect the load to "Load" terminals or to the screened output socket on
the top of the instrument, see B or C.
4. Feed the control voltage to the "Compressor Input" terminal. If neces -
sary use an amplifier which has a linear frequency characteristic for
the amplification of the control signal, approximately 1 volt being
required for full utilization of the compressor.
5. Set "Compressor Voltage" and "Output Voltage" to maximum (fully clock-
wise}.
6. Feed the voltage to be measured to the recording instrument, e.g. the
Level Recorder Type 2305.
7. Set the "Compressor Speed" switch in one of the positions: 30, 100, 300
or 1ooo db/sec.
8. Regulate the desired output voltage by turning "Compressor Voltage"
knob counterclockwise.
Note: \Vhen the Beat Frequency Oscillator is used in conjunction with the
Level Recoitder Type 2305 the writing speed of the Level Recorder should be
kept belo~ the regulation speed of the compressor.
It is also possible to obtain di.fferent regulation characteristics dependent on
the position of the potentiometer marked "Output Voltage" . This can be seen
from Fig. 2.

22
 Rep:~.ote
Control.
In the main description of the apparatus several forms of remote control are
mentioned. To carry out any one of these methods use must be made of the
"Remote Control" jack on the front panel, the appropriate connections being
made to the pins of the six-poled socket. Fig. 10 shows the different pins
on the socket.

Oscillator Stop
( +100 Volts d. c. approx.)
Internal
d c
Marking Contact

e b External Modulation

f a
Ground Magnetic Clutch
MIO•' .

Fig. 10. ''Remote Control'' jack, viewed looking towards front panel.

Hemote con trol of th e magnetic clulch can he obtained by setting or breaking

a connection between a and f, providing the clutch control switch is at the
position "Clutch Off" .
For external modulation it is necessary to connect the external generator
between chassis and terminal b, having the "Modulation Frequency" switch
set to any position except "Mod. Off."
For remote interruption of the output signal (stopping of the fixed oscillator)
the terminal c should be connected to terminal f (ground). This arrangement
is used, for instance, when reverberation measurements are carried out
autol?atically by employing tl::) B & K Level Recorder Type 2305. A special
switch in the Rec·order then connects terminal c to ground when the radiated
, signal has to be interrupted.
Terminals d and e are in connection with an internal contact used for mark-
ing a certain frequency within the Oscillator's frequency range and further
to interrupt the signal output when the frequency scale pointer is outside
the scale.
Note: When delivered from the factory, each B.F.O. is supplied with a
6-poled plug containing the necessary connections for the function of the
internal contact.

Trouble Shooting.
If th~ B. F. 0. is not ~orking properly when switched on, check the follow
• ing:-
1. · That 6-poled plug for the "Remote Control" jack is in position.
 2. That scale-pointer is not situated in the uncalibrated section of the main
dial, i.e., between 20ooo c/s and 20 c/s.
3. That scale-pointer is not on the "marked" frequency or on a section
chosen for "partial blocking of frequency range", see pages 9 and 10.

Accessories
Output Transformer TU 0005.
This transformer is designed to allow symmetrical output from the altenu-
ator out'pqt of the B.F.O. 1014. (Symmetry better than o.1 %). The output
impedance' is 600 Q and the distortion o.5 % at 20 c/s with maximum output
voltage from the B.F.O. (12.6 V). The accuracy of the Transformer is ± o.2
db in the frequency range 10 c/s to 35 kc/s. In addition a core material has
been chosen for the transformer, which makes it possible to "preload" the
secondary winding with a current of 100 rnA without causing additional
distortion for frequencies above 300 c/s. The transformer ratio is ylo : 1.

24'
 Applications
The field for use of the Beat Frequency Oscillator Type 1014 is so extensive
that only a few of the possible applications are illustrated in the following
pages, these being classified into three sections, showing the instrument being
used as a power source for:-
(A) Electronic Measurements
(B) Acoustical Measurements
(C) Mechanical Measurements.

GROUP A. ELECTRONIC MEASUREMENTS

Measurement of Frequency Response of Four-Terminal Networks.
The object to be tested, e.g. a filter, transmission line, transformer etc. is fed
from the Beat Frequency Oscillator Type 1014 output. Then point-by-point
measurements can be taken by means of the Audio Frequency Voltmeter
• Type 2409 (or 2410) or Microphone Amplifier Type 2603 or 2604.

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Brilol & Kl-

Fig. 11. Measurement of frequency response of four-terminal A.F. network.

25
If an automatic recording of the frequency response is wanted, the Level
Recorder Type 2305 should be used. The mechanical coupling between the
motor in tqe Level Recorder and the tuning capacitor of the B.F.O. is
effected with a Flexible Shaft UB 0040 which is delivered with the B.F.O.
The measuring arrangement which is employed to obtain the frequency
characteristic of an A.F.-filter is shown in Fig. 11.
Should the compressor circuit be used to regulate the output signal from
the Oscillator it is advisable to verify that the voltage at the "Compressor
Input" is approximately the required 1 volt. When it is intended to use the
equipment for automatic recording of frequency characteristics, the input
of the Level Recorder may first be connected to the input of the compressor,
and a recording of the compressor input voltage made for the complete fre-
quency raf\ge in which measurements are to be taken. With the compressor
working correctly the resultant recording should be a straight line. If this
is the case the input to the L~vel Recorder can then be disconnected, and the
desired measurements carried out.

A.C. Bridge Measurements.

By employing the B.F.O. Type 1014 and a Frequency Analyzer Type 2107 as
an indicating instrument selective measurements of components in an A.C.
bridge can be obtained.
The only requirement the bridge must satisfy is that one diagonal point can
be grounded as shown in Fig. 12. This requires the bridge to be supplied from
the B.F.O. via a screened transformer e.g. TU 0005, the B.F.O. being grounded
at one terminal.

Fig. 12. Tile B.F.O. Type 1014 used as voltage source for AC Bridge Measure-
m ents . Th e Output Transformer TU 0005 provides a symmetrical output from
the B.F.O.

The balancing transformer TU 0005 contains two precisiOn resistors and

serves as one half of the bridge circuit. A total output impedance from the
,
transformer of 600 Q is obtained, and the balance between the two trans-

26
 former "arms" is better than o.1 %. The voltage transmission loss of the
transformer when loaded by 600 [J is approximately 16 db and the distortion
less than 1 % for a transformer input voltage of approximately 12 V.
Due to the selectivity of the Frequency Analyzer it is well-suited as an
indicating instrument in a bridge circuit. The decibel scale on the instru-
ment meter will often prove useful when it is desired to measure the quality
of different components placed within the bridge.

Measurement of Temperatures and Temperature Differences.

Electrical measurements and recordings of temperatures and temperature
variations can be carried out by using the combined BFO Type 1014 and Level
Recorder Type 2305 i.e. the Automatic Frequency Response Recorder Type
3304 (see Fig. 13), in conjunction with Negative Temperature Coefficient
(N. T. C.) resistors.
A practical example of this is shown in Fig. 14a where the temperatures on
both sides of a wall are being measured "simultaneously" by the equipment,
allowing the heat loss or insulation value of the wall to be calculated. This
joint recording is possible by use of the two -channel selector which is in-
corporated in the recorder section of the Type 3304 .
• To make accurate temperature measurements by means of thermistors the
following method should be adopted:-
1. The signal voltage must be constant, this being obtained by using the
compressor circuit.

Fig. 13. Automatic Frequency Response Recorder Type 3304.

27
2. The current has to be determined by measuring the voltage drop across
a fixed resistor which is connected in series with the respective termistor
(see sketch 14a). The obtained voltages are fed to the two input terminals
of the above mentioned selector in the Level Recorder section of the
Type 3304.

Heat
®
Source

Fig. 14. Measurement of temperatures on both sides of a concrete wall.

(a) Measuring arrangement.

3. Prior to the final measurements the set-up must be calibrated, this being
done by placing the thermistors in a bath of warm oil the temperature
of which has been taken with an ordinary thermometer. During the
cooling period the voltage drop across each resistor is taken at requisite
intervals and plotted on the side of the paper to be used.
As the ther~istors have a negative temperature co-efficient a decrease
in temperature will cause the voltage drop across the fixed resistor to
fall as can be seen on the calibrated curve in Fig. 14b.
Fig. 14c shows the recordings of a "simultaneous" measurement taken
f
with the arrangement as in 14a. The wall having firstly been warmed on
one side by a constant heat source to the desired temperature, then re-

28
 ~ D 0 D 0 D 0 0 D D D D D 0[
lllllllllll
~
~on 50 Ln 30 20
h
_l
'1
)

""n I
"L..
t;..,
I)...,
n-.
lh_
lll-'lr.
II-...,_
II th.
II I fTh- h.ltlln. on
..... &. .I.&. lllllllllh
.... .._...... l
·~··
~--

..
_

• AU.::IU m V I I I"----'

Fig. 14 ( b) Calibrati on Curve.

0 D D D 0 D D n 0 0 0 D 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Fig. 14 (c) Si m ultan eous rec ord ing of th e temperature on both sides of
th e wall .

29
 moving the source whereby the temperature gradually falls on the heated
surface but still rises on the other surface due to the "inertion" of the
heat transmission through the structure, see Fig. 14c.

Measu._-ement of Gain in A.F. Amplifiers.

The measurement of distortion and frequency response of A.F. amplifiers
may be carried out in the same manner as for four-terminal networks, the
descripUon for the arrangement being given in the initial paragraph to tHis
section.
Frequerltly it is important to determine the linearity of an amplifier i.e. to
measure the gain for different values of input voltage. As the attenuator
circuit of the Beat Frequency Oscillator Type 1014 is very accurately eali-
brated it is an extremely useful instrument in carrying out gain measure-
ments.

~ --:.- •
-o- •
i
·:· :·
""'"i= JL .;~-~ .:§_

Amplifier -"7i:=
1014 2804
t60~57

Fig. 15. Measurement of gain in an A.F. amplifier.

The output voltage from the amplifier under test should be measured with an
Audio Frequency Voltmeter Type 2409 (or 2410), or a Microphone Amplifier
Type 2603 (or Type 2604) an example of the arrangement being given in
Fig. 15.

GROUP B. ACOUSTICAL MEASUREMENTS

Frequency Response Recording of Loudspeakers.
Loudspeaker tests may be carried out either in an anechoic chamber, or in
the open air. In the open air, noise is generally present, and a completely
sound absorbent room should preferably be used.

30
 Absorbing material

~0

.
I@-
: e::
:: :.

. '\;;P ~"""'!!!~-----__.

2803 3304

Fig. 16a. Measuring arrangement used for recording the frequency

characteristic of a loudspeaker.

.... ........
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
.... ..... "'"'

10 100 1000 10000

160559

Fig. 16b. Recording of the frequency characteristic of a loudspeaker mounted

in a cabinet. (The m easurement was not carri ed out in a completely dead
room, and the effect of reflections can be seen from the recording).

31
The loudspeaker under test should be fed with a constant voltage or current,
the latter producing a mechanical force of constant amplitude which is
applied to the diaphragm.
Fig. 16a shows a set-up for recording the frequency characteristic of a
loudspeaker. The loudspeaker is fed from the B.F.O. section of the Auto-
matic Frequency Response Recorder Type 3304 via a series resistor the
voltage drop across which is led to the Compressor Input of the B.F.O. A
constant current will therefore be obtained in the circuit when the voltage
across the resistor is approximately 1 volt.
The output voltage from the Condenser Microphone Type 4131 is fed to the
Level Recorder in the Automatic Frequency Response Recorder via an
Amplifier. This amplifier can be a Microphone Amplifier Type 2603 (or 2604)
or a Frequency Analyzer Type 2107 or 2112. The Amplifier should be
switched to have a linear frequency characteristic.
An advantage gained by employing a Frequency Analyzer as an amplifier is
that distortion measurements can be carried out with the same measuring
set-up. Fig. 16b shows a recording obtained with the described set-up.

Recording of the Frequency Response of Microphones.

Fig. 17a shows a typical arrangement for automatically recording the fre-
quency response of a microphone.

4131

i•
•.
.,

~·= _Q_ .;~-~ _g_ ·•i '·

-~·:=-
2804 2104

f6H60
A
Fig. 17a. Measuring set-up for automatic recording of the frequency response
of microphones.

32
In the set-up depicted, the microph~nc to be tested is connected to the Level
Recorder Type 2305, via a Microphone Amplifier Type 2604, the originating
sound source being a loudspeaker which is fed from the B. F. 0. Type 1014.
As the sound pressure in front of the microphone under test has to be kept
constant, it is necessary to place it relatively close to another microphone
(in this case a Condenser Microphone Type 4131) which is coupled to a
second Microphone Amplifier Type 2604, the output of which is fed to the
"Compressor Input" of the BFO ensuring a constant sound source. It is
essential that the two microphones are symmetrically placed in th ~ radiated
sound field and the correct compressor speed selected. If the acoustical delay
time required for the sound to travel from the loudspeaker to the micro-
phone is r, this period must be small in comparison to the time constant
determining the compressor speed. Under normal circumstances these con-
ditions are easily fulfilled. l

.........
DD D DDOIJDIJDIJDOOO OO OOODOODO DDO D DD DD D DDIJDDDODDD

..c
-~

"'.,......., ............ c ....

'"
100 1000 10000 (1on/:ztn) A I C l..lft.

Fig. 17b. Recording made with th e set-up shown in 17a.

To give reliable measurements the room to be used need not be fully anechoic
as the regulating effect of the compressor will compensate for any minor
reflections set-up . However, for correct operation of the regulation circuit,
the reverberation time of the room must not be too long and a low scanning
speed should be used for the frequency sweep.
In Fig. 17b will be seen a recording showing the frequency response of a
microphone recorded by employing the previously outlined system.

Recording the Frequency Characteristic of Hearing Aids and Earphones.

A recommended set-up for the testing of the above units is displayed in
Fig. 18a. By this method it is possible to automatically record the frequency
characteristics of the components under well-defined acoustical conditions.

33
The B.F.O. 1014 feeds the earphone under test which is placed in the
Artificial Ear Type 4151. The ear consists basically of a base mounted on
a board and a replaceable acoustical coupler. Different types of couplers are
available. A DB 0138 2 cm3 which conforms to ASA Z 24. 9. 1949 and the new
IEC standards is suitable for measurements on insert type of earphones.
For headsets and similar external earphones a 6 cm3 can be supplied e.g.
DB 0160 (N . B. S. type) or DB 0161 (A. S. A. type).
(a)

+
-.- : ·e:

·;·~~
.•;,;-,·!- . -~~~~·:·
21.03 1014

(b)

-
DDDDDDDDDDDDDDDDDOODDDOODDDDDDOODOODOODDDDDD

100 20000 c/1 40000 A I C U...

( l6:'1/2tt1 ) A I C lJn.
10 100 1000 10000

Fig.18.
(a) Measuring arrangement.
(b) Recording of the frequency characteristic of an earphone.

A B&K Condenser Microphone 4132 is placed in the coupler and measures

the S. P. L. produced by the earphone. The output from the Microphone is
fed to the input of the Amplifier Type 2603 and the amplified signal led to
a Level Recorder Type 2305 to obtain a graphic recording, see also Fig. 18b.

34
Checking of Hearing Aids.
An arrangement for the checking of Hearing Aids is illustrated in Fig. 19a.
This set-up makes it possible to automatically record the frequency character-
istic of a complete hearing aid, under what are approximately free field
conditions.

l61t767

Fig.19.
(a) Arrangement for automatically checking the frequency characteristic of a
hearing aid.

The hearing aid earphone under examination is placed on the ear of the
Hearing Aid Test Box Type 4212, which consists of an external artificial ear,
a regulating microphone, a built-in loudspeaker, the latter two of which are
enclosed in a small anechoic chamber. The chamber is effectively insulated
against both airborne and impact noise, allowing measurements to be taken
down to 50 db re 2 X 10-4 .ubar approximately.

35
 The hearing aid and the regulating microphone are placed symmetrically in
the sound field. The regulating microphone is connected to the Microphone
Amplifier Type 2603, which amplifies the signal and then applies it to the
Compressor· input of the iF.O. Type 1014. This combination enables the
sound pressure level on the hearing aid to be kept constant without in-
fluencing the practically free sound field conditions.
The B. F. 0. Type 1014 supplies the required power for the loudspeaker in
the chamber, while a B & K Condenser Microphone, which is placed in the
Artificial Ear, is used for tpe measurement of the acoustic output from the
hearing aid. The microphone is connected to a Microphone Amplifier Type
2603, and the amplified voltage is led to the input of a Level Recorder Type
2305.

IJ o
1
o o o o o o o o o : o o o o o o ~ o o. o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o
BrD~I•r +.502:5

-----...
~h~~~cr;r,stjc
of hearmg ajd
.... max . amplification

reduced amplification

QP 1123 10 100 1000 10000

16(068

Fig. 19. (b) Recording by 19 (a). Taken for different settings of the hearing
aid volume control.

Fig. 19b shows typical characteristics of a hearing aid device automatically

recorded with the arrangement described in 19a. (N .B. Recordings are taken
for two different settings of the hearing aid volume control).

Measurements on Air Filters, CarbureUor Inlets, etc.

To carry out measurements ·o n the above and other such items as mufflers,
silencers, etc. it is necessary to provide a high but constant sound level
source. This can be readily obtained by using the Constant Sound Pressure
Source Type 4211 in conjunction with the B. F. 0. 1014. The B. F. 0. 1014
gives high regulation of the signal (by the use of the compressor circuit), even
when operating at maximum rated power.
An arrangement utilising the Type 4211 in conjunction with the combined
B. F. 0. 1014/Level Recorder 2305, (i.e. Automatic Frequency Response

36
Recorder Type 3304) is shown in Fig. 20, the item under test being an air
filter. Fig. 21 shows the resultant curves which give an indication of the
attenuation of sound waves in the air filter. Curve A is measured with the
Sound Pressure Source 4211 open and Curve B with the filter mounted on
the Source opening. The sound pressure level at the air filter input in both
cases being 110 db with reference to 2 X 10-4 ,ubar, and to avoid directional
influences the measurements were carried out in a highly reflective hard-
walled chamber.
In many instances there will be a requirement to find the most suitable
position to mount absorbent material in order to silence the .object on test.
When this situation arises measurements should be carried out under "free

~.-
if ~ ~-~:.
,....... ~···
@
~ .!;

Fig. 20. Arrangement for determining the sound attenuation in an air filter.

-
OD D DDDD DD DDO OOODDDDDDDDDDDDDDDDDDOOOOOOOOOOOOOOOOOO
!rO~jatr .
5025

z-L..; _rop.Sp.: _
LIJ..ft; _Lc.No.: _

''"-
w•. s,..: _
-""'"- -"'

-
s.,: _ _
""' --~s.....• - oo~

OP 0123 10 100 1000

1'61()69

Fig. 21. Determination of the air filters sound attenuation.

Curve (a) Measurement without air filter.
Curve (b) Measurements with air filter mounted on top of the Constant Sound
Pressure Source 4211.

37
 receiver room transmitter room

4131 4131
L.
2812 2812

Fig.22.
(a) Measuring arrangement for automatic reading of the sound insulation
properties of a wall.

field conditions" or in an anechoic chamber allowing the directional pattern

of the unit under examination to be obtained.

Testing the Qualities of Airborne Sound Insulation.

A means of automatically carrying out these test is shown in Fig. 22a. The
wall under test is placed between two rooms, which are termed "the tranll
mitting room" and "the receiving room" respectively.
In each of the two rooms separated by the wall is placed a Type 4131
microphone individually coupled to a Cathode Follower Type 2612. Two
extension cables connect the microphone units with the Two-Channel Micro-
phone Selector Type 4408. The Microphone Selector is remotely controlled
by the two-channel switching device, which is "built-in" to the Level
Recorder portion of the Automatic Frequency Response Recorder Type 3304.
In this case the Recorder is using a 50 db Range Potentiometer and the

38
necessary sound is generated by the BFO 1014 section of the Type 3304
in conjunction with a loudspeaker. The Beat Frequency Oscillator should be
frequency modulated and the loudspeaker (or loudspeakers) placed so that a
sound field, as diffuse and isotropic as possible, is built up.

"'·--""" -_
w.s,..:_s..... _ -
...,, _ _ F,..Scol.o _
1000 XIOOO c!-«10004
("11/2'rii) .A. I
QP 0123 10 100 1000 10000
~6(0717

Fig. 22 (b) Recording obtained with a measuring set-up as in (a).

N.B. 50 db Range Potentiometer is used in Level Recorder.

By means of the Microphone Selector which is connected to the Microphone

Amplifier Type 2603, the different sound levels picked up in the two rooms are
taken alternately and amplified before being fed to the Level Recorder. The
result is that two independent curves are automatically reproduced on the
recording paper, enabling the sound level difference between the two sides
of the wall to be read off in decibels. The sound absorption of the receiving
room must be taken into account.

Measurement of Reverberation Time.

One of the more important factors •i n determining the acoustic qualities of
a room is the measurement of the room's reverberation time. The Beat
Frequency Oscillator includes special functions, such as the compressor

Fig. 23. Measuring equipment for the automatic recording of reverberation

time. Compressor of the B. F. 0. employed .

39
circuit and the possibility of frequency modulation , which makes it very
suitable for this type of measurement. The compressor circuit serves to
keep the sound radiated in the room at a constant value throughout the
frequency range of the measurements. Frequency modulation of the signal
mdiated in the room ensures that a great number of eigentones of the room
are excited in the frequency band covered by the frequency modulated
signal. The resuJtant recorded decay curves will in this manner appear with
a smooth slope. That would not be the case when a pure sine-wave signal
is radiated in the room, as distinct standing waves would arise. The fre-
quency modulation is easily adjusted in frequency swing and modulation
frequency by the controls "Modulation Swing" and "Modulation Frequency"
respectively.
Various measuring arrangements for reverberation measurements can be set
tip where the B.F.O. is an integrating part. Here will be discussed an arrange-
ment which wotks automatically and where the m easured decays are re-
corded by the B & K Level Recorder Type 2305. The set-up is illustrated in
Fig. 23. The B.F.O . Type 1014 and the loudspeaker constitute the trans -
mitting part, whereas one of the B & K Condenser Microphones, the Audio
Frequency Spectrometer Type 2112 and the Level Recorder Type 2305 make

Frontal Connections
2112 2305 1014

Ground o--------.,---o Ground

(a)

2112 2305 1014

84 A2

Ground Ground

(b)
Fig. 24. Connection between instruments.
(a) Connections between remote control jacks.
(b) Electrical circuit of the remote arrangement.

40
up the recetvmg part. As the amplifier for the Microphone is chosen the
Spectrometer whioh makes a selective reception in 1h or Yt octave bands
possible, thereby reducing the influence of the room's background noise.
A sufficient dynamic range is in this manner obt.a ined when measurements
are carried out in rooms where the background noise cannot be removed.
The measuring arrangement shown allows decay curves of the reverberation
to be recorded automatically throughout the frequency range 25 c/s to 20ooo
cis with intervals of 1h octave. All the decays are registered on ' a frequency
calibrated part of recording paper being only 250 mm in length (refer Fig.
26). If greater spacing between the individual decay curves is required, the
recording has to be made on non-frequency calibrated paper. Below is given
a brief d·e scl'iption of the principal working of the two types of m:e asurements.

Frequency Calibrated Paper. For recording the decay of the sound in the
room the sound source has to be disconnected at definite intervals, this is
achieved by stopping the oscillator in the B.F.O. To ensure that only the
part of the measurement is recorded · which is of interest, the writing pen
should lift from the paper in the interval between two decays, and as
selective reception is utilized, the filters in the Spectrometer should be
switched in successively. The disconnecting of the sound source, the lifting
of the pen and the switching of the filters in the Spectrometer can all be
automatically controlled by a special switch in the Level Recorder. (The
Two-Channel Selector). The necessary connections between the different
instruments are shown in Fig. 24. The connections to the respective Remote
Control Jacks are shown in Fig. 24a, while F~g. 24b gives the electrical
circuits for the remote controlling arrangement. ,

Overlapping junction.

Fig. 25. Making up of paper loop.

4:1
When placing a loop of 50 mm paper width (Fig. 25) in the Level Recorder
with a length of 49·5 mm (i.e. two chart lengths minus 5 mm; 5 mm being
the distance between two perforated holes) it is possible to have the curves
for the different frequencies placed with a spacing of 1h octave as shown
in Fig. 26. By cohesively synchronizing the paper movement with the fre-

-
OOODDDOODOOOODOODDOODDDDODDDDDDDDODDDOODODODOOOODDD
BrO~Jc:.r.
5025 ~ro.~•q_ .W6r., lro.lloq.. lS

~ ~·lil!l l !l!l l l l! l li !I I!I I!I I I !I I I I I TI=!0!1Ss!el~ != l~ l

f1me mthe .,
1
"illjcpfJtB.
Mod Freq J2C/s" "
,_..,, _..,l!!Q_"' ..
·
~....~:.~ ...
:::..'"'~::!::E.·..
aP 0123 10 ioo -
1000 10000
"""' "' .......
, •• ,..... •
c ....

f6f85-+

Fig. 26. Example of recording of decay curves.

Compressor arrangement used.

quency scanning of the B. F. 0., with the filter switching on the Spectrometer
and with the switching off moment of the sound, the starting points of the
decay curves will correspond to the center frequency of the respective filters,
represented bY'' small squares on the top of the preprint of the recording
paper QP 0123, QP 0223 and QP 0323, see Fig. 26. It is possible, lo a certain
degree, to keep the sound pr:e ssure level at the point of measurement
independent of loudspeaker and room response by utilizing the compressor
circuit of the B. F. 0. as indicated in Fig. 23. This method ensures that all
the decay curves commence at the same level on the recording paper.

Non-Frequency Calibrated Paper. When a larger spacing than 5 mm between

the decay curves is desired (vide example in Fig. 27), the recording paper
loop used in the Recorder has to be made accordingly shorter as the length
of this determines the spacing. For example, a loop length of 490 mm gives

o oo c~o~oo~c~coco~o~cccco c ~=oncooccoocooc oooo ooo oor

I= Potentiometer Range ' so 31SC/s 1000t/s 4000Qs ~ 100~0 C/s ----=

~ Rectifier :
I= Lower Limiting Freq.
RMS
so
400 ~ T•0.3sec.
~ Writing Speed :
I= Paper Speed :
~Drive Shaft Speed :
10012 T• 0 . 4Ssec. ~-
~ Loop Length: 490mm
i= Measuring room : Information Department
I= Date : 20-1-61 B.E. B.
-
16fB5>

Fig. 27. Decay curves at 10 mm intervals recorded on a loop of 490 mm.

42
10 mm spacing between the curves. In such instances the recoiding has to be
carried out on the lined recording paper, e.g. QP 0102, QP 0202 or QP 0203,
and it is necessary to " mark" one or more frequencies on the paper. The
marking can be readily done by means of the Level Recorder 's "Event-
Marker" arrangement.

If only a few reverberation curves are to be taken, the situation may not
warrant the use of automatic measuring, in the se circumstances use should
be made of the pressbutton marked "Oscillator Stop" on the B. F. 0. Also,
when it is desired to record the decay curves with a spacing less than 1/:{
octave, the described function of the automatically working arrangement
cannot be used immediately. The manually or remotely operated "Oscillator
Stop" may then be utilized.

Use of the Protractor SC 2361.

The Protractor has been designed to facilitate the determination of rever-
beration time from recorded decay curves on the 50 mm width paper. It is
divided into four sections marked " 75 db 10 mm/ sec.", " 75 db 30 mm/ sec.",
"50 db 10 mm./sec.", and "50 db 30 mm/ sec.". When one of the se four
combinations of " Range Potentiometer" and " Paper Speed" has been em-
ployed during the measurements, the reverberation time can be read directly
in seconds.
1. The Protractor is held so that the printing is readable. The proper section
is chosen and its left limiting line (thick diagonal) is placed on top of the
portion of the recorded decay curve to be measured, and in such a
manner that the centre of the Protractor coincides with one of the
horizontal lines on the recording paper. Refer Fig. 28.

Range Potentiom1t1r : 50db

Paper Speed : 100 mmjs1c. Reading :- S.!ilec.•0.1•0.55sec

16f8G6

Fig. 28. Use of Protractor SC 2361.

Ten times higher paper speed used than stated on the protractor section
"50 db, 10 mm!sec.". Reading th en divided by 10, i.e. o.55 sec.

43
 2. The reverberation time in seconds is then read on the scale at the point
through which the horizontal line passes. Vide Fig. 28.
The decay curves should preferably be approximated into a straight line
making it easier to determine -the average slope.
If paper speeds other than 10 and 30 mm/ sec. have been used, the determined
reverberation times should be multiplied or divided by factors of 10.
Example.
50 db Range Potentiometer.
Paper Speed 100 mm/sec.: Use the section "50 db 10 mm/sec." and divide the measured
result by 10, see also Fig. 28.

Absorption Measurements of Sound Insulation Material.

The B. F. 0. Type 1014, in conjunction with the Standing Wave Apparatus
Type 4002, enables the sound absorbing properties of different materials
to be evaluated and their sound absorbent coefficients to be determined.
A lay-out is shown in Fig. 29, the B. F. 0. Type 1014 feeding the loudspeaker
which is mounted in the end of the tube of the Standing Wave Apparatus

161962
400'1

Fig. 29. Absorption measurements on sound insulating material by means of

Standing Wave Apparatus Type 4-002. Spectrometer Type 2112 and B.F.O.
Type 1014-.
Type 4002. Included in the apparatus is a probe type microphone which is
mounted on a trolley, allowing the open end of the probe to be stationed
at any point along the central axis of the tube. The output of the micro-
phone unit is connected to the input of the Frequency Analyzer Type 2107
or the Audio Frequency Spectrometer Type 2112. The Analyzer Type 2107
can be continuously tuned through the band 20 to 20ooo cjs with a constant
percentage bandwidth, the 3 db bandwidth being variable in steps from 6 %
to 29 %. On the other hand the Spectrometer 2112 has 33 :% octave and
11 octave filters with center frequencies from 25 c/s to 40 kc/ s, and 31.5 cjs
to 31.5 kc/s respectively. Both equipment s have meter scales which directly
indicate the sound absorption coefficient.
The material to be tested is placed in the termination end of the Standing
Wave Apparatus and when the set-up is operated, due to the sound reflection
from the sample, standing waves are produced in the tube. If 't he termination
of the tube was made to consist of a totally rigid material, a complete
reflection of the sound wave with minima equal to zero would be obtained.
On replacing the rigid termination with an absorbent material only part of
the wave will be reflected and the minima will no longer be zero. Thus by
measuring the ratio between the maximum and minimum sound pressures
the absorption co-efficient of the sample for 0 degree incidence sound can
be found.

GROUP C. MECHANICAL MEASUREMENTS

Strain Measurements on Vibrated Objects.
In the measuring of mechanical strain on objects under vibration, it is
essential that the vibration acceleration is kept constant within the range
of frequencies at which measurements are being taken and that inherent
resonances in the system have no effect on the magnitude of the driving
force.
The illustration in Fig. 30a shows a tes t rig for strain measurements of
small mechanical constructions, the BFO. 1014 section of the Automatic
Frequency Response Recorder Type 3304 feeding the shaker, the object under
test being placed on the shaker table.
To keep the acceleration constant a controlling system is utilized. This
system consists of an Accelerometer Type 4308 mounted on top of the test
object. As the acceleration has to be constant and under control the output
voltage is connected via a Cathode Follower Adaptor JJ 2612, Cathode
Follower 2613, and a Microphone Amplifier 2603 to the compressor input
of the B. F . 0. The filter switch on the Type 2603 should be set to position
" Linear" .
By using the built-in meter on the Amplifier Type 2603, the output voltage
of the Accelerometer can be observed and the acceleration calculated from

45
the Accelerometer's sensitivity curve. From this the driving force can be
calculated, using Newton's equation F = m X a, where F = the driving force,
m = the mass, and a = the acceleration.
To measure the mechanic~! strain in the object under test a Strain Gage

JJ 2612 2613

""i = ~ .;::·~· g
~ :.'C."!..
@
2103

Fig. 30a. Arrangement for measurement of vibrations in small mechanical

constructions.

oooooooooooooooooooooooooo~
50
db

---~-+

20

10

0
50 100 200 C/s 300 ~0

Fig. 30b. Recording of the mechanical strain of a bar. Measured with an

arrangement as shown in Fig. 30a.

46
is used. This is a pick-up device which is comprised of a looped resistance
(or resistances) sandwiched between insulating material which is cut in the
form of a strip, and which can be glued on to the test object. The object
when subjected to mechanical strain is distorted and this will alter the Gage
resistance, the variation being register ed by a sensitive measuring bridge
arrangement, e.g. the Strain Gage Apparatus Type 1516. (For further · informa-
tion refer to manual).
The output voltage from the Strain Gage Apparatus is then fed to the input
of the Level Recorder of the Type 3304, to give an automatic recording.
An example of such a recording, taken on a thin metal bar, showing the
mechanical strain and indicating its resonant frequency, is shown in Fig. 30b.

The A.F. Response and Spectrum Recorder Type 3326.

DESCRIPTION
The apparatus is a combination of the Beat Frequency Oscillator Type 1014,
the Audio Frequency Spectrometer Type 2112 and the Level Recorder Type

Fig. 31. Photograph of the A.F. Response and Spectrum Recorder Type 3326.

47
2305, all being housed in one unit. The complete assembly gives a compact
means of carrying out and automatically recording almost any desired
electrical, electro-acoustical, or acoustical measurement in the audio fre -
quency range.
Fig. 31 shows a photograph of the equipment. For a comprehensive technical
description of the Spectrometer _and the Level Recorder, reference should
be made to their respective manuals. However, a brief outline of their bask
principles is given in the following paragraphs.

The Audio Frequency Spectrometer.

The basic design of the Spectron~eter is shown in Fig. 32. The input amplifier
section consisting of three stages is supplied with a large amount of negative
feedback, thus ensuring that a low source impedance (approx. 10 ohms) is
coupled to the filter system.

" Input "Filter Input" " Recorder"

Potentiometer " "Extension Filter "

Fig. 32. Block diagram of the Spectromete:-.

The filter system is made up of 33 band-pass filters and three weighting

networks . The center frequencie s of the filters being: 25 - 31.5 - 40 - 50
- 63 - 80 - 100 - 125 - 160 - 200 - 250 - 315 - 400 - 500 - 630-
800 - 1ooo - 1250 - 1600 - 2ooo - 2500 - 3150 - 4ooo - 5ooo -
6300 - 8ooo - 10ooo - 12500 - 16ooo - 20ooo - 25ooo - 31500 -
40ooo when switched for 1 /a octave analyses, and: 31.5 - 63 - 125 - 250 -
500 - 1ooo ---: 2ooo - 4ooo - 8ooo - 16ooo - 31500 in the case of octave
analyses. Switching from filter to filter is accomplished by a rotary switch in
the output d rcuit, and a large illuminated scale is used for filter id entification.
ThP output amplifier, similar to the input, contains three stages also

48
supplied with a large amount of negative feedback. The metering circuit
connected to the output amplifier can be switched to measure half the peak
to peak, average or RMS value of the input signal.
The linear frequency range covered is 2 c/s to 45ooo c/s and the three
weighting networks conform to the IEC-proposed standards for precision
sound level measurem ents.

The Level Recorder.

The Level Recorder has been designed to record signal levels in the
frequency range 10 c/s to 200ooo cjs as well as d . c. The operation of the
instrument is based on the servo-principle. Fig. 33 shows a block diagram
of the instrument.

IS9J99

Fig. 33. Block diagram of the Level Recorder.

Fundamentally the Recorder consists of an interchangeable range potentio-

meter; a direct coupled amplifier; the special B & K rectifier circuit which
gives RMS, Average or Peak detection of the input signal, a DC chopper
amplifier and an electro-mechanical writing system.
When the magnitude of the voltage applied to the input terminals is changed
a current will flow through the driving coil of the writing system thus
moving the stylus, which is mechanically coupled to the range potentiometer.
By the movement of the stylus the voltage delivered from the potentiometer
to the AC amplifier will be altered until a stable condition is regained. In
this way it is possible to obtain different ranges of voltage variations
which can be recorded, by employing different range potentiometers. Loga-
rithmic range potentiometers are available for voltage level ranges of 10, 25,
50 or 75 db. Two linear potentiometers cover the ranges 10 to 35 m V '' nd
10 to 110 m V respectively.
The recorder is capable of writing on two different widths of recordjng
paper, 50 mm and 100 mm. To change from one width to the other it is

49
only necessary to release a snap-lock arrangement on the moving arm
which holds the stylus.
The writing speed is determined by the amount of damping applied to the
writing system which is selectable by the rotary switch "Writing Speed".
This is variable in 15 spot settings shown by the figures outside the switch.
The large figures denote the settings for 50 mm p'lper while the small
figures are for recordings on paper of 100 mm width.

Recording Paper.
Different types of pre-printed recording paper to be used on the Level
Recorder in conjunction with the Beat Frequency Oscillator Type 1014 and
Spectrometer Type 2112 combination are available, the paper being manu-
factured with a printed logarithmic frequency scale covering the range from
10 c/s to 40ooo c/s.

Fig. 3.4. The various typ es of pre-printed recording paper.

Three main types of paper, having various features, and which come in
two widths, can be supplied. Also included in the range is polar diagram
recording paper to be used when the Level Recorder is required for this
application (see Level Recorder Manual). Two types of writing are catered
for, either ink or stylus, the applicable surfaces being treated accordingly.
\Vhile paper for ink writing is available, with pre-printed line~ or outlines
of frequency diagrams and is obtainable in the widths of 100 mm or 50 mm.
The polar diagram type comes in packs of 100 sheets and has a 100 mm rarlius.
The waxed paper comes in two types but only in 50 mm widths, i.e. white
waxed, or red waxed, and are intended for use with a sapphire stylus. The
white waxed paper consists of coated black paper and exhibits printed lines
or frequency di~grams as the case may be. The red waxed type is comprised
of transparent pa:per with a thin covering of red wax, having printed lines
or frequency diagrams superimposed on it. Waxed paper is particular useful

50
when high writing speeds have to be used, giV~ng a very clear definition
of the recording. When using these papers the sapphire stylus will leave
a thin black line on the white waxed paper and a transpar(}nt line on the
red waxed paper, the latter being specially made to enable ,blue prints to
be taken. It is necessary to double-copy the latter, as the scales being
printed in black will not show up if a direct blue print is take~ from it.

Copying of Recorded Information.

Curves or data, recorded on the white paper in ink, are ideal for re-
production by photostatic coyping or blue printing. The frequency range
lines are outlined in orange, making it very suitable for photostatic copying
To allow good blue printing or other methods where light is passed through
the paper, the lines of the frequency diagram are impenetrable to light. To
give clear curves on the prints, it is prefcrabl~ to use black ink in the pen.

Fig. 35. Photo of Photographic Negative QF 0009 with transparency frequency

diagram.

When it is desired to make copies of data recorded on the transparent, red-

waxed paper, the use of a Photographic Negative QF 0009 and a double-
copying process is necessary.
The recommended method to "double-copy" is as follows:-
1. Fasten the recorded paper strip at the left edge with scotch tape to a
board or table.
2. Place negative on top of recording, making certain that the recorded
paper lines and the lines on the transparent negative coincide, then
tacking by tape at righthand edge to table.
3. Fold recording and negative paper to either side, placing the photostatic
paper in the vacated space and fix to table.
4. Replace the recorded curve on top of the photostatic paper.
5. Expose the combination by illumination.
6. Turn recording paper aside, replacing with negative, and maintaining it
in the correct position, again carry out the exposure procedure.
7. Take the photostat and develop.
N.B. By illuminating the recording longer than the negative, a more pro-
nounced picture of the curve will be obtained.

51
 OPERATION
General.
The instruments contained in the A. F. Respon se and Spectrum Recorder
can be used separately, or together, in various combinations. The logarithmic
frequency scale of the B. F. 0 . 1014 allows it to be completely synchronized
with the Spectrometer.

Synchronization.
To fully synchronize the three units the following sequence of operation
is recommended. Switch on the power of each instrument and connect the
" Recorder" terminal of the Spectrometer to the "Input" terminal of the
Level Recorder, then calibrate as follows:-
BFO 1014 Section.
Calibrate the BFO as in " A. Calibration" under Operation on page 15.
S pee trome ter.
1. "Input Switch" to " Direct".
2. "Meter Range" to "Ref.".
3. "Meter Switch" to " Fast" "RMS".
4. "Range Multiplier" to "x 1, 0 db" .
5. " Function Selector" to "Linear, 2- 40ooo c/ s".
6. "Automatic Switching" to " Off " .
Other knobs in any position.
Meter should show a deflection to the red mark on the scale. If
necessary any deviation can be recorded by means of the potentiometer
marked. " Sensitivity Amplifier Input" situated on the front panel.
Adjust with screwdriver.
Level Re~order.
(N.B. In this case a 50 db potentiometer is used. For other ranges refer
manual for Level Recorder Type 2305).
Set control knobs to the following positions:-
1. "Potentiometer Range db" to "50".
2. "Rectifier Response" to "RMS".
3. " Lower Limiting Frequency " to " 20".
4. " Writing Speed" :
50 mm paper: 500 mmj sec. (large figures ).
100 mm paper: 1000 mmjsec. (small figures ).
5. "Paper Drive" to "Stop" and " Forward" positions.
6. Motor to "On".
7. Set "Input Attenuator" to " 10" .
8. Using the " Input Potentiometer" adjust stylus to full deflection
- 4 db (e. g. using · 50 db Range it will be 50 db- 4 db = 46 db).
9. Insert the desired type of frequency calibrated paper. (If necessary
refer Level Recorder Manual).

52
 10. Pull the Synchronizing Gear Lever (1 : 10) marked "X" in Fig. 36
to outer position.
11. With a screwdriver turn the screw " S" on Fig. 36 until marking
cut is in vertical position.
12. Set the "Paper Speed" to 10 mm/sec. (small figures). The spring
loaded knob is operated by pulling, turning and dropping to correct
position.

p1 p1
z 5
z

X 2305

Drive
Shafts

Fig. 36. Front and side views of Level Recorder.

13. The toggle switch "Paper Drive" is set to "Start" whereby the
paper should start moving. If not, press the pushbutton "Single
Chart - Continuous Recording" and release it again, the paper will
move and after a chart length or less automatically stop.
14. Move the recording paper by means of the finger wheel "Z" shown
in Fig. 36 until stylus rests on 10 c/s line.
At this stage final synchronization of the units takes place by firstly syn-
chronizing the Spectrometer and Level Recorder sections as follows:-

Set the control knobs of the Spectrometer to:-

1. "Function Selector" to " 1h Octave - 0 db".
2. "Filter Switch" to one step before (counter-clock-wise) the position
"12.5".
3. "Automatic Switching" to "On".
Then the control knobs of the Level Recorder to:-
1. "Paper Drive" to Stop.
2. Press pushbutton marked "Single Chart" and hold in. (Paper will
move and the reference voltage commences to record). Release push-
button when paper has moved to about the "200" ci s line.
3. Units are correctly synchronized when switching from the 80 cjs
to the 100 cjs filter takes place on the 90 cjs line.

53
 4. As a means to see how far the paper has to be shifted, it is re-
commended to draw a line, by means of the "100 mV Ref." button
on the front plate of the Recorder, at the point where the paper has
stopped. By using this line as a reference the paper can be shifted
the appropriate distance to give correct synchronization.
5. To check the synchronization, run the recording until the pen is
stopped on for example the 2ooo cjs line. When correctly syn-
chronized, the switching of the 800 cjs to the 1ooo cjs filter should
now take place at the 900 cjs line. If this is not the. case repeat
from item 2.
6. Finally reset the Writing Speed on 250 mm or lower if the large
figures are being used, and to a figure of 500 mm or less if using
the small figures.
To complete the synchronization sequence the BFO must be synchronized
with the frequency calibration on the paper.
1. Rotate main scale of BFO manually until it corresponds to the
frequency denoted by the stylus, firstly moving it to a higher fre-
qen'cy and then rotating it back until it arrives on the desired
frequency. This will take up any possible backlash with the gears
of the BFO.
2. Move "Clutch Switching" of the BFO to the "On" position.
All units should now be in complete synchronization and on
switching the toggle switch of the "Paper Drive" on the Recorder
to "Start", the combination will operate in complete unison.

APPLICATIONS

Applications for the combined B. F. 0. 1014 and Level Recorder 2305 have
been described, and illustrated, under this heading in the earlier phase of
this manual. Now one important use for the treble combination, B. F. 0. +
Level Recorder + Spectrometer (i.e. the A.F. Response and Spectrum Re-
corder Type 3326) will be described, followed by an other example using
the Spectrometer and Level Recorder with the exclusion of the B. F. 0. 1014
section.

Automatically Recording Harmonies.

The Type 3326 combination is well-suited for the automatic recording of
harmonic distortion, as the Spectrometer can be switched to select any
specific harmonic component of the fundamental in use, up to and including
the 5th. In Fig. 37 the illustration shows an arrangement for the investigation
of harmonic distortion in hearing aids.
As the distortion produced in the electronic assembly of such a unit (i. e. the
amplifier only) is small, in comparison to distortion produced by the electro-

54
 Fig. 37 (a) Measuring arrangement to measure harmonic distortion in
hearing aids.

acoustical transducing stage, this portion must also be included in the

examination.
Basically the set-up is the same as described undeT "Checking of Hearing
Aids", page 35, with the exception that the output of the Artificial Ear on
the Hearing Aid Test Box Type 4212, is fed to the input of the Spectrometer,
instead of to the Microphone Amplifier Type 2603. \Vhen synchronization
of the units as described on page 52 has been carried out, th e measurements
can commence.

55
In the case where the harmohics of a four-terminal network are to be
automatically recorded, the synchronization of the units should have been
completed and the Single Chart recording method should be utilized .
The harmonics are measured by setting the filter switch on the Spectro-
meter so that it runs ahead of the frequency scanning of the B. F. 0 ., the
selected frequency difference being in accordance with the harmonic which
is going to be measured.
By means of the Reverse/Forward switch the recording is returned by the
si:Rgle chart length already run ·o ff; and the process repeated until all the
required harmonics are recorded on the same chart as the fundamental.
The dips seen on the curve are due to the shape of the Spectrometer filter,
and the manner of scanning the frequency range . When properly syn-
chronized the "open " dips will occur with a depth that corresponds to
around 3 db.
During measurements the sensitivity of the Spectrometer or the Level
Recorder should not be altered. If, however, circumstances require an
alteration to be made e. g. to obtain a clear recording of some of the

0 -

~ D D DODDDDOOODODDDDDD~DDDOODDDDODD DO DDDDDDDDODDDODOO
BrU:!!,!Iatr . . . . . !10 too.~•q. Joa.I6Q. M.l~ll- \0 1S
25

~~' --
Measuring of
....
fundamental .. "'
and harmonics
1n heormg a1ds ,. ,

Fundamental

2nd harm

z-L...'.~IOS
~....u.. F.__so- - 3rd harm . ·.
w•. .,._ 2o_o__
.... .,._ 10_ _
.........,.Fotq.Seol.loy•_l_o o

QP 1123 100 1000 10000

l6fU1

Fig. 37 (b) Recording obtained with set-up as shown in 37 (a).

harmonics, the gain of the Spectrometer can be increased by means of the

"Range Multiplier" switch. (The "Meter Range" switch should not be used
as it will tend to overdrive the amplifier stages). When using the Range
Multiplier for this means, it must of course be taken into account in the
evaluation of the final results.

Vibration Measurements.
By using one of the Accelerometer Sets Type 4308/09/10/11 with a Vibration
Pick-up Preamplifier Type 1606, or one of the Microphone Cathode

56
Followers, vibrations in buildings, machinery, ships, etc., can be measured.
In Fig. 38a a layout is shown, where the Type 3326 is being used to auto-
matically record vibration measurements taken on the base of an electrical
motor, the Spectrometer allowing an analysis of the vibrations to be made .
The output from the Accelerometer is fed via the Preamplifier 1606 to the
"Condenser Microphone" input of the Spectrometer. Information on the
calibration of the arrangement is given in the manual for the Type 1606.

@•
• :i::

·i·~!
.x:;;p@
~·:;;'i:!- · ·~~~··

Fig. 38a. Set-up for vibration measurements on a motor installation.

Fig. 38b shows a recording of the vibrations from a synchronous motor

which uses a 50 cjs mains supply. In this recording a 50 db Range Potentio-
meter was used in the Level Recorder, 0 db reference being equal to an
acceleration of 25 cmjsec.2.

Noise Measurements.
Noise within factories , offices, cities, airports , etc. varies with the time of
day. Therefore it is a firm requirement that recordings of the noise should
be carried out over a considerable period of time as a single recording
taken at a particular instant would provide incorrect inform,ation.
Fig. 39 shows a recording taken of the noise level in a m~~hanical work-
shop over a selected period of time by using one of the Weighting Networks

57
~ oooooooooo:ooooooooooooooooooooo o ooooooooooooooooo
lrD:!!!Iaor • .50~ ~.o.~•r;. , ~.G.~•KI- ~oo.~•K~-

...
10 75

- " ' -- -
~e~~~~f~~gtn .. .,
on a electncg[
motor frame

~~~
.,~ BE!;l_
'-" RM5
z-w...~ 1;; S
~: ·"~
:~~ 2go
Mu~~~pt,F,.Scoleby~_j_ o 010 30000 c/1 .0000 A I C lill.
(1611/ 2111lA I C IJA. ·
UP 1123 10 100 1000 10000

Fig. 38b . Recording obtained with measuring set-up as shown in Fig. 38a.

in the Spectrometer. As can be seen the noise level varies considerably with
time and a reliable anai.ysis cannot be obtained by only taking a few
measurements. It is therefore necessary, when doing such a test, to record
continuously and then to statistically assess the final result.
To measure, analyse and record such noise the Spectrometer + Level
Recorder (i.e. the receiving part of the A. F. Response and Spectrum
Recorder Type 3326) in combination with a B & K Condenser Microphone
can be used.

,----------------,,-------------------- ---- --------------------------~

o o o o o o o o o o o o o o o o o o o o o o o o octo o o o o o o o o o o o o o o o o o o o o o o o o
8rD:!!!,Ic.t . . s.o.~•r.., .w•IQw ~.o.~•r..,.

QP 0123 10 100 1000

1'619H

Fig. 39. Recording of the noise level in a mechanical workshop over a period
of time.

58
 Specification
Frequency Range: 20-20ooo c/s.

Frequency Scales:
Main Scale: Logarithmic from 20-20ooo c/s.
Tolerance ± o. 7 degrees of theoretical logarithmic curve. Vernie1
driven.
Increment Scale: Range -50 to +50 c/s of main scale reading. Both
scales illuminated.

Frequency Accuracy:
Main scale: 1 % ± 1 c/s.
Increment scale: ± o.5 c/s.

Outputs:
Matching: Switchable matching impedance for 6, 60, 600 or 6ooo ohms
load.
Max. power output 2.5 watts approx.
Attenuator: Variable in steps of 10 db (within ± o.2 db} from 125 !f-V to
12.5 V. Continuously variable by potentiometer within each step.

Output Voltage Accuracy: In frequency range 20 to 20ooo c/s.

Better than ± o.3 db on "Attenuator Output".
Better than ± 1 db on "Load" 1 watt loaded.

Voltmeter: Vacuum-tube voltmeter. Moving-coil. Illuminated mirrored scale.

Highly accurate, better than 1.5 % of full-scale deflection. Perfectly safe-
guarded against overload.

Distortion:
Frequency in c/s ....... 20 200 2ooo 20ooo
"Attenuator" terminal.
No load with 10 V
output approx . . .. ..
.. . 1.0% o.1 ·% o.1 o/o 0.7%
"Load" terminal.
(Loaded 1 watt) ....... . 2.0 •% 0.3% 0.3% 1.2 •%

Automatic Output Regulator: Output voltage automatically regulated when

required. Built-in compressor amplifier maintains regulation up to 45 db
and a constant voltage, current or sound pressure to within ± 2 db in

59
 frequency range 50- 20ooo c/s. Input impedance 100 kohms. Regulation
speed variable in steps: 30- 100-300 and 1ooo db/sec.

Frequency Modulation: Continuously variable modulation swing 0 to ± 200

cis. Selectable modulated frequency by built-in saw-tooth oscillator of
1-2-4-8-16 and 32 c/s.

Oscillator Stop: Push-button Oscillator Stop for noiseless switching in rever-

beration measurements. Remote control available.
Frequency Scan: Worm gear in oscillator permits variable capacitor to be
driven from motor of Level Recorder Type 2305. Connection achieved
by flexible shaft. Magnetic clutch for set and release of drive. Clutch can
be remotely controlled. Accurate synchronization with Level Recorder
Frequency Calibrated Paper.
Tubes: 4 X 12AT7 (ECC81) - 3 X 6AU6 (EF94) - 6AL5 (EAA91) - 6BQ5
(EL84) - OA2.
Power Supply: 100- 115- 127- 150- 220- 240 Volts AC. 50-400 c/s.
Power consumption approximately 70 watts.
Dimensions:

EY,cl. dials and knobs I Height

I
Width
I
Depth

Centimetres 50 40 20
I I
Inches 20 16 8
I I

Weight
I 22 kg
I
49lbs.

60
B K

Printed in Copenhagen, Denmark

January 1962