Experiment No.

Measurement of Noise
Object
1. To measure equivalent continuous sound level LeqA (10 min.).
2. To draw the percent time occurrence of the particular sound levels in the laboratory.
Introduction
The Basic Sound Level Meter
A sound level meter is an instrument designed to respond to sound in approximately the same
way as the human ear and to give objective, reproducible measurements of sound pressure
level. There are many different sound measuring systems available. Although different in
detail, each system consists of a microphone, a processing section and a read-out unit. The
microphone converts the sound signal to an equivalent electrical signal. The most suitable
type of microphone for sound level meters is the condenser microphone, which combines
precision with stability and reliability. The electrical signal produced by the microphone is
quite small and so it is amplified by a preamplifier before being processed. Several different
types of processing may be performed on the signal. The signal may pass through a weighting
network. It is relatively simple to build an electronic circuit whose sensitivity varies with
frequency in the same way as the human ear, thus simulating the equal loudness contours.
This has resulted in three different internationally standardized characteristics termed the "A",
"B" and "C" weightings. The last stage of a sound level meter is the read-out unit which
displays the sound level in dB, or some other derived unit such as dB(A) (which means that
the measured sound level has been A-weighted). The signal may also be available at output
sockets, in ether AC or DC form, for connection to external instruments such as level or tape
recorders to provide a record and/or for further processing.
Calibration.
Sound level meters should be calibrated in order to provide precise and accurate results. This
is best done by placing a portable acoustic calibrator, such as a sound level calibrator or a
piston-phone, directly over the microphone. These calibrators provide a precisely defined
sound pressure level to which the sound level meter can be adjusted.
Detector Response.
Most sounds that need to be measured fluctuate in level. To measure the sound properly we
want to be able to measure these variations as accurately as possible. However, if the sound
level fluctuates too rapidly, analogue displays (such as a moving coil meter) change so
randomly that it is impossible to get a meaningful reading. For this reason, two detector
response characteristics were standardized. These are known as "F" (for Fast) and "S" (for
Slow). "F" has a time constant of 125 milliseconds and provides a fast reacting display
response enabling us to follow and measure not too rapidly fluctuating sound levels. "S" with
a time constant of 1 second gives a slower response which helps average-out the display
fluctuations on an analogue meter, which would otherwise be impossible to read using the "F"
time constant. Many modern sound level meters have digital display, which largely overcome
the problem of fluctuating displays, by indicating the maximum RMS value measured within
the preceding second. Selection of the appropriate detector characteristic is then often dictated
by the standard upon which the measurements are to be based.
The Impulse Sound Level Meter.
If the sound to be measured consists of isolated impulses or contains a high proportion of
impact noise, then the normal "F" and "S" time responses of the simple sound level meter are
not sufficiently short to give a measurement which is representative of the subjective human

response. For such measurements, sound level meters having a standardized "I" (Impulse)
characteristic are needed. The "I" characteristic has a time constant of 35 milliseconds, which
is short enough to enable detection and display of transient noise, in a way which takes into
account the human perception of impulsive sounds. Although the perceived loudness of short
duration sound is lower than that of steady continuous sound, the risk of damage to hearing is
not necessarily reduced. For this reason, some sound level meters include a circuit for
measuring the peak value of the sound, independent of it's duration. A Hold Circuit is also
incorporated to store either the peak value or the maximum RMS value. Some standards
require the peak value to be measured while others ask for a measurement using the "I" time
constant. In either case the Hold circuit makes reading the measurement easy.
Energy Parameters.
As sound is a form of energy the hearing damage potential of a given sound environment
depends not only on it's level, but also it's duration. So to assess the hearing damage potential
of a sound environment, both the sound level and the duration of exposure must be measured
and combined to provide a determination of the energy received. For constant sound levels,
this is easy, but if the sound level varies, the level must be sampled repeatedly over a well
defined sampling period. Based on these samples, it is then possible to calculate a single value
known as the Equivalent Continuous Sound Level or Leq which has the same energy content
and consequently the same hearing damage potential as the varying sound level. For an Aweighted Leq the symbol LAeq is used. In addition to determining the hearing damage potential
of a sound, Leq measurements are also used for many other types of noise measurements, for
example community noise-annoyance assessments. The formulas for Leq determination are

where T is the duration of sound exposure time, and t i is the time within which the L i levels
occur.
Quest 1800.
An advanced microprocessor technology, coupled with a practical, compact design. Utilizing
the 1800, slow, fast, impulse and peak response are possible. Manual and automatic octave
band analysis are possible with the Quest Octave Band Filter Sets.
Features:
Computer/Printer interface
Multi-functional capability
Direct Readings of Many Parameters
Overload Indicator
Multiple Response Settings
Procedure
Results
Discussion
References