NOISE & VIBRATION

Topic 2 :
Perception Of Sound

Ir Dr Zainal Fitri B Zainal Abidin

July 2018
 Loudness Vs Frequency.

• Loudness is the characteristic of a sound that is primarily a

psychological correlate of physical strength (amplitude). It is defined as
"that attribute of auditory sensation in terms of which sounds can be
ordered on a scale extending from quiet to loud".

• Loudness is a subjective term; sound-pressure level is strictly a physical

term. Loudness level is also a physical term that is useful in estimating
the loudness of a sound (in units of sones) from sound-level
measurements.

The sone is a unit of how loud a sound is perceived. The sone scale is linear.
Doubling the perceived loudness doubles the sone value
 Loudness Vs Frequency.

Equal - loudness contours of the human ear. These contours reveal the relative lack of sensitivity of the ear to
bass tones, especially at lower sound levels. Inverting these curves give the frequency response of the ear in
terms of loudness level. (After Robinson and Dadson.8)
 Loudness Vs Frequency.
A sound-pressure level of 30
dB yields a loudness level of
30 phons at 1,000 Hz, but it
requires a sound -pressure
level of 58 dB more to sound
equally loud at 20 Hz .The
curves tend to flatten at the
higher sound levels. The 90-
phon curve rises only 32 dB
between 1,000 Hz and 20 Hz.

The phon is a unit of loudness

A comparison of the ear’s response at 20 Hz compared to
level for pure tones. Its that at 1,000 Hz. At a loudness level of 30 phons, the
purpose is to compensate for sound-pressure level of a 20-Hz tone must be 58 dB higher
than that at 1,000 Hz to have the same loudness. At 90
the effect of frequency on the phons loudness level, an increase of only 32 dB is
perceived loudness of tones required.
 Loudness (Phons Vs Sones).

A loudness of 2 sones is then 10 dB higher; a loudness of 0.5 sones is 10 dB lower

The graphical relationship between the physical loudness level in

phons and subjective loudness in sones.
Area of Audibility

The auditory area of the human ear is bounded by two threshold curves, (A) the
threshold of hearing delineating the lowest level sounds the ear can detect, and (B)
the threshold of feeling at the upper extreme. All of our auditory experiences occur
within this area.
Loudness Calculation
 Example Loudness Questions

Noise was measured at a location in a factory and

resulted in the octave band spectrum levels in the table
below.
Frequency - Hz 63 125 250 500 1K 2K 4K 8k

100 101 97 91 90 88 86 81

a) What is the loudness level in Sones & Phones?

 Example Loudness Questions

Noise was measured at a location in a factory and resulted in the octave band spectrum levels in the table below.

Frequency - Hz 63 125 250 500 1K 2K 4K 8k

100 101 97 91 90 88 86 81

a) What is the loudness level in Sones?

 Example Loudness Questions

Noise was measured at a location in a factory and resulted in the octave band spectrum levels in the table below.

Frequency - Hz 63 125 250 500 1K 2K 4K 8k

100 101 97 91 90 88 86 81

a) What is the loudness level in Sones?

Freq- 63 125 250 500 1K 2K 4K 8k

Hz
100 101 97 91 90 88 86 81

Sones 28.5 38 35.3 28.5 33 35.3 38 33

L= 38 +03(28.5+35.3+28.5+33+35.3+38+33)
L= 107.5 sones
 Example Loudness Questions

b) What is the loudness level in Phons?

Freq- 63 125 250 500 1K 2K 4K 8k

Hz
100 101 97 91 90 88 86 81

phons 90 92.5 92.5 90 90 92.5 90 90

 Pitch vs Frequency

Pitch, a subjective term, is chiefly a

function of frequency, but it is not
linearly related to it. Because pitch is
somewhat different from frequency,
it requires another subjective unit—
the mel

Pitch (in mels, a subjective unit) is related to

frequency (in Hz, a physical unit) according to this
curve obtained by juries of listeners.
 Occupational Noise Exposure –
Permissible Exposure Limit
Noise Level Duration of Exposure Noise Level Duration of Exposure
dB(A) - Slow Permitted Per day (Hours – dB(A) - Slow Permitted Per day (Hours –
minute) minute)
85 16 - 0 96 3-29

86 13 -56 97 3 -2

87 12 - 8 98 2 - 50

88 10 - 34 99 2 - 15

89 9 - 11 100 2-0

90 8-0 101 1 - 44

91 6 - 58 102 1 - 31

92 6-4 103 1 - 19

93 5-17 104 1-9

94 4-36 105 1-0

95 4-0 106 0 - 52

Malaysia Regulation OSHA

 Occupational Noise Exposure

Audiograms showing serious loss centered on 4

OSHA permissible noise exposure times kHz, presumably resulting from years of
exposure to high-level sound in the control room of
a recording studio.

Maximum Allowed Exposure time to an equivalent noise level

Occupational Noise Exposure

EFU
 Occupational Noise Exposure

There are different regulation concerning on allowed noise exposure.

• Europe & Australia has noise level increased up to 3 dB(A)
• USA & Malaysia has noise level increased up to 5 dB(A)
• USA Navy has increased up to 4 dB(A)

EFU
Occupational Noise Exposure
 Occupational Noise Exposure

Noise dosimeters measure and store sound

pressure levels (SPL) and, by integrating
these measurements over time, provide a
cumulative noise-exposure reading for a
given period of time, such as an 8-hour
workday. Dosimeters can function as
personal or area noise monitors. In
occupational settings, personal noise
dosimeters are often worn on the body of a
worker with the microphone mounted on the
middle-top of the person’s most exposed
shoulder.[3]
 Occupational Noise Exposure (Legal
Requirement – Table Method)
Action level (exchange rate = 5)
• Equivalent continuous sound level of 85
dB(A) or daily dose equal to 0.5
• 90 db(A) or daily dose = 1.0

PEL (Permissible Exposure Level)

• Continuous steady noise – 90 dB(A) for 8
Hours
• Maximum Limit (Intermittent noise) not
exceeding 115 dB(A) at any time
• Peak Limit (impulsive noise) not
exceeding 140 dB(A)
Occupational Noise Exposure (Table
Method)
Occupational Noise Exposure (Table
Method)
 Occupational Noise Exposure –
Example Questions 2
What is the allowable time of exposure to a noise level of LAeq = 99dB(A) using

(a) European criteria

(b) Malaysian criteria

 Occupational Noise Exposure –
Example Answer 1
What is the allowable time of exposure to a noise level of LAeq = 99dB(A) using

(a) European criteria

(b) Malaysian criteria

 Occupational Noise Exposure –
Example Question 2

What is the equivalent noise exposure level LAeq of 8

hours, if the measurement at the Seremban factory are as
follows :

90 db(A) for 6 hours, 85 dB(A) for 0.5 hours and 100 dB(A)
for 0.5 hours

What is the action that the company has to take?

 Occupational Noise Exposure –
Example Question 2
What is the equivalent noise exposure level LAeq of 8 hours, if the measurement at the Seremban factory are as follows :

90 db(A) for 6 hours, 85 dB(A) for 0.5 hours and 100 dB(A) for 0.5 hours. What are the action that the company has to take?

Using the noise dose formula Nois Duration of Nois Duration of

e Exposure e Exposure
Level Permitted Level Permitted
dB(A Per day dB(A Per day
)- (Hours – )- (Hours –
Slow minute) Slow minute)
85 16 - 0 96 3-29

86 13 -56 97 3 -2
Dose for 7 hours = (6/8 +0.5/16 + 0.5/2) 100
87 12 - 8 98 2 - 50

88 10 - 34 99 2 - 15
= 103.125 100 2-0
89 9 - 11
90 8-0 101 1 - 44

91 6 - 58 102 1 - 31
Dose for 8 hours = (8 x 103.13 )/ 7 103 1 - 19
92 6-4
104 1-9
93 5-17
= 117.86 105 1-0
94 4-36
106 0 - 52
95 4-0
 Occupational Noise Exposure –
Example Question 2
What is the equivalent noise exposure level LAeq of 8 hours, if the measurement at the Seremban factory are as follows :

90 db(A) for 6 hours, 85 dB(A) for 0.5 hours and 100 dB(A) for 0.5 hours.

Leq (Malaysia n=5) = 90 + 16.61(log(117.86/(12.5x8)

= 91.18 dB(A)

The company must provide PPE as it exceed noise exposure level

 Occupational Noise Exposure –
Example Question 3

If a disco band in Kuala Lumpur plays at an average

level of 105dB(A) for 4.5 hours each evening and
recorded music plays at 95dB(A) for 1.5 hours while the
band takes breaks, what is the average LAeq,8h
exposure level suffered by the employees?
 Occupational Noise Exposure –
Example Question 3 – Table Method
If a disco band in Kuala Lumpur plays at an average level of 105dB(A) for 4.5 hours each
evening and recorded music plays at 95dB(A) for 1.5 hours while the band takes breaks,
what is the average LAeq,8h exposure level suffered by the employees?
Using the noise dose formula Nois Duration of Nois Duration of
e Exposure e Exposure
Level Permitted Level Permitted
dB(A Per day dB(A Per day
)- (Hours – )- (Hours –
Slow minute) Slow minute)
85 16 - 0 96 3-29

86 13 -56 97 3 -2
Dose for 6 hours = (4.5/1 +1.5/4) 100
87 12 - 8 98 2 - 50

88 10 - 34 99 2 - 15
= 487.5 100 2-0
89 9 - 11
90 8-0 101 1 - 44

91 6 - 58 102 1 - 31
Dose for 8 hours = (8 x 487.5 )/ 6 103 1 - 19
92 6-4
104 1-9
93 5-17
= 650 105 1-0
94 4-36
106 0 - 52
95 4-0
 Occupational Noise Exposure –
Example Question 3
If a disco band plays at an average level of 105dB(A) for 4.5 hours each evening and recorded music
plays at 95dB(A) for 1.5 hours while the band takes breaks, what is the average LAeq,8h exposure
level suffered by the employees?

Leq (Malaysia n=5) = 90 + 16.61(log(650/(12.5x8)

= 103.5 dB(A)
 Occupational Noise Exposure –
Example Question 3 – Calculation
Method
If a disco band in Kuala Lumpur plays at an average level of 105dB(A) for 4.5 hours each
evening and recorded music plays at 95dB(A) for 1.5 hours while the band takes breaks,
what is the average LAeq,8h exposure level suffered by the employees?

Using under Malaysia criteria of :

L =5
 AUDITORY OF SOUND

The portion of the auditory region The portion of the auditory region utilized for typical
utilized for typical speech sounds. music sounds
 Hearing Protection Devices

Type of hearing protection

• Flexible foam ear plugs
• Ear Muff

Designing Noise Reduction Rating (NRR)

Insufficient Attenuation
85 dB(A)
Acceptable Attenuation
80 dB(A)
Good Attenuation
Noise Level inside the protected ear 75 dB(A)
Acceptable Attenuation
70 dB(A)
Too High Attenuation
 Hearing Protection Devices

Double protection Noise Reduction Rating (NRR)

• Take the highest NRR between the two

• De rate (NRR-7)/2
• Add 5 to the finale de rated value
• Subtract the noise exposure value with the final value to get attenuation
 Hearing Protection Devices

Noise Reduction Rating (NRR) Example

• What is the attenuation after putting on ear plugs with NRR of 29 in

a working environment with the noise level at 95 dB(A)
 Hearing Protection Devices

Double Noise Reduction Rating (NRR) Example

• What is the attenuation after putting on ear plugs with NRR of 29

and ear muff with NRR 20, when travelling in a helicopter with noise
level of 98 dB(A)
 White Noise

White noise is analogous to white

light in that the energy of both is
distributed uniformly throughout the
spectrum. In other words, white
noise energy exhibits a flat
distribution
of energy with frequency

Pink noise is noise having higher

energy in the low frequencies. In
fact, pink noise has come to be
identified specifically as noise
White noise pink noise
exhibiting high energy in the low-
frequency region with a specific
downward slope of 3 dB per octave
 Reverberation

Reverberation is define as the persistence of a sound after its source has

stopped, caused by multiple reflection of the sound within a closed space

Reverberation time measured with pure sine signals at low frequencies reveals slow sound decay (long reverberation
time) at the modal frequencies. These peaks apply only to specific modes and are not representative of the room as a
whole. High modal density, resulting in uniformity of distribution of sound energy and randomizing of directions of
propagation, is necessary for reverberation equations to apply. (Beranek,1 and Schultz.2)
 Reverberation

The buildup and decay of sound in a room. (A) The direct sound arrives first at time = 0,reflected components arriving
later. (B) The sound pressure at H builds up stepwise. (C) The sound decays exponentially after the source ceases.
 Reverberation- Idealised Growth &
Decay of sound

The growth and decay of sound in a room. (A) Vertical scale in linear sound pressure units. (B) The vertical scale in
logarithmic units (decibels).
Reverberation Time (Steady State)
 Reverberation Time (Steady State)

Combination of reverberation + direct sound

 Reverberation Time (Steady State)

Intensity (in one direction in diffuse field)

 Reverberation Time

The length of the decay dependent on strength of the source and the noise level. (A) Rarely do practical circumstances
allow a full 60-dB decay. (B) The slope of the limited decay is extrapolated to determine the reverberation time
 Measuring Reverberation Time

A, sound sources

Can use Impulse excitation or white noise

B, Equipment Impulse excitation

C Measurement

Equipment arrangement for measuring the

reverberation time of an enclosure. (A)
Recording decays on tape on location. (B) • Play sound source up to 110 db and
Later recording decays for analysis. measure the decay until a full 60 db decay
 Reverberation Time

Optimum” reverberation time for churches.

The upper area applies to the more Optimum” reverberation time for concert
reverberant liturgical churches and halls. Symphony orchestras are near the top
cathedrals, the lower to churches having of the shaded areas; lighter music is lower.
services more oriented to speech. A The lower shaded area applies to opera and
compromise between music and speech is chamber music.
required in most churches.
Reverberation Time

or

ά = absorption coefficient
C = speed of sound
S = Total surface area
V = Volume
 Reverberation Calculation

Example
Reverberation Tutorial Question 1
Reverberation Answer 1
Reverberation Answer 1
Reverberation Answer 1
 Reverberation Answer 1

Q=2
Situation Directivity Directivity Index
Factor D

Free Space
1 0
Centred in a
2 3
larged flat surface
Centred at the
4 6
edge form
junction of two
flat surface
At the corner
8 9
formed by the
junction of three
large flat surface
 Reverberation Tutorial Question 2

An electric motor produces a steady state reverberant

sound level of 74dB re 20μPa in a room 3.05 × 6.10 ×
15.24m3. The measured reverberation time of the room
is 2 seconds.

(a) What is the acoustic power output of the motor in dB

re 10-12 W?
(b) How much additional Sabine absorption (in m2) must
be added to the room to lower the reverberant field by
10dB?
Reverberation Tutorial Answer 2
Reverberation Tutorial Answer 2
 Reverberation Tutorial Question 3

•It is proposed to add sound absorbing material to the walls and

ceiling of the room to reduce the interior noise levels produced by a
machine mounted on the floor in the centre of the room. Assume that
there are no other significant sound sources. If the room size is 10m
× 10m × 5m and the Sabine sound absorption coefficient for all
surfaces in the 250Hz octave band is 0.08 before addition of the
absorbing material and will be 0.5 on the surfaces covered after
addition of the sound absorbing materials, what is the expected
noise reduction (in dB) 3m from the machine in the 250Hz octave
band. Assume that the floor is concrete and that the machine
radiates noise omni-directionally (same in all directions).
Reverberation Tutorial Question 3
Reverberation Tutorial Question 3
 Reverberation Tutorial Question 4

A room of dimensions 8m × 6m × 3m high has an average surface

absorption coefficient of 0.05, apart from the ceiling which is covered
with acoustic tiles having an absorption coefficient of 0.15 (random
incidence values, for the octave band centred at 125Hz).

(a) Estimate the average reverberant sound pressure level due to a

broadband source in the room which radiates 25mW of acoustic
power in the 125Hz octave band.

(b) At what distance from the source do you expect the direct and
reverberant sound pressure levels to be equal, for the room
described above? (Assume the source is non-directional.)
Reverberation Tutorial Question 4
 Reverberation Tutorial Question 4

Situation Directivity Directivity Index

Factor D

Free Space
1 0
Centred in a larged
2 3
flat surface
THE END