Contents

 Types of noise
 Measurement of noise and indices.
 Theory of sound transfer: Airborne, impact, flanking.

 Techniques of sound control

 Absorption, insulation principles,
 sound reduction index, simple calculation on sound
insulation.
NOISE
 Noise is unwanted sound.
 Effects on the listener or on the environment:
1. Loss of hearing is one result of excessive exposure to noise.
2. Quality of life in general decreases in a noisy environment
such as in areas near busy roads or airports.
3. Interference with desirable sounds such as speech or music
can be annoying and in some situations, dangerous.
4. Distraction from a particular task can cause inefficiency and
inattention.
5. Expenses are incurred by the measures needed to try and
combat noise. Business may suffer loss of revenue in a noisy
environment.
 Measurement of Noise
The acceptance of noise depends on individual hearing sensitivity
and living habits. The acceptance of noise is affected by the
external factors below:

1) Type of environment
Acceptable levels of average noise are affected by the type of
activity. Example: library has different requirements to those vs
workshop.

2) Frequency structure of the noise

Some frequencies are found to be more annoying or more harmful
than others. Example: Jet engine vs motocycle

3) Duration of the noise

A short exposure to a high noise level is less likely to annoy or to
damage the hearing than a longer exposure.
 Sound Level Meter
 Sound level meter is an instrument to give constant
and objectives measurements of sound level.

 Sound level meter can be small enough to be hand-

held and are supplied in several grades of accuracy.

 The meter may have ‘fast’ and ‘slow’ response settings

but may not be fast enough to accurately record an
impulse sound, such as gunfire.
 Frequency Components
 Most practical noise is sound which contains a spectrum of
different frequencies which are detected by the
microphone of a sound level meter.
 Human hearing judges some frequencies to be more
important than other frequencies. Therefore, the
interpretation of difference frequencies needs
consideration.
 If a noise contains pure tones of a single frequency, then it
is usually more annoying than broadband noise with a
spectrum of frequencies.
 Pure tones are often present in the noise given off by
industrial equipment such as high speed fans or other
machinery and by electrical generators and transformers.
 Statistical Measurement
 During a chosen period of time, such as 12 hours, it is
possible to record many instantaneous readings of
sound level, in dB(A).

 The variations in these readings can then be combined

into one single number or ‘index’ by a statistical
process using a percentile level, such as 10 per cent.

 A percentile level, such as L10, T’is the noise level

exceeded for 10 percent of a given measurement time T.
Unit: dB(A)
 Equivalent continuous sound level, Leq
 Another method of assessing noise that varies in sound
level over time is to use an average value related to total
energy.
 Although human hearing does not judge loudness in terms
of energy, the Leq measurement of accumulated sound
energy is found to correlate well to the annoyance caused
by noise and also to hearing damage.
Equivalent continuous sound level, Leq,T is that
constant sound level which, over the same period of
time T, provides the same total sound energy as the
varying sound being measured.
Unit: dB(A)
 Effect of Noise
 Noises of various levels may produce both psychological and physiological
effects:
65 dBA – up to this level noise or unwanted sound may create annoyance, but its
result is only physiological (nervous effects).Above this level
psychological effects, such as mental and bodily fatigue, may occur.
90 dBA – many years of exposure to such noise levels would normally cause
permanent hearing loss.
100 dBA – with short period exposure to this level, the aural acuity may be
impaired temporarily, and prolonged exposure is likely to cause
irreparable damage to the auditory organ.
120 dBA – cause pain
150 dBA – causes instantaneous loss of hearing.

 The acceptable level of noise depends not only on objective, physical factors
but also on subjective, psychological factors. Whenever a noise is disturbing or
not depends on the state of mind or expectation of the listener.
 Eg.: in a sleeper train, the monotonous noise, even at 70 to 80 dBA will not
disturbing, but in a quiet home, if the listener is badly ‘tuned’, even the ticking
of a clock at 20dBA may cause great annoyance.
 NOISE CONTROL
Can be group into three main categories:
1. sound source – may be from outside of the
building such as from a road or maybe within a
building, such as noise from occupants.
2. sound path – may be through the air from the
source to the building, or the path maybe
within the building.
3. receiver – the receiver may the building itself,
may be a particular room, or maybe the person
hearing the noise.
Noise levels in and around
buildings are also affected by wider
issues such as:
◆ the design of quieter vehicles and
machinery.
◆ the location of industry and
transport.
◆ the type of construction techniques
in use.
The major noise sources in an industrialised
society are:
 Road traffic
 Railways
 Aircraft, particularly airports
 Industry: factories, workshops, etc
 Office machines (typewriters, accounting machines,
etc)
 People in residences; conversation, singing, music,
radio, records, tv, etc.
 Motorised appliances in general use (lawn mowers,
portable tools, kitchen implements, etc.)
 Noise Control Actions
 Noise from many industrial sources including factories
and construction site can be controlled using the
following actions:
➢ Elimination of noisy equipment and work methods
➢ Substitution of quieter machinery and methods
➢ Isolation of the plant by position, mounting and enclosures.
➢ Engineering modifications such as silencers and hush kits.
➢ Work practices such as short periods of high energy sound
➢ Personal protection such as hearing defenders.
• The above actions help to prevent damage to the hearing
of workers and to reduce noise pollution of the
environment.
NOISE TRANSFER
(1) AIRBORNE SOUND:
sound which travels
through the air before
reaching a partition
example
voices, radios, musical instruments,
traffic and aircraft noise.
AIRBORNE SOUND

Source
Flanking
paths
Floor

Wall
Airborne
sound
received
NOISE TRANSFER
(2) IMPACT SOUND:
sound which is generated
on a partition.
footsteps, slammed doors
Example?
and windows, noisy pipes
and vibrating machinery.
IMPACT SOUND

Source
Flanking
paths
Floor

Wall
Impact sound
received
SOUND CONTROL
1. Sound absorption
2. Sound insulation
Sound Absorption
 reduction in the sound energy reflected by the
surfaces of a room

 little effect on noise control but has an important

effect on sound quality
Sound Absorption

Sound
Absorbent reflections
lining reduced

Corridor or
Noise control by duct
absorption
Sound Insulation
 the reduction of sound energy transmitted into an adjoining air
 principle method of controlling both airborne and impact sound in
buildings
 Insulation principles
1. mass – heavyweight structures with high mass transmit less energy
than lighweight structure.
2. Completeness – the completeness of a structure depends upons
airtightness and uniformity.
3. stiffness – a physical property of a partition and depends upon
factors such as the elasticity of the materials and the fixing of the
partition. High stiffness can cause loss of insulationat certain
frequencies.
4. isolation.
 The effectiveness of each technique of insulation can differ with the type of sound
but in most constructions all the principles are relevant
SOUND REDUCTION INDEX (SRI)
 a measurement of the insulation
against the direct transmission of
airborne sound.
 The difference in sound levels on
either side of a partition as shown in
the figure given can be used as an
index of airborne sound insulation.
 Partition

Incident sound energy Transmitted sound

(100 per cent) energy
(0.01 per cent)

95 dB 50dB
Sound Reduction Index (SRI)

SRI = 10 log 1
T
Where, SRI = sound reduction index
T = transmission coefficient

UNIT : decibel (dB)

CALCULATION OF SOUND
INSULATION
 the airborne sound insulation of a partition depend
upon the amount of sound energy transmitted across
the partition.
 The proportion of energy transmitted through the
partition is measured by the transmission
coefficient, T, where

T = transmitted sound energy

incident sound energy
WORKED EXAMPLE 1:
At a certain frequency a wall transmits 1 per cent of
the sound energy incident upon it. Calculate the
sound reduction index of the wall at this frequency.
Using T = transmitted sound energy
incident sound energy
T = 1
100 SRI = 10 log10 1
= 0.01 T
= 10 log 1
0.01
= 10 log (100)
= 10 x 2
SRI = 20 dB
COMPOSITE PARTITION
A window placed in a well-insulated wall can greatly reduce the overall
sound insulation of the wall. The overall transmission coefficient can
be calculated using the transmission coefficients and areas of the
individual components such as windows and doors.
Then, T0 = (T1 x A1) + (T2 x A2) + (T3 x A3)
A1 + A2 + A3

Where, T0 = overall transmission coefficient

T1 = transmission coefficient of one component
A1 = area of that component etc.

The overall sound reduction index for the complete partition can
then be calculated using the overall transmission coefficient.
WORKED EXAMPLE 2:
A wall of area 10m2 contains a window of area 2m2. The
SRIs are: 50dB for the brickwork and 18dB for the
window. Calculate the overall SRI for the wall.
Brickwork : T1 = ?, A1 = 10 – 2 = 8m2, SRI1 = 50dB
Window : T2 = ?, A2 = 2m2, SRI2 = 18dB

SRI1 = 10 log (1/T1) SRI2 = 10 log (1/T2)

50 = 10 log (1/ T1) T0 = (T1 x A1) + (T2 x A2)
18 = 10 log (1/ T2)
5 = log (1/T1) A1 + A2
1.8 = log (1/T2)
Antilog 5 = (1/T1) (1 x 10-5 x 8) + (0.016 x 2)
Antilog 1.8 = (1/T2)
8+2
T1 = 1/antilog 5 = 1 x 10-5 T2 = 1/antilog 1.8 = 0.016 = 3.208 x 10-3
SRI0 = 10 log (1/T0)
= 10 log (1/3.208 x 10-3)
= 24.94 dB

Overall SRI = 25 dB
Work example 3:
800 units of sound energy are incident upon a wall and 10 of
these units are transmitted through the wall.
(a) calculate the SRI of this wall
(b) If a window has a SRI of 33 dB, then calculate the
transmission coefficient of this wall.

Work Example 4:
An external brick cavity wall is to be 4m long and 2.5 m high.
The wall is to contain one window 1.2 m by 800mm and one
door 750mm by 2m. The relevant sound reduction indexes
are: brickwork 53 dB; window 23 dB and door 20dB. Calculate
the overall SRI of the completed partition.