Building Construction & Materials Notes for Maharashtra Engineering Services, State PSC, PSUs,

RRB JE, SSC JE & All other Competitive Exams Related to Civil Engineering

19. Acoustics
Acoustics is the science of sound as applied to buildings & its scope includes not only the
design & construction of an enclosure particularly auditoriums with proper acoustical
condition & correction of acoustical faults in existing ones but also absorption & dissipation
of exterior & interior noises & insulation against sound both air borne & structure borne.
There are three characteristics of audible sound. These are:
1) Frequency higher 20,000 cycles per second to a minimum of 20 cycles per second.
2) Intensity sound energy per sq. cm. per second
3) Tone piano, cornet.

which represents the level of equal loudness of all frequencies. Thus sound is measured
mparing it aurally with a standard pure tone of a thousand cycles per
second adjustable intensity. Zero dB means sound pressure of 0.0002 dyne per sq.m.

Reverberation of Sound:
It is the persistence of sound. Reverberation time is the time taken by sound wave to
decay to 60 dB of its original strength. Reverberation time is property which may be
calculated with reasonable accuracy in the design stage & which may be measured in the
completed building.
A difficulty arises in deciding an appropriate reverberation time particularly if the room
has to be used for many purposes Scientist W.G. Sabina gave formula to find it.
According to him:
Time in seconds = 0.161 V/A
Where, V = Volume of enclosure in m3
A = Total absorption of sound in the enclosure given is m2 Sabina or sq.m.

Where s is the area of different parts of an enclosure like walls, roofs, furniture, curtains

The ratio of energy absorbed to that incident on the surface is called the sound
absorption coefficient of the surface. The unit of absorption is called Sabina (sq.m.).
Thus the Sabina is to so 1 sq.m. of surface area with a coefficient of 1.0 or 1 sq.m. of
open window.

Optimum Time of Reverberation:

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For sound film theatres or for auditorium for public address system, the time should be
selected as short & taken near the lower edge of the shaded part, for concert halls &
churches time should be longer & be taken near upper limit, room used for both speaking
& music, a middle value should be taken.

The selected should be applied to the rooms in question when the most probable
sized audience is present i.e., approximately 2/3rd of full capacity.

Recommended values of optimum reverberation time for different enclosure are :

Enclosure Optimum Reverberation Time
1. Cinema building (Hall) 0.6 to 1.2 seconds
2. Lecture halls, convocation halls & 1.5 to 2 seconds
for similar activities
3. Chamber music 1.6 to 2 seconds
4. Drama plays 0.8 to 1.2 seconds
5. Churches 1.8 to 3 seconds
6. Multipurpose hall 2 to 3 seconds
Table-18: Optimum Reverberation Time

Absorption of Noise:
The ratio of energy absorbed to that incident on the surface is termed as sound
absorption coefficient of the surface. Materials which absorb sound are called sound
absorbing materials & are classified as
i. Porous sound absorbers
ii. Non-porous absorbers

Porous sound absorbers are those which are characterized by its pores which are open
to the air in the room. An impinging airborne sound wave cause the air in the connected
pores to vibrate but owing to the constricted nature of air flow within the material loss of
sound energy occurs. Hence sound energy absorbed is due to friction.

Non-porous absorbers are those which absorb sound due to gap between the rigid wall
& an airtight panel mounted at some distance from wall.

Following are the absorbing coefficients of different materials used for acoustics for
500 Hz frequency.
Material Absorbing coefficient
Wood 0.03
Plaster 0.033
Seats 0.016 (of an auditoria or other enclosure)
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Brick wall painted 0.017

Glass 0.027
Audiences seated 0.28 to 0.4 (higher values for female with
sarees)
Straw board 0.1
0.22 (for 1000 Hz frequency)
0.56 ( for 2000 Hz frequency)
Jute board 0.1
0.33 ( for 1000 Hz frequency
Foam concrete 0.12
Rock wool 0.39
Cotton wool loose 0.89 0.61
Carpet (5 mm thick) 0.15
Timber board 0.13
Plywood 3 mm 0.10
Ventilating aperture 0.3
Curtains 0.15
Table-19: Absorbing Coefficient

Perfect acoustic condition in room is obtained when an average sound rises to a suitable
intensity in every part of the enclosure with no echo or distortion of sound & then dies
out quickly enough so as not to interfere with succeeding sounds.
In the case of concave shaped reflecting interiors surface there is a possibility for
reflected sound wave to meet at a point called sound foci. This can be avoided either
putting absorbing materials at such areas or by planning hall such that its radius of
curvature is large enough to have its centre of curvature outside of an enclosure.
Due to high concentration of reflected sound at sound foci there exist some spots with
deficiency of reflected sound waves. These spots of low sound intensity causing
unsatisfactory hearing for audience, called as dead spots. This defect can be stopped by
using sound reflectors.
Resonance: Resonance is a phenomenon of the growth of vibratory motion in an elastic
body under periodic forces timed to its natural rates of vibration. To understand this, let
us consider that if a sound wave strikes an object & is reflected from it, its strength is
reduced.
The object to which the wave strikes will also start vibrating & will strengthen the
reflected wave if the vibration of sound waves corresponds to the natural rate of
vibration of the object. This strengthening effect is called Resonance.

Conditions for Good Acoustics:

The following are the conditions which will affect acoustic properties in an auditorium.
1) The initial sound should possess adequate loudness.
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2) The sound should be clear & distinct.

3) The sound should be evenly distributed.
4) The sound should reach the audience in the same pitch & tone as it is produced.

The sound should be clear & distinct: The sound produced on a stage is absorbed by the
audience to some extent & thus row by row the energy received by audience is less.

The sound should be evenly distributed: To improve this, the speaker should be at a
higher level than the audience & the audience should be made to sit on a sloping floor
the farthest end being at higher level. This allows every audience to receive more
portions of spherical waves. The sound passing on the back of speaker is lost unless a
hard surface (reflecting surface) is provided on his back & then the audience should be
arranged in a semicircular form.

For even distribution of sound in a hall one must consider the reflecting properties of
surfaces. Due to good reflection the interference of sound waves will take place. This
interference may either cause reinforcement of waves or neutralization.

The sound should reach the audience in the same pitch & tone as it is produced: Thus
some parts may have reinforced sound while in some parts we may have comparatively
quite. Secondly reflection may cause the echo. The human ear cannot register
intervals smaller than 1/15th of a second or less, these will not cause echo. Hence no
audience should be placed beyond 10 m from a good reflecting surface or otherwise
good absorbent should be provided on rear walls to avoid echo.

Acoustical Defects:
Following may be the possible defects due to reflected sound:
1. Deflection time reverberation
2. Echo formation
3. Sound foci
4. Dead spots
5. Loudness of sound
6. External noise

1. Deflection time of Reverberation:

Each listener is elevated with respect to the person immediately in front of him so that
the listeners head is about 12 cm above the path of sound which would pass above the
head of the person in from of him.
In the case of staggered seats, this may be reduced to 8 cm.

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To satisfy the above condition, the angle of elevation of the inclined floor should not
be less than 8º.
When more accurate values are considered necessary, the slope of the floor may be
calculated by the following formula:

Where,
H = Height of sound source above normal head level
r = Back to back distance between rows of seats
h = Head clearance, in relation to sound source, the difference in height between one
row of people & the next
S= = Horizontal distance from the source to the last row, which does not require
elevator.
= elevations of the first, second & nth rows behind the row which is at a
distance S from the source.
Importance in the Acoustical Design: Deflection time of reverberation is the common
defect in most of the auditoriums. In this case, successive sounds produced by the
source overlap each other & sound is not distinctly audible & therefore this deflects can
be corrected.

2. Echo Formation: This defect is also more common when the reflecting surfaces
are curved & so many far away from the source.

3. Sound Foci: Reflecting concave surfaces cause concentration of reflected sound

waves at certain spots & when these foci are considerably increased they cause
acoustics defect in the hall.

4. Dead Spots: Because of sound concentration at certain spots, intensity of sound at

other spots gets decreased. Sound at these points is called dead spots & is
insufficient for hearing.

5. Loudness of Sound: It is caused when the source of sound is not good at reflecting
& hall has been provided with excessive sound absorption.

6. External Noise: This defect occurs due to inadequate sound insulation of the hall.

Absorbent Materials:
The ideal absorbent materials is that which has high coefficient of absorption for all the

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again it should be a structural material easy to apply, durable in use & should suit the
general scheme of decoration.

The absorbent materials obtainable in the market can be conveniently divided into
three categories:
1) Material manufactured on a cementatious base: These are special acoustic
plastors & acoustic stones. The former can be spray painted with any decorative
colour & the later is obtainable in various colours. These are used for new
buildings.

2) Materials of soft nature: These can be used only on those surfaces where they
are not likely to be damaged. These include acoustic boards, fibre, tiles, glass,
slag, wool, etc. These are more efficient but are not capable of resisting forces.
These can be used on new as well as old buildings.

3) This class is really a combination of the two. These are soft & highly
efficient absorbent material over which a hard cementific material or a metal
piece is fixed. This hard force material is perforated with holes of 3 mm diameter.
These materials are costly but are good in efficiency & at same time they are
durable in use.

Noise Insulation:
Noise in general sense, is the unwanted sound originating in or outside the building. It
is heard by ear only & the transmission to ear through the bones on the body is ignored.

If two noise levels are represented by I1 & I2, the ratio of two levels expressed in bels
(unit for comparing two noise levels) is :

th
of a

The scale of intensities of audible sound commonly met, covers a range of 130 db.
Taking into account the sensitivity of ear, a more representat
which unlike db, represents the loud of equal loudness at all frequencies.

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