4. • Reverberation is the persistence of
sound in the enclosed space , after the
source of sound has stopped.
• Reverberant sound is the reflected
sound , as a result of improper
absorption.
• Reverberation may results in confusion
with the sound created next.
4
5. However some reverberation is
essential for improving quality sound.
The time during which the sound
persists is called the reverberation time
of sound in the hall.
As per Prof. W .C. Sabins reverberation
time ‘t’ is given by formula :-
t= 0.16V /A where V=volume of room in cubic
meters
A= total absorbing power of all the
surfaces of room/ hall.
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8. Reverberation time & quality of sound
Reverberation time should remain within
limits as per Indian Standard Code: 2526-
1963.
8
Sr.
No.
RECOMMENDED TIME IN
SECONDS
ACOUSTICS
1 0.50 to 1.50 Excellent
2 1.50 to 2.00 Good
3 2.00 to 3.00 Fairly good
4 3.00 to 5.00 Bad
5 Above 5.0 seconds Very bad
9. Remedies
The reverberation time can be controlled by
the suitable choice of building materials and
furnishing materials.
Since open windows allow the sound energy
to flow out of the hall, there should be a
limited number of windows. They may be
opened or closed to obtain optimum
reverberation time.
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11. Echo's
Not all sound that hits matter is
absorbed. Some of it is reflected. That
means sound bounces off the solid
matter the way a tennis ball bounces off
a wall. Sound reflected back to its
source is an echo.
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12. An echo is produced when the reflected
sound wave reaches the ear just when
the original sound from the same source
has been already heard.
Thus there is repetition of sound.
The sensation of sound persists for
1/10th of a second after the source has
ceased.
Thus an echo must reach after 1/10th
second of the direct sound
12
13. Multiple echoes may be heard when a
sound is reflected from a number of
reflecting surfaces placed suitably.
This defect can be removed by selecting
proper shape of the hall .
And by providing rough and porous interior
surfaces to disperse the energy of echoes.
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14. Remedies
This defect is avoided by selecting proper
shape for the auditorium. Use of splayed side
walls instead of parallel walls greatly reduces
the problem and enhance the acoustical
quality of the hall.
Echoes may be avoided by covering the
opposite walls and high ceiling with
absorptive material.
14
16. Some times shape of the hall makes sound
waves to concentrate in some particular
areas of hall creating a sound of large
quality.
These spots are called sound foci.
This defect can be removed by
Geometrical design shapes of the interior faces.
Providing highly absorbent materials on critical
areas (curved spaces).
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18. This defect is the out come of formation of
sound foci.
Because of high concentration of reflected
sound at sound foci , there is deficiency of
related sound at some other points.
These spots are known as dead spots where
sound intensity is so low that it is insufficient for
hearing.
This defect can be removed by suitably placing
diffusers and reflectors.
Right proportions of internal spaces.
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21. External noises from vehicles , traffic engines ,
factories , machines etc. may enter the hall either
through the openings or even through walls and
other structural elements having improper sound
insulation.
This defect can be removed by proper planning
of the hall with respect to its surroundings and
by proper sound insulation of external walls.
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22. Acoustical design of halls
The initial sound from the source should be of
adequate intensity so that it can be heard
throughout the hall .
For halls of big sizes suitable sound
amplification system should be installed.
The sound produced should be evenly
distributed so that there is no dead spots and
sound foci.
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23. Acoustical design of halls
The boundary surface should be so
designed that there are no echoes or near
echoes.
Desired reverberation time should be
achieved by proper placement of
absorbents on wall.
The out side noise should be eliminated.
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24. Noise
Noise is unwanted sound which masks the satisfactory
hearing of speech and music.
There are mainly three types of noises that are to be
minimized.
They are (i) air-borne noise,
(ii) structure-borne noise and
(iii) internal noise.
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25. (i) Air-Borne Noise
The noise that comes into building through air from
distant sources is called air-borne noise.
A part of it directly enters the hall through the open
windows, doors or other openings while another part
enters by transmission through walls and floors.
Remedies
The building may be located on quite sites away from
heavy traffic, market places, railway stations, airports
etc.
They may be shaded from noise by interposing a
buffer zone of trees, gardens etc.
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26. (ii) Structure-Borne Noise
The noise which comes from impact sources on the
structural extents of the building is known- as the
structure-borne noise. It is directly transmitted to the
building by vibrations in the structure. The common
sources of this type of noise are foot-steps, moving of
furniture, operating machinery etc.
Remedies
The problem due to machinery and domestic
appliances can be overcome by placing vibration
isolators between machines and their supports.
Cavity walls, compound walls may be used to increase
the noise transmission loss.
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27. (iii) Internal Noise
Internal noise is the noise produced in the hall or
office etc.
They are produced by air conditioners, movement of
people etc.
Remedies
The walls, floors and ceilings may be provided with
enough sound absorbing materials.
The gadgets or machinery should be placed on sound
absorbent material.
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