REVERBERATION:
The sound
produced in a room or a hall suffers multiple reflections from the various
walls, ceilings and floor of the wall, so that in addition to the direct sound
. A listener in the hall hears a series of sound waves.
The listener feels the
persistence of sounds for sometime even the original sound has ceased. This is
known as reverberation.
REVERBERATION
TIME:
It is time taken
for the sound to fall below the minimum audibility measured from the
instant when the source has stopped.
SABINE’S
LAW:
This law states
that the standard reverberation time is taken by the intensity of sound to fall
to one millionth ( 10 -6)
of its original value after the source of sound is cut off.
T = 0.165V/ E al s l
Where,
V= Volume of the hall
E al s l – Total absorbing power of the room or coefficient
of absorption.
MEASUREMENT
OF REVERBERATION TIME:
Sabine used an
organ pipe of frequency 512Hz as his source of sound stopwatch. The stopwatch
was start ear instant when the source of sound ceased to act and stopped at the
instant when the sources just becomes inaudible. The ( ‘T’) can be determined.
IMPORTANCE
OF REVEBERATION TIME:
The
reverberation time in a hall should not be too large and also should not be too
short. If the reverberation time is too short, the sound may not be
sufficiently loud in all portions of the hall should “dead” on the other hand.
These Satisfactory
times are ,
|
SPEACHES
|
0.5
SECONDS
|
|
Music
|
1 to 0.2 Seconds
|
|
Theaters
|
1.1 to
0.15 Seconds
|
CONTROL
OF REVERBERATION TIME :
·
The Reverberation time can be controlled
by the following factors.
·
Providing windows and ventilators.
·
Covering the ceilings, part of the walls
and even back of the chairs with absorbent
material like felt, glass wool etc.,
·
Using heavy curtains with folds and covering
the floor with carpets.
·
Having a good sized audience.
REQUIREMENT
OF A GOOD SOUND ABSORBING MATERIALS :
A good sound
absorbing material should have the following requirements :
Ø It
should have high sound absorbing efficiency.
Ø It
should be efficient over a wide range of frequencies.
Ø It
should be easy to fix.
Ø It
should be cheap and easily available.
Ø It
should have a good resistance to fire.
ABSORPTION
CO- EFFICIENT AND ITS MEASUREMENT:
The
Coefficient of absorption of materials is the ration of sound energy absorbed
by the surface to that of the total incident sound energy on the surface. Absorption
coefficient = Sound energy absorbed by surface / Total sound energy incident of
the surface.
|
MATERIALS
|
ABSORBTION
CO-EFFICIENT
|
|
Marble
|
0.01
|
|
Glass
|
0.02
|
|
Brick wall
|
0.03
|
|
Wooden floor
|
0.06
|
|
Carpet
|
0.15 to 0.30
|
|
Ordinary Chair
|
0.17
|
|
Hair or Flat ( 2.5 cm thick )
|
0.58
|
|
Open Window
|
1.3
|
|
Human Body
|
4.3 – 4.7
|
CALCULATION OF REVERBERATION TIME
GIVEN DATA:
Size
of stage = 15.7 x 4.60m
Size
of hall = 15.7 x 22.80m
Reverberation time :
Area of stage
= 15.7 x 4.60m
=
78.4 m2
Volume of stage =
78.4 x 5.60
=
439.04m3
Area of hall =
15.7 x 22.8
= 364.80m2
Volume of hall =
364.80 x 6.50
=
2371.2m3
Total volume =
439.04+2371.2
=
2810.24m3
Floor Area =
( 22.8 x 16 )+( 16x 4.9 )
=
443.2m2
Ceiling area =
( 22.8 x 16) + ( 16 x 4.9 )
=
443.2m2
Stage =
(16 x 5.6) + 2(4.9 x 5.6)
=
144.48m2
Hall =
( 16 x 6.5 ) + 2 ( 22.8 x 6.5 )
=
400.4m2
Total =
544.88 m2
Coefficient of wall =
0.30
Coefficient of ceiling = 0.8
Coefficient of floor =
0.06
Area ( A ) =
( 0.3 x 544.88 )+ ( 0.8 x 443.2 )+ ( 0.06 x 443.2)
(544.88+443.2 +443.2)
=
0.278
Reverberation time : T = 0.167 V/ as
S = 544.88 + 443.2 +
443.2
=
1441.28
T = (0.167 x 2810.24) / (0.278
x 1431.28)
=
1.18 Sec < 1.5 Sec.
Hence Ok.
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