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Acoustics
Room Acoustics In some areas such as lecture theatres, concert halls and conference rooms the acoustics are important. The shape of an auditorium is significant since sound may be directly or indirectly transmitted to the audience. The diagram below shows an elevation of an auditorium with a shape to give improved room acoustics. Sound directed to audience
Sounding Board or sloped ceiling
SOUND SOURCE
Raised speaker improves direct sound transmission Raked seating improves audience direct path to sound so
AUDIORIU! S"A#E
Reverberation Time If sound in a room is reflected from several surfaces it will reach the receiver at different times and in the worst cases an echo is set up. The reverberation time is a standard to indicate the time taken for a sound to decay by 60 decibels. It is also the time taken for the sound intensity to die away to one millionth of its original intensity, the symbol is ‘t’ and the units are in seconds. The table below shows some typical reverberation times
Small Rooms
Medium Rooms
!ar"e Rooms
750 m3
750 7500 m3
#ver 7500 m3
0.! #.00 #.!0
0.! " #.00 #.00 " #.%! #.!0 " %.00
#.00 #.00 " %.00 %.00 or more
Use
Speech $ulti"purpose $usic
Sabine $ormula
Sabine devised an e&uation which relates reverberation time 't( with volume ')( of a room and the total absorption 'a( of the room.
t
*
0.#6) a
The absorption 'a( is found by obtaining the sound absorption coefficient + α for each material in the room and multiplying it by the areas +s of each material and adding all the absorption(s together.
t
*
0.#6) Σ α s
%&le '
-alculate the reverberation time for a hall of volume 000m / at !001 given the following surfaces finishes. Item
Area 2 Absorption
Area 's( +m%
Absorption 'α'
3rick wall
!%0
0.0%
'0()
4laster
00
0.0%
')
-arpet
/00
0./0
*0
-urtain
0
0.0
+,
-anvas Scenery
%00
0./0
-0
Acoustic 3oard
#00
0.0
70
/00 5o
0.m% unitsseat
'+0
Seat occupied
at !001
α
. s
(s 3*+()
*
0.#6) Σ α s
t
*
0.#6 2 000 /7%.
t
*
'(-3 sec
t
8or this si1e of room a reverberation time of #.6/ seconds is suitable for music but not for speech.
%&le +
-alculate9 a the reverberation time for a hall of volume %!0m/ at !001 given the following surfaces finishes. b The e2tra area of curtain re&uired to reduce the reverberation time to #.# seconds.
Item
Area 2 Absorption
Area 's( +m%
Absorption 'α'
3rick wall
%#0
0.0%
)(+
4laster
/00
0.0%
-(0
-arpet
%%0
0./0
--(0
-urtain
60
0.0
+)(0
-anvas Scenery
#%0
0./0
3-(0
Acoustic 3oard
#/0
0.0
*'(0
%!0 5o
0.m% unitsseat
'00(0
Seat occupied
at !001
α
. s
s 3+7(+
Answer +a t
*
0.#6) a
t
*
+ 0.#6 2 %!0 /%.% * #./ seconds
Answer +b $ake rev time #.# seconds :earranging the Sabine formula9 a
.mprovin" Room Acoustics To improve a room(s acoustics for a particular purpose it will be necessary to alter the total absorption /a( A number of different types of absorbent may be re&uired or it may be possible to concentrate on one material or area to improve the reverberation time. 8ibre"boards and similar soft materials are very efficient absorbers at hi"h fre1uencies but not at low fre&uencies. A thin panel concealing a space lined with absorbent is an efficient absorber at lo2 fre1uenc. The position of any absorbing material is important and back walls are fre&uently the first choice. Sometimes a simple solution to improve reverberation time is the addition of curtains or to improve e2isting curtain material. The table below shows some absorbing materials. .T%M
A3S=:4TI=5 -=;88I-I;5T '+5 4
500 4
+000 4
0
0
0.00
Audience /padded seats per person
0.#
0./
0.
Seats /padded per seat
0.0<
0.#6
0.#7
6ric2or
0.0%
0.0%
0.0
$loor tiles /hard
0.0/
0.0/
0.0!
8laster
0.0%
0.0%
0.0
9indo2 /5mm
0.0%
0.0#
0.0!
0./0
0./0
0./0
0.%0
0.#0
0.0
0.0
0.#!
0.#0
0.#0
0.0
0.!0
Air
$ibreboard 2ith space behind Suspended plaster:board ceilin" 8l panel over air space 2ith absorbent ;urtains : heav
8artition .nsulation The amount of Sound reduction in decibels +d3 through a partition can be defined as the Sound :eduction Inde2. A formula for S:I is given below.
S.:.I. in d3 * >here9 T * average sound transmission
#0 log#0
$
* area 2 transmission coefficient for material
See formula below?
TA) 2 A
* T# 2 A# @ T% 2 A%
%&le 3
-alculate the sound reduction of the partition as shown below. (indo)* +m )ide , $.%m high - &.&$
'.%& m Brick )all #lastered /oth sides Single lea0 otal thickness - +%&mm - 1 , $&2 $%.&& m
$ 2 #0" S.:.I. in d3 * #0 log#0 7.%#7 -asio calculators " ;nter # divide by 7.%#7 2 #0 " +;C4 button for #0 " and @" button to change signthen e&uals button then Dog button then 2 #0.
S.:.I. in d3 * /0./! d3
%&le )
-alculate the sound reduction of the partition as shown below.
S.:.I. in d3 * #0 log#0 #6.6 S.:.I. in d3 * %%. d3
The table below shows some typical Sound :eduction Inde2 +S:I figures for partitions.
Tpe of 8artition
%<0mm cavity wall with both outer faces plastered Single sheet +% o1 glass in wood or sealed metal frame Single 6mm plate glass in wood or sealed metal frame Eouble gla1ing, two sheets of +%o1 glass in frame with mm airspace.