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Ken:
Dad, is designing sound of a hall same as determining
the reverberation time?
Dad:
Reverberation time is the most important, but it is one
thing.
Ken:
What other factors are there in designing it?
Dad:
Well, let’s think about what is the first step to
design sound. Designing sound includes industrial design
for the sound to shut doors of automobiles, fold cell
phones and designing space for conversation or listening
to music, and determining structure and height of sound
barrier which I talked about the other day relating to
sound diffraction and diminishing noise with sound
insulation and acoustic absorption in a broad sense. It
is also designing of sound to determine thickness of
slab and wall of a condominium for quiet dwelling
environment.
Ken:
You could hear sound of water bickering from a
loudspeaker on a street. It is also designed, isn’t it?
 Dad:
Yes, it is. It is called “Soundscape”,
meaning scenery of sound, whose concept is that as
vision comes with scenery, so does hearing and sound
from a loudspeaker is designed to stir the imagination
of the site. However, natural sound on a street does not
always strike a chord.
Ken:
You are not always aware of sound around you, but
if it is not designed appropriately, it gets too loud or
resounding to listen to what you really want.
Dad:
Yes. Now I will tell you about designing interior
sound.
In designing interior sound, it is important to decrease
noise inside at first. Designing echo to fit the purpose
of the space follows it. There are varieties of sizes
and purposes for rooms from gymnasium and indoor stadium
to a meeting room which is ordinary as a room. To put it
simple, reverberation time depends on the size of a
space and acoustic absorption rate of materials of walls
and ceilings.
Ken:
What is the acoustic absorption rate?
Dad: Generally
speaking, when sound hits on such materials as concrete whose surface is
solid, most of it reflect without absorption.
In this case, the absorption rate is 1-2% at least. In contrast, such
materials as thick curtain and carpet absorb sound to some extent.
In case of high frequency, absorption rate is about 30-40%.
Ken: Isn’t there a
material which absorbs sound 100%?
Dad: There is a
material whose absorption rate is close to 100%. You may have heard it.
There is a room which absorbs any sound which is higher than a certain
level of frequency in complete. It is called anechoic room. It is made
of walls and ceilings with glass wool or absorption material made of
glass fiber in the form of wedge.
Ken: I wonder how it
sounds like. But for what purpose do they need that kind of room?
Dad: If you measure
noise discharged from equipment in a room with echo, it is more than
what is from the equipment alone because there is echo on walls or
floors besides noise from the equipment. In order to measure noise from
equipment precisely, you need a room without echo or an anechoic room.
Ken: You told that
reverberation time depends on the size of a room. Does a large room have
longer reverberation time than a small?
Dad: Yes, it does.
Reverberation time is proportional to the size of a room and is in
inverse proportion to equivalent absorption area
Ken: Equivalent
absorption?
Dad: What equivalent
absorption area means is a value of absorption rate (rate of material
absorbing sound: if 0%, complete reflection, if 100% complete
absorption) multiplied by the area of the material used. In case of a
room, total value of each absorption rate multiplied respectively by the
area of each material or wall, ceiling and floor which compose a room is
the equivalent absorption area of the room.
Ken:
Then, a room with large dimension with lower-rate
absorption material has long reverberation time, and a
small room with high-rate absorption material has short
reverberation time, is it correct?
Dad: You’ve got
right. Compare a gymnasium and a living room for instance. When you are
humming in a bathroom, you feel easy on the ears with echo. Even though
tiny space, it is tiled inside and the equivalent absorption area is
very small.
Ken:
I see. Then if you design a hall to play music, you’d
better to make its reverberation time longer. It is
effective if you use a material with low absorption
rate.
Dad:
It is not so simple. If the reverberation time is
too long, it is hard to catch subtle sound or words of
songs in case those are to be heard. Design of
reverberation should be according to the purpose of each
hall with care.
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