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.