Sound design

Dad, is designing the acoustics of a concert hall about determining the reverberation time, like you taught me the other day?

Reverberation time is the most important factor, but it's not the only one.

What else do we need to decide?

Right, let's start by thinking about what sound design entails. Sound design can encompass a wide range of things, from the sound of a car door closing or a cell phone folding, to the design of spaces where humans hear sounds like conversations and music. And, as I mentioned when I taught you about diffraction, determining the structure and height of sound barriers, or reducing noise in noisy areas through sound insulation and absorption, are also part of sound design in a broad sense. Even determining the thickness of floors and walls in an apartment building to maintain a quiet living environment is part of sound design.

Sometimes you hear the sound of water coming from speakers in the city, right? That's also part of the sound design, isn't it?

That's right. That's called a soundscape, which translates to "sound landscape." Just as there are landscapes in sight, there are also landscapes in sound, and it's designed to evoke sound images that are appropriate for the situation. But even if natural sounds are played in the city, it often doesn't resonate with people.

We don't often pay attention to sound because it's all around us, but if it's not designed correctly, the sound can be too loud or reverberate too much, preventing us from properly hearing the sounds we really want to hear.

That's right. So, let me tell you about the acoustics of a room. When designing the acoustics of a room, the first thing to do is to reduce the noise that is already present, and then design the reverberation in a way that suits the purpose of the room. Rooms vary in size and purpose, from large spaces like gymnasiums and indoor arenas to ordinary rooms like conference rooms. In short, reverberation time is determined by the size of the space and the sound absorption rate of the materials used for the walls and ceiling.

What is sound absorption coefficient?

That's right. Generally speaking, hard-surfaced materials like concrete don't absorb most of the sound when it hits them; it just reflects it. In that case, the sound absorption rate is low, around 1-2%. On the other hand, materials like thick curtains or carpets absorb sound to some extent. Especially at high frequencies, the sound absorption rate is around 30-40%.

Is there no material that can absorb 100% of sound?

There are materials that are close to 100% sound-absorbing. I've never heard of them, though. An anechoic chamber is a space that almost completely absorbs sounds above a certain frequency. It's usually made of walls and ceilings formed from glass wool, a sound-absorbing material made of glass fibers shaped into wedges.

What would it be like to be in a space that completely absorbs sound? But why would you need a room like that?

The reason is that when you measure the noise emitted by a device in a room with reflections, the measured sound will be louder than the sound coming from the device alone because of the reflections from the walls and floor in addition to the sound emitted by the device itself. Therefore, to accurately measure the noise from a device, you need a room without reflections, in other words, an anechoic chamber.

You said that reverberation time is determined by the size of the space, but does that mean that the larger the room, the longer the reverberation time?

That's right. Reverberation time is proportional to the size of the space and inversely proportional to the equivalent sound-absorbing area.

Equivalent sound absorption? ?

Equivalent sound absorption area is a value obtained by multiplying the sound absorption coefficient (the ability of a material to absorb sound, 0%: perfect reflection, 100%: perfect sound absorption) by the area of the material. In the case of a space, the sum of the values obtained by multiplying the sound absorption coefficient of each material by its area for all interior materials that make up the room—walls, ceiling, and floor—represents the equivalent sound absorption area of that room.

So, a large space with a lot of materials that have low sound absorption will have a long reverberation time, and conversely, a small room with a lot of materials that have high sound absorption will have a short reverberation time, right?

That's right. For example, you can see this by comparing a gymnasium to a living room. Even in a small space, humming in the bath feels nice because it creates an echo, and although the space is small, the interior is tiled or something similar, resulting in a small equivalent sound-absorbing area.

I see. So, if a hall is also used for performing music, then the goal is to increase the reverberation time. That means we should use materials with a low sound absorption coefficient, right?

It's not that simple. If the reverberation time is too long, it becomes difficult to hear subtle sounds, and in music, for performances where the lyrics need to be understood, a long reverberation time makes it difficult to hear the words. That's why reverberation design needs to be done carefully, according to the purpose of the hall.