sound design
Oto-kun
Dad, is the design of sound in a hall about determining the reverberation time you taught me recently?
father
Reverberation time is the most important element, but it's not just that.
Oto-kun
What other things do we need to decide?
father
That's right, let's first think about what sound design entails. Sound design includes the sounds of industrial products, such as the sound of a car door closing or the sound of a folding mobile phone, as well as the design of spaces where humans listen to sounds, like conversations and music. Additionally, when I taught you about diffraction before, we discussed sound barriers. Deciding on the structure and height of sound barriers, and reducing noise in loud areas through sound insulation and absorption, also falls under the broad definition of sound design. Determining the thickness of floors and walls in apartments to maintain a quiet living environment is also part of sound design.
Oto-kun
Sometimes, the sound of water flows from the speakers in the city, but that sound is also designed, right?
father
That's right. It's called a soundscape, which translates to 'the landscape of sound.' Just as there are visual landscapes, there are also sound landscapes designed to evoke auditory imagery that fits the environment. However, even when natural sounds flow through the city, they often don't resonate.
Oto-kun
Sound is all around us, so we often don't pay much attention to it, but if proper design is not implemented, the sound can be too loud or resonate too much, making it hard to hear the sounds we really want to hear.
father
That's right. Let me tell you about the design of indoor sound. The first important aspect of indoor sound design is to minimize the noise present in the space, and next, to design the reverberation in a way that suits the purpose of the room. Indoor spaces vary in size and purpose, ranging from large areas like gyms and indoor sports arenas to regular rooms like conference rooms. In simple terms, the reverberation time is determined by the size of the space and the sound absorption rate of the materials used on the walls and ceiling.
Oto-kun
What is the sound absorption rate?
father
That's right. Generally, hard materials with surfaces like concrete reflect most of the sound that hits them without absorbing it, resulting in a low absorption rate of about 1-2%. In contrast, materials like thick curtains or carpets do absorb some sound. Particularly at higher frequencies, the absorption rate is around 30-40%.
Oto-kun
Are there no materials that absorb 100% of sound?
father
There are materials that are close to 100% effective. I haven't heard of 'oto' though. Sounds at frequencies higher than a certain point are absorbed almost completely in a space called an anechoic chamber. Typically, it is made from walls and ceilings shaped like wedges using a sound-absorbing material called glass wool.
Oto-kun
What would it be like in a space that completely absorbs sound? But what is the purpose of needing such a room?
father
You see, when you measure a noise-producing device in a room with reflections, the sound measured will be louder because it includes not only the sound from the device but also the sound reflected off the walls and floor. Therefore, to accurately measure the noise from the device, a room without reflections, that is, an anechoic chamber, is necessary.
Oto-kun
You said that the reverberation time is determined by the size of the space, but does that mean that larger rooms have longer reverberation times?
father
That's right. Reverberation time is proportional to the size of the space and inversely proportional to the equivalent absorption area.
Oto-kun
Equivalent sound absorption??
father
The equivalent absorption area is the value obtained by multiplying the absorption coefficient (the ability of a material to absorb sound, where 0% means complete reflection and 100% means complete absorption) by the area of the material. In the case of a space, the equivalent absorption area of a room is the total of the products of the absorption coefficients and areas of all interior materials that make up the walls, ceiling, and floor.
Oto-kun
So, a space with a large volume and many materials with low sound absorption rates has a long reverberation time, while conversely, a small room with many materials that have high sound absorption rates has a short reverberation time.
father
That's right. For example, if you compare a gymnasium and a living room, you can understand this. Even in a small space, singing a tune in the bathroom feels nice as if it has an echo, because although the space is small, the interior is tiled, resulting in a small equivalent sound absorption area.
Oto-kun
I see. So if the hall is going to perform music, we should increase the reverberation time. That means we should use materials with a low absorption rate.
father
It's not that simple. If the reverberation time is too long, it becomes difficult to hear fine sounds, and in music, for performances where understanding the lyrics is necessary, a long reverberation time makes it hard to catch the words. Therefore, reverberation design needs to be approached carefully according to the purpose of the hall.