Try this simple experiment: Go to an open field, and make some noise. Shout, sing, bang two rocks together, whatever. You’ll note that although the sound may be loud, you can also feel it dissipate into the open air. The openness makes sounds seem empty. Now repeat the experiment indoors. You’ll hear a closeness and fullness to the sound as the room surfaces return some of the sound to you. You are enveloped by the sound; it is all around you; it is more involving. The room embellishes the sound. As you explore different rooms, you’ll observe that each room has a different sound quality. Simply by listening, you can tell whether a room is “dead” or “live” and whether the room sounds “dull” or “bright.” Furthermore, if you listen carefully, you can hear if you are standing near a reflective wall, or far from it. You can also detect directionality and room size; for example, you can hear if your room has a high ceiling.
We conclude that rooms imprint their sonic characteristics on sounds within them. This is logical because sound emanating from a source will travel outward, strike room surfaces such as the floor, ceiling, and walls, and bounce back. The characteristics of each surface thus affect the sound that is returned to the listener. Some sound components are uniquely free of room effects. Imagine that you are sitting near a noisy machine. Some sound radiates from the machine and travels directly to your ears. Because that direct sound does not strike a room surface, it is not affected by the room. (However, its high-frequency response will be slightly reduced as it travels through air.) On the other hand, other sound from the machine strikes a room surface and returns to your ears; that sound is affected by the surface characteristics.
In any case, rooms have their own “sound” because they impose their own characteristics on audio signals contained within them. Let’s think about that for a second. It’s actually kind of remarkable. Sound such as music coming from headphones will sound the same everywhere. No matter what acoustical environment we are in, the headphones sound the same. That’s because the room is not part of that playback signal path. But sound such as music from a loudspeaker will sound different in every acoustical environment. Every room where you set up the loudspeaker will cause the sound you hear to be different—sometimes dramatically different; that is because the room is now part of the signal path. Also, in the same room, the loudspeaker will sound different when it is placed in different locations in the room and it will sound different as you move around the room. By the same token, when you are recording a musical instrument, the sound you receive at the microphone will be different in every room and the recorded sound will sound different as the instrument or the microphone is moved. Clearly, acoustical environments such as rooms are a big deal.
We are familiar with the idea of an electrical signal passing through a black box that changes the signal passing through it. We can imagine that the box has knobs and buttons that let us manipulate the changes. A room operates the same way on an acoustic signal, and we can just as surely manipulate the changes it imposes. Instead of knobs and buttons, we use room size and geometry, glass fiber, drywall, carpet, ceiling tile, and other common and specialized construction materials to tune the room to the desired result. Getting the desired result is what these posts are all about.
It’s also worth noting that with a box with knobs and buttons it is easy to effect changes and vary them at will. A room’s acoustical characteristics, on the other hand, are more permanent. And, we won’t fully know how the room sounds until after it is built. Clearly, it’s important to be able to predict the room’s response with reasonable accuracy when it still only exists as a set of blueprints. Doing this requires knowledge, experience, and some mathematical tools. Finally, all the design work in the world won’t yield the desired result unless the construction is done right. Relatively small differences in construction technique can spell the difference between good and bad acoustical performance.