Every room has its own acoustical characteristics based on size, shape, and surface material. Since sound waves travel, they are affected by obstacles which either stop further travel (absorption), or cause it to "bounce" and travel in different directions (reflection). Both conditions affect the ambient sound of the room, which is picked up by recording microphones. Depending on personal sound preference and the type of microphone and its placement, room acoustics can add or detract to the recorded sound. In the case of a vocal booth, which is a room designed expressly for the recording of vocals, treatment of the booth will control the activity of sound waves.
When sound waves hit a hard surface, they are deflected off the surface and reflect or "bounce" back at slightly different times. This slight time-delay results in reverberation and echo effects which are picked up by the microphone. Although artificial reverb and echo effects are often added to vocals during recording to add depth, they can be controlled by the engineer. When natural room reverb and echo is present, it can only be tolerated as-is, and is known as a "wet" or "live" room. Vocal booths are treated with sound absorbing materials when the natural room sound is detrimental to recording, but some rooms are purposely left non-treated to achieve a certain sound.
Sound waves hitting a soft surface are absorbed, and are not subject to reflection. Vocal booths that are fully treated with sound absorbing materials are called "dry" rooms, as the only sound picked up by the microphone is that of the voice with no other ambient sound. Dry vocal booths give the recording engineer the advantage of unadulterated sound, which facilitates easier manipulation with effects if desired, but some engineers (and vocalists) prefer vocal booths with some sort of natural sound qualities. This is achieved by partial booth treatment, in which some booth surfaces are treated with sound-absorbing material, while others are not treated at all.
Vocal booths that are partially treated usually contain some method for sound "diffusion." Diffusion treatment allows sound waves to bounce and travel to create a more "live" sounding room, but in a controlled manner. Sound diffusion involves the placement of a hard but irregularly-surfaced object in the direct path of sound waves. When the waves hit the diffuser, they are broken up into smaller frequencies and reflected. This results in room reverb and echo, but the effect is less pronounced because of the smaller sound waves.
Bass frequencies are the longest and lowest sound frequencies, and tend to gather in corners before being reflected to other areas of the room. Reflection of low frequencies results in exaggerated bass in a room, and can disrupt higher frequencies. For this reason, "bass traps" are used to control the problem, and consist of placing sound-absorbing material in the corners. Bass traps prevent low frequency reflection, allowing higher frequencies to be heard.
Vocal booths can be treated with commercially available products, typically in the form of foam blocks or roll material which are glued or stapled to room surfaces, but ordinary materials may also be used. Sound-absorbing materials can include carpeting, blankets or egg crates. Sound reflecting materials can be made from old windows, countertops, sheet metal or other solid materials. Sound diffusers can be purchased or fabricated with strips or blocks of wood or other materials fastened to the booth wall. Experimentation is the key to vocal booth treatment however, as preferences will vary from person-to-person, and results will vary from room-to-room.