Chapter One: An Acoustics Primer
11. What is reverberation (reverb)?
The sound waves that reach the listener's ear directly from the sound source are often referred to as the direct sound. These waves reach the listener's ears first in most acoustic environments. The first reflected sounds to reach a listener's ears are called early reflections. Since they travel a longer path, compared to the direct sound, the amount of time it takes the first reflected sounds to reach our ears gives us clues as to the size and nature of the listening environment. Because the reflected sound may continue to bounce off of many surfaces, a continuous stream of sound fuses into a single entity, which continues after the original sound ceases. The stream of continuing sound is called reverberation. The rate of build-up of echo density is proportional to the square root of the volume of the room.
The time-domain and frequency-domain reverb characteristics of an enviroment can be represented by its impulse response, which is equivalent to subtracting the original sound from its reverb and storing it. Combining digital sound files with an impulse response file in a process call convolution will result in something equivalent to playing the sound in that hall. Many high-end digital reverb units have stored impulse responses from famous concert halls. Later, we will see how digital filters are also measured by their impulse responses.
Because of the inverse-square law described above, reverberated sounds will eventually lose enough energy and drop below the level of perception. The amount of time a sound takes to die away is called the reverb time. A standard measurement of an environment's reverb time is the amount of time required for a sound to fade to -60 dB. (This time is often called T60.) Concert halls will normally have much longer reverb times than small rooms, but maybe not as much as tunnels. Rooms with lots of reverb are called "wet" and those without are called "dry." The reverb time in a concert hall, and the variable decay time of different frequencies, keeps acoustic designers up at night. Special chambers for acoustic research, and for the recording of special sound examples, are called anechoic chambers (an=no, echoic=echoes), which should have reverb times of 0.
Diffusion, often a setting on reverb units, refers to higher frequencies spreading and dying out more quickly than lower frequencies, something we use as an aural cue to the size of a space. A football field will have a higher degree of diffusion than a small studio. Humidity plays a large factor in diffusion as well.
Almost all studios have either hardware or software reverb units. The controls include reverb time (how long it will take the reflections to completely die away), pre-delay time (how long after the direct sound will the first early reflection arrive), and diffusion (how much more quickly will the higher frequencies die away). Many reverb units also have filters that allow the user to tune the acoustic characteristics of the imaginary environment.
Reflections from surfaces that stand out from normal reverberation levels are called echoes. An echo of prominent amplitude, close in time to the original sound may be referred to as a slapback echo. Concert halls with a focusing flat back stage wall may produce slapback echoes with sharp loud sounds, such as percussion.