Reverb, of one type or another, is probably the most widely used effect in modern mixing. In today's recording environments music is usually made from a combination of artificially produced sounds and close-miced recordings. The recordings may have been made at different times and in different places. This approach gives us control over practically all aspects of our mix but if we want it to have any kind of coherence we will need to use some reverb to help to recombine the sounds and bind them into a performance with the impression of an ambient space. Using the correct reverb can make the disparate multi- tracked parts sound whole again as well as bringing colour and three-dimensionality to the sound. Conversely, using too much reverb or the wrong reverb can cause the mix to sound cluttered, messy and distant.
When we make a noise in an enclosed space the sound radiates outwards. It hits obstacles such as walls, tables and chairs and one of the following will occur.
- Reflection: If sound encounters a hard surface it will generally reflect back into the room.
- Absorption: Softer surfaces will tend to absorb sound although precisely how much absorption occurs and which frequencies are affected depends largely on the nature of the materials.
- Diffraction: Sound will bend round or be shadowed by objects. This will spread the sound out in many new directions.
Generally there will be a distinct initial reflection after which further reflections build up quickly as the sound gets rebounded around the room and becomes more diffuse. The time between the sound and its initial reflection depends on the size of the room. On a reverb unit it is usually known as pre-delay. The complex mass of repeats and echoes that follow the initial reflection are what we recognise as reverb. Each line on the diagram below represents the amplitude of a reflection of a discrete sound. There are a number of early reflections followed by a more dense mass of intermingling later reflections. These diminish in amplitude over time.
The exact point when reverb ends, is difficult to define, so a measurement known as RT60 is used. This is the time it takes for the reverb to decay by 60dBs from its original value. This is known as the reverberation time. Reverb can radically change the nature of a sound. The sound of a hand clap in a living room, a church or a bathroom, for example, would all be very different. Reverb can be bright or dull, have a long or short decay time, it can be dense or sparse, it may or may not have a distinct early reflection.
These characteristics, and so the resultant sound, are determined by such factors as the size and shape of the enclosed space, the absorptive qualities of the materials that form the room boundaries and the quantity, size, shape and reflectivity of the objects within the room.
To illustrate the last point, if you've ever been to a band sound-check and then to the subsequent gig you will have noticed that the room sounds much more reverberant and generally brighter during the sound check than the gig. When the room is full of people high frequencies are much more readily damped and a large part of the reflected sound is absorbed by the mass of bodies making up the audience (assuming they have turned up!) giving a tighter sound.