Reverb – Background
Everything we hear is a combination of the sound travelling directly from the source to our ears and reflections of the sound from surfaces and objects in our environment. This means that a reflected wave has to travel further to reach our ears and so it arrives at our ears later than the direct sound, and from a different direction. Normally our brains decipher this information to obtain information about our environment, and our position within that environment.
An Anechoic Chamber is a special room that produces no reflections, due to both architectural design and the use of special acoustically absorbant materials. If you stand in an anechoic chamber and clap, the lack of reverb is strange, and somewhat unsettling. The human brain is not used to dealing with a reflectionless room. The lack of information about our environment is an uncomfortable sensation, especially when our eyes are telling the brain that our ears should be detecting some information.
Reverb, or more formally Reverberation, cannot be removed from a signal. So, unless you are deliberately trying to capture the natural sound of a particular room or hall on the recording, it is common for sound engineers to use close miking techniques to minimise the reflected sound and maximise the direct sound during recording. This allows the engineer to manage the reverb on the track by artificially applying reverb using either an analogue or digital effects unit.
Each environment has a characteristic reverb. For example a small room has a short reverb time, so much so that it is not always very noticeable. Conversely, a large hall has a long reverb time as it will take a significantly longer time for the reflections to die away. The absorbtion of the walls, floor and ceiling will colour the reverb as the absorbtion of different surfaces varies with frequency.
When a sound is emitted within a room, the reverb will be delayed by the time to the first reflected wave. This time is called the Pre-Delay. Next the reverb enters a phase called Early Reflections, as the first and clearest reflections of the initial sound reach the listener. Lastly the reverb enters a phase of Dense Reflections as the amplitude of the building number of reflections fades away as the energy of the sound is absorbed by the surface materials of the environment.
Each time the sound is reflected the surface that it collides with will absorb some of the sound energy, thus reducing the amplitude. But the frequency response of different surface materials means that each material will absorb a different amount of sound energy at different frequencies.
As mentioned, effects processors are used to artifically apply reverb to a source signal. These effects processors can be either analogue or digital effects units. Plate or Spring reverbs are commonly used in both domains, although both are simulated in the digital domain.
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