Chapter 2: What Is A Compressor?
The natural sounds of everyday life, which can be caused by anything from, say, the falling of a leaf to the roar of a jet engine, vary extremely widely in sound level. This variance is known as the “dynamic range” and is the difference — expressed in decibels (dB) — between the loudest and quietest sounds in a piece of audio, such as music or speech, or that can be reproduced by a piece of audio equipment without distortion.
The human ear has an automatic gain control that enables it to accommodate all of the sounds ranging from the threshold of hearing to near the threshold of pain, a dynamic range of approximately 120dB. However, a dynamic range of this magnitude, even if it were possible in audio equipment, is not required in practice for comfortable listening. A sound pressure level (SPL) of 96dB would, in a domestic environment, almost certainly annoy the neighbours. While, at the other end of the dynamic scale, a typical ambient noise level of at least 40dB SPL makes it impossible to use very quiet levels in recorded or broadcast sound media. So, it is almost always necessary to compress the dynamic range of natural sounds to fit them into a window (usually about 30dB to 40dB) suitable both for the equipment and for comfortable listening.
The compressor is a valuable tool for the control of the dynamic range.
Principles of compression
One of the principal uses of compression is the control of level in vocals. Many singers train for years to achieve the degree of breath control necessary for an even tone and expressive performance. Whereas, other vocalists rely on an instinctive voice production technique, which may need technological assistance to maintain a consistent level that sits correctly in the programme.
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| Unprocessed signal | Time |
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| 3:1 Compression above threshold | Time |
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| With make up gain added | Time |
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Figure 1: Compression - signal level graphs
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Operator Manual | 5 |
