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DN540
Operator Manual 5
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.
Figure 1: Compression - signal level graphs
Unprocessed signal
Time
Threshold level
Signal level
3:1 Compression above threshold
With make up gain added
Time
Threshold level
Signal level
Time
Signal level
Threshold level