Basics
In simple terms, a compressor is an automatic volume control. Loud sounds over a certain threshold are reduced in level while quiet sounds remain untreated-- (this is known as downward compression, while the less common upward compression involves making sounds below the threshold louder while the louder passages remain unchanged). In this way it reduces the dynamic range of an audio signal. This may be done for aesthetic reasons, to deal with technical limitations of audio equipment, or to improve audibility of audio in noisy environments.
In a noisy environment, background noise can overpower quiet sounds (such as listening to a car stereo while driving). A comfortable listening level for loud sounds makes the quiet sounds inaudible below the noise; a comfortable listening level for quiet sounds makes the loud sounds too loud. Compression is used in order to make both the soft and loud parts of a sound more tolerable at the same volume setting.
Compression reduces the level of the loud sounds, but not the quiet sounds, thus, the level can be raised to a point where the quiet sounds are more audible without the loud sounds being too loud. Contrast this with the complementary process of an expander, which performs almost the exact opposite function of a compressor, i.e., an expander increases the dynamic range of the audio signal.[1]
A compressor reduces the gain (level) of an audio signal if its amplitude exceeds a certain threshold. The amount of gain reduction is determined by a ratio. For example, with a ratio of 4:1, when the (time averaged) input level is 4 dB over the threshold, the output signal level will be 1 dB over the threshold. The gain (level) has been reduced by 3 dB. When the input level is 8 dB above the threshold, the output level will be 2 dB; a 6 dB gain reduction.
A more specific example for a 4:1 ratio:
- Threshold = −10 dB
- Input = −6 dB (4 dB above the threshold)
- Output = −9 dB (1 dB above the threshold)
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