As an aside, an expander is a device which increases the dynamic range of a signal. For example, a 10dB change in the input signal might result in a 20 dB change in the output signal, thus “expanding” the dynamic range.
There are several other terms related to compression that can be demonstrated by referring to Figure 2. The amount of compression or gain reduction is typically given in dB and is defined as the amount by which the signal level is reduced by the compressor. Graphically, this can be understood by looking at the difference in levels between what would have been the uncompressed (the output from an amplifier) output level and the compressed output level. This value is what is displayed by the
As mentioned previously, the compression ratio is defined as the ratio of the increase of the level of the input signal to the increase in the level of the output signal. In this example, the input level is increased by 10 dB while the output level only increases 5 dB. This would be a compression ratio of 2:1. Lower ratios such as 2:1 result in more gentle compression. (Note that a compression ratio of 1:1 is no compression at all).
Typically, compressors let you choose a threshold. This is the point at which gain reduction starts to take place. When an audio signal is below this threshold the compressor acts like an amplifier and there is no gain reduction. Above the threshold the slope becomes less than 45 degrees, indicating gain reduction and hence compression.
The point at which a compressor transitions into compression is commonly called the knee. In practical compressors, this transition is more gentle than what is depicted in the diagram.
Many modern compressors provide a control which adjusts the threshold directly. In the case of the
Output Level (dB)
+10
0
Compression region
10 dB of compression
2:1 Compression 
knee
0 | +10 |
Input Level (dB)
Figure 2 - Input/output curve of a compressor with a ratio of 2:1 and a threshold of
6
