Compressor

Reduces the dynamic range of a signal.

Parameter	Description
Thres Hold	Certain level for threshold, in db. Varies from -92 to 0.
Rate	Rate of mixing signals, in percent. Varies from 1 (minimal effect) to 100 (maximal effect).
RMS Time	Time for root-mean-square calculation, in ms. Usually RMS Time is equal to 100 ms.

Tip:

See also: Apply a Special Effect or Filter
N db = 20 lg (Pn/P0)

N - number of db.
Pn - Level of sound pressure in pascals.
P0 = 2*105 pascals.
A compressor is basically a variable gain device, where the amount of gain used depends on the level of the input. In this case, the gain will be reduced when the signal level is high which makes louder passages softer, reducing the dynamic range. This effect equals dynamic volume regulator.
This behavior is opposite to that of the Expander.