Compressor?

[gspiess]

Indulge me with this simplistic explanation:
Picture a bell curve that represents sound level during a musical passage. Now picture placing your hand at the top of the curve and pressing down. By "compressing" the curve, you are reducing the difference in sound level between the quietest and loudest passages; thus, the loud passages are not as loud. You have "smoothed out" the curve.
But the width of the curve has not changed, so the length of the passage is the same. If we raise the overall volume, but keep compressing the curve, the effect is a ringing note will sustain at a near constant volume without going beyond the top of the compressed curve. Electric guitarists in particular like this effect to be able to hold long notes.
Another real life example comes from our church. Years ago we had a pastor who was a bit on the charismatic side. He would go from almost whispering to shouting at the top of his lungs during a sermon. Being mic'd up, he drove the sound team crazy and made babies cry until they put a compressor on his channel. They could turn up the volume enough to hear him speaking softly, but when he cut loose with shouting, it didn't rattle the steeple.

[Explorer]

What does a compressor actually do ?

A compressor brings down the volume peaks, so the dynamic range stays between certain levels.

There are different parameters controlling the volume at which the volume limit kicks in (threshold), how quickly the compression starts working (attack, which can be set somewhere between hard knee/instant and soft knee/gradual), and how quickly compression rolls off once the volume drops below the threshold (release).

There are all kinds of audio pages explaining how to use those controls to emphasize different characteristics of different instruments, from fingerstyle acoustic guitar to vocals to hi-hats/percussion. Quite a few have audio examples to really get across what is happening. If anyone is really interested, you could probably easily find such searching for "audio compressor example).

[craig.collas]

I use a barber compressor wit a blend control to balance the string volume
using a k&K

[EdHanrahan]

... They could turn up the volume enough to hear him speaking softly, but when he cut loose with shouting, it didn't rattle the steeple.

Thus, many bass guitar amps include compressors. Some have controls, while some are pre-set mostly, IMHO, to protect the steeple, ehr, uhm, speaker.