Audio Compressors and Limiters

Audio compressors limit a signal's dynamic range to within settings chosen by the sound engineer.

Why would you want to do that?

Does that not take a way from producing a faithful recording of the original sound?

Consider some of the challenges you face as a recording engineer:

When signal levels get too high for your recording equipment to handle, you'll hear distortion on the output. Worse, they can even damage your equipment (or your ears!)
If one instrument, (for example, an amplified acoustic guitar) has a naturally wider volume range than the other instruments in the band, it'll risk not be heard above the drums during quiet passages, but can be overpowering when played at full strength.

Audio compressors and limiters help by reducing the maximum levels of sudden transient sounds that could distort your recording.

They are also used for cutting tracks and adjusting the mix. By smoothing out sudden or large volume changes, and automatically adjusting the dynamic range and balance of a track.

So how does that work?…

The audio compressor is designed to automatically:

reduce the gain when the signal level is high - making louder passages softer
increase the gain during quieter sections - so the overall dynamic range is smaller

Other Factors:

Benefit - Added Sustain
Compression can also increase an instrument's sustain. It amplifies the incoming signal to maintain a constant level, so after twanging a string, a little compression will attempt to maintain the string's sound level.

Hazard - Reduced Expression
While adding sustain to your arsenal, compression by its nature, reduces your dynamics, making it difficult to accent notes and phrases. So don't overdo the amount of compression you set just for the sake of sustain, or you'll get a lifeless resulting sound. (There are better ways to add sustain.)

Ducking, Cross-limiting, or auto-fading
If you need to have a signal's level controlled by a different signal, it is called 'ducking' (it 'ducks' a signal out of the way) or cross limiting.

For example: While music is playing, using the microphone will cause the level of the music to drop so that it's easier to hear the singer. When mixing in the studio, a ducker can also be used to make certain instruments pop out of the mix.

Improving Vocals
A 'de-esser' is a limiter that only monitors a specific frequency range. It only reduces the level of frequencies in a selected range. This allows you to reduce unwanted sounds - for example sibilants from vocal mics.

Where Should I place a Compressor in the Signal Path?
When using audio compressors with other effects, many players put it first in the chain.

Why? - Because it gives you a good signal to work with.

When the compressor is on and the output level is increased, any unwanted noise will be amplified along with the instrument's sound. Other signal processors can introduce more noise. So if the compressor is placed after those effects units, it will end up amplifying their noise, too.

Audio compressors are used in your recording studio for audio recording, production work, noise reduction, as well as for live performance applications.