Compression just might be the most important effect to master, but it's also easily abused. Compression is the reducing of the dynamic range of audio. Dynamic range is a natural part of sound. When you manipulate it, you are changing a big part of the sound. If done wrong, it can sound unnatural and harsh.

Breathing and Pumping

These are two common compression no-no's, and both are the results of overcompressing. Let's talk about how you can avoid them. Pumping typically occurs with bass drums, which have sudden loud bursts and can get too loud. Compression can help tame this, but if the kick drum is only one of many sounds fed into a compressor, you might have a problem. The bass drum will kick the volume up, and the compressor will respond by turning everything down with it, suddenly. Since the kick drum is rhythmic, the effect is noticeable because the volume pumps up and down in rhythm. The easiest way around this is to put the kick drum on its own separate compressor, or lengthen the attack time on the compressor.

Breathing is a side effect that occurs when vocals are overcompressed. If the compression is set too high, there is little difference in volume between the loudest sung phrases, and the quietest inhale. Breathing occurs when inhales and exhales of air are as loud as the rest of the sung parts. Not the nicest sound. You can get around this by setting the ratio lower and adjusting the threshold so that the quiet parts stay quieter, and the loud parts get squeezed slightly.

Overcompression: Hitting the Wall

Compression is the limiting of dynamic range. Dynamics are also referred to as nuance. If you overcompress a final mix, making the whole song very loud and not allowing it to dip down and have nuance, your music will suffer. NO ONE LIKES TO READ SENTENCES WITH ALL CAPS. IT'S ACTUALLY ANNOYING. That is a dramatic example, but that's what a listener hears when your mix is overcompressed with no dynamic range. This is also very prevalent in dance/house/club–style music. Don't let your music fall victim to the current trend of louder is better — it's not always so. Subtle compression will help a mix sit and sound nice, but overcompression will kill it.