Compressors are used in many audio applications and would probably be next in line to the most important piece of equipment in a recording studio. They are used on vocals, guitars, drums, keyboards, and post production.

Compressors are used to reduce the dynamic range (how loud it gets) of a sound source (guitar, voice, keyboard). They do this by reducing the volume of a sound when it gets loud. This will DECREASE the dynamic range making it smaller or fitting the sound into a smaller sonic area.

When using a compressor with other effects, many players put it first in the chain. First, it gives you a good signal to work with. When the compressor is on and the output level is increased, the noise will be amplified along with the instrument's sound. Other effects can introduce more noise, so if the compressor is placed after those effects, it will end up amplifying their noises, too. To reduce all this noise while not playing you need to use a NOISE GATE.

You can increase the sustain of an instrument, lets say guitar, because the compressor will try to maintain a constant output level for a given input level. You can get the "STING" or "POLICE" sound by using a compressor near it's limits. Because compressors reduce dynamic range they make Accents much less noticeable. So you should be careful not to kill the sound of the instrument.

For example: If you have a singer that can't seem to sing directly into the microphone (they move around) a compressor will help. So would nailing him to the wall, but a compressor is much more humane.

You can also use compressors as a LIMITER where if a signal gets to loud the compressor/limiter will kick in and reduce the volume so it doesn't distort. In a digital studio limiters are especially useful. Recording Drums would be rather risky without a limiter on the microphones (limiter = a compressor setup in limit mode). Drums have a lot of spikes (transient sounds) that can go beyond the digital brick wall. When this happens you pretty much lost the recording.

Ducking a Signal

Doing voice-overs for radio commercials you will probably run into this. For instance if you want the background music to be reduced while the announcer is speaking then ducking is a way to do this. You can also apply ducking while recording a vocalist that is having trouble hearing themselves. The music will be reduced while the singer is singing and then get louder when he stops singing.

In essence ducking is the dynamic control of sound 'A' (Music) using the signal from sound 'B' (the voice). Most compressors have this option in the form of a 'Side Chain' input. Plug the output of voice into the 'Side Chain' input and connect the music to the input of the compressor. Adjust as required (see below):

A 'De-Esser' is a frequency dependent compressor. You may be saying "What?". A de-esser will reduce the volume of the 'Sss' frequencies while letting the rest of the voice pass through un-changed. This is very useful for voice overs and vocals where the Sssss' can be noticed.

How to Use a compressor:

You will need:

• A compressor.
• An audio source to be compressed (eg. mic, guitar, or output from a sound card, etc).

Plug the audio source in to the compressors input jacks.

Adjust the levels so you're not overdriving the compressor.

Set the threshold level. This is the point at which the compressor will kick in and take effect. Signals below this level will not be affected. Signal levels above the threshold will be reduced according to the compression ratio.

Set the compression ratio. Ratios of 5:1 or less will produce fairly smooth compression; ratios of 10:1 or more will produce more severe cutting off. For Voice a 2:1 or 3:1 should be ok.

Set the attack time. This is the delay between detection of a signal above the threshold, and the commencement of compression (ie. the time it takes to "attack" the signal).

Set the decay time. This is the time taken to release the signal from compression.

Adjust any other settings on the compressor. If you don't know what they are, try to put them on automatic, or disable them.

Example:
Set the compressor to a threshold of 0db, and a compression ratio of 3:1. In this case, all signals below 0db will be unaffected, and all signals above 0db will be reduced by 3db to 1 (ie. for every 1db input over 0db, 1/3db will be output).

Compression is one of the staples of virtually every recording studio, from basement demo stations to world-class super-studios. But just what do you do with it? Here are five ways of using compression to squeeze the most out of your music:

1) Tame the wild vocalist. By adding a bit of compression on vocals, you can both make up for a singer's tendency to move close to or away from the mic, and to temper the occasional peaks that might otherwise overload your recording medium. This is especially important for digital recordings - digital clipping will irreparably ruin a take in an instant!

2) Add sustain. Guitarists know this one well - by adding compression with a long release time, you effectively lengthen the tail of the signal. On attack, the peak is squashed, and as the signal decays, the compressor opens up, allowing more of the signal through. Don't limit yourself to guitars, though. Play around and see what works.

3) Add punch and clarity to the bass. By reigning in the bass signal, a compressor helps give the sound definition and power. When you really want your bass line to stand out, a healthy dose of compression may be just the ticket!

4) Special effects. One of the hazards of compression is that the lowering and raising of volumes can lend a breathing or pumping sound to the compressed track. However, if you like to live on the cutting edge of composition, this can be used to good effect (pardon the pun!). Get a good rhythmic groove going that really works the compressor, and experiment with the results.

5) Compress the entire mix. By adding high-ratio compression (or limiting) as you mix down to stereo, you increase the apparent overall level of your tune, and give it maximum punch. But beware - too much of this can completely suck the dynamics out of your music, leaving it loud but lifeless!

So what do you need to get started? There are dozens of solutions, but here are some of my favorites from across the price range:

For high-end quality, the Avalon VT-747SP is the way to go. At $2,199.99, it's not inexpensive, but its outstanding tube/discrete design, attention to detail, and painstaking construction all add up to one amazing piece of gear.

Supporting the mid-range price zone, the Joemeek SC2.2 and dbx 1066 provide pro-studio grade compression using discrete technology to keep the costs down. Both are recognized leaders in the world of studio compression.

For the budget-minded, you can't go wrong with the PreSonus Blue Max. At under $150, you get a half-rack space mono compressor that lets you go fully automatic with built-in presets ideal for those new to compression. For the already-initiated, full manual mode lets you work compression magic to your heart's content!

Whatever level you choose, these sonic tools will always have a place in your rack, as each compressor adds its own unique signature to the sound. Try 'em and see!