Compression

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Dynamic range compression is one of the most common effects used during mixing, yet it is one of the most difficult for beginners to understand. In a nutshell, compressor makes loud parts softer, and softer parts louder. However, it is often used to shape the wave in more complex ways. For instance it can be used to increase the transient of the wave while increasing the length of decay to create a snare that really snaps and has a long ring out.

1 Compressor Settings
2 Compression vs. Limiting
3 Soft Knee vs. Hard Knee
4 What Is the "Right" Amount of Compression?
5 Potential Problems
6 General instrument specific guidelines
7 Different types of compressors
8 Side chained compression and radio
9 Parallel Compression
10 References
11 External Links

Compressor Settings

Most compressors have the following controls, though they may be labeled slightly differently:

Threshold. This is the level at which gain reduction begins to happen. Usually measured in dB. Lower threshold values increase the amount of compression, as less signal is required for gain reduction to occur. A threshold of -20 dB would initiate compression any time the signal went above it.

Ratio. This is the ratio of change between input level and output level once the threshold is reached. For example, a ratio of 4:1 means that an input level increase of 4 dB would only result in an output level increase of 1 dB. Example: if you take the -20 dB threshold and take your input signal that reached -12 dB with no compressoin (8 dB above the threshold), if you compress it at 4:1 (8 / 4 = 2) the signal would only reach -18 dB (-20 + 2 = -18).

Attack. The amount of time it takes for gain reduction to take place once the threshold is reached. The ratio is not applied instantaneously but over a period of time (the attack time) Usually measured in microseconds or milliseconds. Use longer attack times when you want more of the transient information to pass through without being reduced (for example, allowing the initial attack of a snare drum).

Release. The amount of time it takes for gain to return to normal when the signal drops below the threshold. Usually measured in microseconds or milliseconds.

Makeup Gain. Brings the level of the whole signal back up to a decent level after it has been reduced by the compressor. This also has the effect making quiet parts (that are not being compressed) louder.

Of course, some compressors have more or fewer controls. Indeed, the classic LA-2A has only an input gain (essentially controls the threshold), peak reduction, and a nob for makeup gain, and a switch for choosing between "Compress" and "Limit". Attack time, release time, and ratio are fixed.

Compression vs. Limiting

Compression and limiting are more or less the same thing. Typically, the term limiter is used to indicate a device that is compressing with a ratio of 10:1 or higher. The word limit is used because it essentially limits the signals ability in any substantial way to go above the threshold.

A Brick Wall Limiter is a special type of limiter with an extremely high ratio (50:1 or greater), usually used with a high threshold that only is engaged right before the signal would distort.

Soft Knee vs. Hard Knee

Knee refers to the shape of the curve at or around the threshold, a soft knee will usually engage within 10 dB of the threshold and begins to apply compresssion at a low ratio increasing gradually until the set ratio when the signal hits the threshold point. Image is the blow up of the square in the previous one. Remember that the knee is different from attack, but that they are related. A soft knee will reduce the percieved change from uncompressed to compressed signal especially at high ratios but a hard knee can be used for effect.

Hard Knee means the curve is sharp, providing near-instantaneous compression above the threshold.

Soft Knee means the curve is rounded, and the onset of the compression around given threshold is more gradual resulting in a more subtle and less percievable sound.

What Is the "Right" Amount of Compression?

Compression is highly dependent on source material, and as such, there is no preset amount of compression that will work for any given material. Some compressors do have presets for certain types of audio, and these can be a good starting point for the inexperienced, but remember that you will still have to adjust the input/threshold for it to work properly because every recording is done with different headroom and dynamics. Generally speaking you will get better results by learning to use compression, and understanding how the controls affect the audio signal. Experiment and listen!

Potential Problems

What happened to my low Noise Floor? When you reduce the peaks of a signal and then add the same relative amount of makeup gain you are raising not only the instrument by x amount of dB, but raising the Noise Floor as well. While usually not an issue in quality recordings, it can become apparent when compressing quiet acoustic recordings, or recording with a low Signal to Noise ratio. That air vent you forgot to turn off in your live room could go from being unnoticeable to being an annoying hum if you compress and raise the makeup gain too much.

What is that pumping sound? The pumping sound you might hear occurs when the compressor initiates say from the hit of a snare but then has too fast a release and the rest of the mix comes up to fast after the hit. To fix this have a slower release, lower ratio, slower attack or higher threshold. They all have a different effect so listen and decide what sounds best and gives you what you're trying to achieve. If you train your ear pretty much all radio signals have a certain "acceptable" amount of pumping.

General instrument specific guidelines

Vocals: fast attack and release, ratio depends on the recording and vocal style. usually a soft knee.
Guitars: fast attack usually somewhat fast release.
Bass: bit slower attack and slower release, so you leave or accentuate the transient of the hit. Of course if you want to smooth out the bass and bury your bass in the track by getting rid of the attack, have a fast attack on the compressor.
Master Fader: Super fast attack and release, with usually a high threshold.
Drums: Compressing drums is an art to say the least. The different amounts and styles of compression can completely and utterly change the way the drums sound. Whether you're going for those HUGE snare cracks or a more subtle tightness, knowing how to compress can save you from a weak sounding mix. The attack and how big you make the transient peak is the most identifiable part of a hit. On the compressor if you have too fast an attack the transient peak will be cut and your drum won't hit hard, but if you have a slower attack that initiates the compressor right after the transient peak it'll accentuate the hit. The compression after initiating and bringing down part of the sustain, the signal falls below the threshold and slowly releases bringing up the decay making the drum last longer and sound larger and more full. This is really a very general overview, you must experiment and listen to find the desired sound.

BOTTOM LINE: Listen! and go extreme before backing down to a good sound, it's the only way to really hear it. don't piddle around .5 dB change of threshold here, go extreme and go way lower or way higher, or crank or lower the ratio and listen to the difference.

TIP: set the compresser to it's fastest release and slowest attack. set the ratio around 4:1 and set the threshold to about 7-10 dB reduction. slowly roll the attack back until it starts to "muffle" the signal. adjust it so that it's just short of that. Then start making the release longer until it starts to decrease the signals perceived volume, set it just short of that. now adjust the threshold and makeup gain to taste.

Different types of compressors

Optical Compression
Tube Compression
VCA Compression
FET Compression
Multi-band Compression
Digital Compression

Side chained compression and radio

If you listen to the radio and hear the music fade down when the DJ begins to talk it usually is not a manual maneuver, but a sidechained compressor being engaged by the voice. If you having a slow release on the compressor it allows a slow fadeup when the voice stops. If you setup a compressor on the audio track and key it to the DJ vocal, it will engage and lower the music when the DJ talks. This can also be used for many other situations.

Parallel Compression

This technique can often be found on drum tracks, but can be employed on any instrument. For drums you would bus the drums or specific drum tracks to a new aux track (in your DAW or on a console) and then compress the aux track. Basically you have two identical tracks running in "parallel" to each other. You would then mix them together till you get your desired sound. Another name for Parallel Compression is "New York Compression".