Velcro City Records

The Art and Science of Compression

In simple terms, a compressor is an automatic volume control. Loud sounds over a certain threshold are reduced in level while quiet sounds remain untreated. This is known as downward compression, while the less common upward compression involves making sounds below the threshold louder while the louder passages remain unchanged. In this way it reduces the dynamic range of an audio signal. This may be done for aesthetic reasons, to deal with technical limitations of audio equipment, or to improve audibility of audio. An audio engineer might use a compressor subtly in order to reduce the dynamic range of source, or they might use a compressor in order to deliberately change the character of an instrument being processed.

Compression reduces the level of the loud sounds, but not the quiet sounds; thus, the level can be raised to a point where the quiet sounds are more audible without the loud sounds being too loud. Contrast this with the complementary process of an expander, which performs almost the exact opposite function of a compressor, i.e., an expander increases the dynamic range of the audio signal.

A compressor reduces the gain of an audio signal if its amplitude exceeds a certain threshold. The amount of gain reduction is determined by a ratio. For example, with a ratio of 4:1, when the input level is 4 db over the threshold, the output signal level will be 1 db over the threshold. The gain has been reduced by 3 db. When the input level is 8 db above the threshold, the output level will be 2 db; a 6 db gain reduction.

A more specific example for a 4:1 ratio:

Threshold = −10 dB

Input = −6 dB (4 dB above the threshold)

Output = −9 dB (1 dB above the threshold)

The signal entering a compressor is split, with one copy sent to a variable-gain amplifier and the other to a path called the side-chain, where a control circuit calculates the required amount of gain reduction. The control-circuit outputs the requested gain-reduction amount to the amplifier. This type of design is known as feed-forward type and is used today on most compressors.

Engineers wishing to achieve dynamic range reduction with few obvious effects might choose a relatively high threshold and low compression ratio so that the source material is being compressed very slightly most of the time. To deliberately soften the attack of a snare drum, they might choose a fast attack time and a moderately fast release time combined with a higher threshold. To accentuate the attack of the snare, they might choose a slower attack time to avoid affecting the initial transient. It is easier to successfully apply these controls if the user has a basic knowledge of musical instrument acoustics.

It should be noted that compression can also be used to lift the soft passages of a selection, pulling the sound toward a compressed “middle”. Hence, loud sounds are pulled back and soft passages are boosted.

Compressor features

Threshold: Threshold is the level above which the signal is reduced. It is commonly set in db, where a lower threshold (e.g. -60 db) means a larger portion of the signal will be treated (compared to a higher threshold of -5 db).

Ratio: The ratio determines the input/output ratio for signals above the threshold. For example, a 4:1 ratio means that a signal overshooting the threshold by 4 db will leave the compressor 1 db above the threshold. The highest ratio of ∞:1 is commonly achieved using a ratio of 60:1, and effectively denotes that any signal above the threshold will be brought down to the threshold level (unless some attack is in force).

Attack and release: A compressor might provide a degree of control over how quickly it acts. The ‘attack phase’ is the period when the compressor is increasing gain reduction to reach the level that is determined by the ratio. The ‘release phase’ is the period when the compressor is decreasing gain reduction to the level determined by the ratio, or, to zero, once the level has fallen below the threshold. The length of each period is determined by the rate of change and the required change gain reduction. For more intuitive operation, a compressor’s attack and release controls are labeled as a unit of time (often milliseconds). This is the amount of time it will take for the gain to change a set amount of db, decided by the manufacturer, very often 10 db. For example, if the compressor’s time constants are referenced to 10 db, and the attack time is set to 1 ms, it will take 1 ms for the gain reduction to rise from 0 db to 10 db, and 2 ms to rise from 0 db

Soft and hard knees: Another control a compressor might offer is hard/soft knee. This controls whether the bend in the response curve is a sharp angle or has a rounded edge. A soft knee slowly increases the compression ratio as the level increases and eventually reaches the compression ratio set by the user. A soft knee reduces the audible change from uncompressed to compressed, especially for higher ratios where the changeover is more noticeable.

Make up gain: Because the compressor is reducing the gain (or level) of the signal, the ability to add a fixed amount of make-up gain at the output is provided so that an optimum level can be used.

Limiting

Compression and limiting are no different in process, just in degree and in the perceived effect. A limiter is a compressor with a higher ratio, and generally a fast attack time. Most engineers consider a ratio of 10:1 or more as limiting, although there are no set rules. Engineers sometimes refer to soft and hard limiting which are differences of degree. The “harder” a limiter, the higher its ratio and the faster its attack and release times.

Brick wall limiting has a very high ratio and a very fast attack time. Ideally, this ensures that an audio signal never exceeds the amplitude of the threshold. Ratios of 20:1 all the way up to ∞:1 are considered to be ‘brick wall’. The sonic results of more than momentary and infrequent hard/brick-wall limiting are usually characterized as harsh and unpleasant; thus it is more appropriate as a safety device in live sound and broadcast applications than as a sound-sculpting tool.

Side-chaining

Side-chaining uses the signal level of another input or an equalized version of the original input to control the compression level of the original signal. For sidechains that key off of external inputs, when the external signal is stronger, the compressor acts more strongly to reduce output gain. The compressor reduces the volume of the sound, a process called ducking. The sidechain of a compressor that has EQ controls can be used to reduce the volume of signals that have a strong spectral content within the frequency range of interest. Such a compressor can be used as a de-esser, reducing the level of annoying vocal sibilance in the range of 6-9 kHz. A frequency-specific compressor can be assembled from a standard compressor and an equalizer by feeding a 6-9 kHz-boosted copy of the original signal into the side-chain input of the compressor.

Multiband compression

Multiband compressors can act differently on different frequency bands. The advantage of multiband compression over full-bandwidth (full-band, or single-band) compression is that changing signal levels in one frequency band (such as from a sporadic low frequency kick drum) don’t cause unneeded audible gain changes or “pumping” in other frequency bands.

Multiband compressors work by first splitting the signal through some number of bandpass filters or crossover filters. The frequency ranges or crossover frequencies may be adjustable. Each split signal then passes through its own compressor and is independently adjustable for threshold, ratio, attack, and release. The signals are then recombined and an additional limiting circuit may be employed to ensure that the combined effects do not create unwanted peak levels.

Serial compression

Serial compression is a technique used in sound recording and mixing. Serial compression is achieved by using two fairly different compressors in a signal chain. One compressor will generally stabilize the dynamic range, while the other will more aggressively compress stronger peaks.

In my experience, compression is an invaluable tool which can be used to get a grip, so to speak, on wild sounds, warm up dull sounds, add a bit of punch to specific sounds in complicated mixes, and to message the peaks of both individual sounds and groups of sounds. Unfortunately compression, in my opinion, is more of a science than an art, but efficient and creative use of both compression and limiting will allow for more efficient use of the finite space we have to work in, as well as prevent wasting valuable overhead.

I definitely advise to pay close attention to where your compressor sit in the chain of effects as placing it before or after certain effects will change the results of how you’ve set it.

Hopefully everything I’ve explained thus far will give you some education, but I’d like to give you a few things to think about when setting compressors. The first thing is, the compressor placed on one particular sound should be set in reference to how that sound interacts with others. If you set a compressor with identical settings on a hand full of sounds, you’ve effective reduced the dynamic range of your production and accomplished nothing. Let’s say you have a kick, a snare, an open hat, a closed hat, and a splash type open hat. I would try setting the kick with a 10 millisecond attack and a release of 200 milliseconds (adjusted accordingly to the actual kick drum sound). The snare I would place with a faster attack time and release time. The function I use this for is to allow my kick to have more immediate presence when the two sounds trigger at the same time. As far as high hats go, your closed hat may need a bit more bite to cut through the mix. If that is the case a higher ration and fast attach time will add a bit of snap to the closed hat which will allow it to have more presence without simply being turned up. The same idea can be applied to the open hat, but since it, typically, is a longer sound, rather than having a 2 ms attack time like the closed hat might have, try 5 ms, setting the release relatively similar to the that of the closed hat. Lastly in this example, the splash is there for to create a certain effect in the loop. You may not need it to have as much bite, rather, you want the decay to have more presence. To achieve this try a faster release time, and a lower ratio.

Practical Compression

As I’ve described in my article “The Art of the Mixdown”, I set my kick drum and a constant in my mix at -7 db. The post gain of my compressor sets the level of my kick to -7 db, while I have a ‘brick wall’ limiter last in the chain of effects set with a threshold of -7 db and an out put ceiling of -7 db as well. From there, I leave it alone completely. As for the other drums in this example, after I’ve equalized them, compressed them, and set them in my stereo mix as per my tastes, I may route their individual buses to a separate insert and use a multiband compressor to get a grip on the loop as a whole. Or I may place a limiter on the channel just to prevent two different drums in the loop (i.e. the closed hat and the open hat) from increase the average peak of the loop. These types of ideas can be used for many purposes. You may find compressing your bassline with the kick drum helps even out the mix and preserve the punch of a bass heavy song (as I’ve learned with engineering dubstep). You may find the multiband compressor is a nice way to spruce up a lot of your high end sounds, such as your upper range melodies. Many simple melodies, or melodies that stay mainly in the high end frequencies can take on a lot of warmth with a properly used multiband compressor, allowing them to sound a lot more full in your over all mix.

What ever your use for these powerful tools, always remember that the compressor and limiter actually damage dynamic range. Be thoughtful in their use. In short use them only as much as needed. As always, slow down, and think the process though. Sometimes the solution to a problem area isn’t compression alone. Perhaps a little stereo placement or equalization can afford you the ability to use less compression. Remember, compression is only one of what I consider the three tools in properly mixing down your music.

John Mundt
Aka: DJ Hero

Solitude Studios
Kaleidoscope Music
Velcro City Records