January 24, 2013, by Jason Corey
From Audio Production and Critical Listening: Technical Ear Training by Jason Corey, published by Focal Press.
In addition to being a utilitarian device for managing the dynamic range of recording media, dynamics processing has become a tool for altering the color and timbre of recorded sound.
When applied to a full mix, compression and limiting can help the elements of a mix coalesce. The compressed musical parts will have what is known in auditory perception as common fate because their amplitude changes share some similarity.
When two or more elements (e.g., instruments) in a mix have synchronously changing amplitudes, the auditory system will tend to fuse these elements together perceptually. The result is that dynamics processing can help blend elements of a mix together.
Here we will move beyond compression as a basic tool for maintaining consistent signal levels to compression as a tool to sculpt the timbre of a track.
Effect of Attack Time
With a compressor set to a long attack time — in the 100-millisecond range or greater — with a low threshold and high ratio we can hear the sound plunge down in level when the input signal goes above the threshold.
The audible effect of the sound being brought down at this rate is what is known as a pumping sound and can be most audible on sounds with a strong pulse where the signal clearly rises above the threshold and then drops below it, such as those produced by drums, other percussion instruments, and sometimes an upright acoustic bass.
If any lower-level sounds or background noise is present with the main sound being compressed, a modulated background sound will be heard. Sounds that are more constant in level such as distorted electric guitar will not exhibit such an audible pumping effect.
Effect of Release Time
Another related effect is present if a compressor is set to have a long release time, in the 100-millisecond range or greater. Listening again with a low threshold and high ratio, be attentive for the sound to come back up in level after a strong pulse.
The audible effect of the sound being brought back up in level after significant gain reduction is called breathing because it can sound like someone taking a breath.
As with the pumping effect, you may notice the effect most prominently on background sounds, hiss, or higher overtones that ring after a strong pulse.
Although compression tends to be explained as a process that it reduces the dynamic range of an audio signal, there are ways to use a compressor that can accentuate the difference between transient peak levels and any sustained resonance that may follow.
In essence, what can be achieved with compression can be similar to dynamic range expansion because peaks or strong pulses can be highlighted relative to quieter sounds that immediately follow them.
It may seem completely counterintuitive to try to think of compressors performing dynamic range expansion, but in the following section we will work through what happens when experimenting with various attack times.
Compression and Drums
A recording with a strong pulse, such as drums or percussion, with a regularly repeating transient will trigger gain reduction in a compressor and can serve as a useful type of sound to highlight the effect of a dynamics processing.
By processing a stereo mix of a full drum kit through a compressor at a fairly high ratio of 6:1, attack and release times can be adjusted to hear their effect on the sound of the drums.
On a typical recording of a snare drum that has not been compressed, there is a natural attack or onset, perhaps some sustain, and then a decay.
The compressor can influence all of these properties depending on how the parameters are set. The attack time has the greatest influence on the onset of the drum sound, allowing an engineer to reshape this particular feature of the sound.
Increasing the attack time from a very short time to a much longer time, the initial onset of each drum hit is audibly affected.
A very short attack time can remove the sense of a sharp onset. By increasing the attack time, the onset sound begins to gain prominence and may in fact be accentuated slightly when compared to the uncompressed version.
Let us explore the sonic effect on a drum kit when listening through a compressor with a low threshold, high ratio, and very short attack time (e.g., down to 0 milliseconds).
With such a short attack time, transients are immediately brought down in level, nearly at the rate at which the input level rises for each transient.
Where the rate of gain reduction nearly matches the rate at which a transient signal rises in level, a signal’s transient nature is significantly reduced.
So with very short attack times, transients are lost because the gain reduction is bringing a signal’s level down at nearly the same rate that the signal was originally rising up during a transient.
As a result, the initial attack of a transient signal is reduced to the level of the sustained or resonant part of the amplitude envelope. Very short attack times can be useful in some instances such as on limiters that are being used to avoid clipping.
For shaping drum and percussion sounds, short attack times are quite destructive and tend to take the life out of the original sounds.
Lengthening the attack time to just a few milliseconds, a clicking sound emerges at the onset of a transient. The click is produced by a few milliseconds of the original audio passing through as gain reduction occurs, and the timbre of the click is directly dependent on the length of the attack time. The abrupt gain reduction reshapes the attack of a drum hit.
By increasing the attack time further, the onset sound begins to gain prominence relative to sustain and decay portions of the sound, and it may be more accentuated than without processing.
When compressing low-frequency drums such as bass drum or kick drum, an increase in the attack time will increase the presence of low-frequency harmonics.
Because low frequencies have longer periods, a longer attack time will allow more cycles of a low-frequency sound to occur before gain reduction and therefore low-frequency content to be more audible on each rhythmic bass pulse.
The release time affects mostly the decay of the sound. The decay portion of the sound is that which becomes quieter after the loud onset.
If the release time is set long, then the compressor gain does not quickly return to unity after the signal level has fallen below the threshold (which happens during the decay).
With a long release time, the natural decay of the drum sound becomes significantly reduced.
When compressing a mix of an entire drum kit, it becomes more apparent that the attack time is affecting the spectral balance of the total sound.
Increasing the attack time from a very short value to something longer, increases the low-frequency energy coming from the bass drum.
As attack time is lengthened from near zero to several tens or hundreds of milliseconds, the spectral effect is similar to adding a low-shelf filter to the mix and increasing the low-frequency energy.
Compression and Vocals
Because vocal performances tend to have a wide dynamic range, engineers often find that some sort of dynamic range control helps them reach their artistic goals for a given recording.
Compression can be very useful in reducing the dynamic range and de-essing a vocal track.
Unfortunately, compression does not always work as transparently as desired, and artifacts from the automated gain control of a compressor sometimes come through.
A couple of simple tips can help reduce dynamic range without adding too many of the side effects that can detract from a performance:
Use low ratios. The lower the ratio, the less gain reduction that will be applied. Ratios of 2:1 are a good place to start.
Use more than one compressor in series. By chaining two or three compressors in series on a vocal, each set to a low ratio, each compressor can provide some gain reduction and the effect is more transparent than using a single compressor to do all of the gain reduction.
To help identify when compression is applied too aggressively, listen for changes in timbre while watching the gain reduction meter on our compressor.
If there is any change in timbre that is synchronized with gain reduction, the solution may be to lower the ratio or raise the threshold or both. Sometimes a track may sound slightly darker during extreme gain reduction, and it can be easier to identify synchronous changes when watching the gain reduction meter of a compressor.
A slight popping sound at the start of a word or phrase may indicate that the attack time is too slow.
Generally a very long attack time is not effective on a vocal since it has the effect of accentuating the attack of a vocal and can be distracting.
Compression of a vocal usually brings out lower-level detail in a vocal performance such as breaths and “ s ” sounds. A de-esser, which can reduce the “ s ” sound, is simply a compressor that has a high-pass filtered (around 5 kHz) version of the vocal as its side chain or key input. De-essers tend to work most effectively with very fast attack and release times.
To purchase Audio Production and Critical Listening: Technical Ear Training click on over to the Focal Press website.
Jason Corey is an assistant professor of audio engineering and performing arts technology at the University of Michigan School of Music, Theatre & Dance and an active member of the Audio Engineering Society.