Ocean's Edge Music

Well, as promised in my last tirade about the use of compressors in mixing live audio, I will attempt and describe the most common set of adjustments available on your average audio compressor/limiter. Though some compressor designers throw in a few more controls to make it even more interesting (i.e. “knee” adjustment, side-chain filters, etc.), the following are the most common.

Threshold – This control determines the signal threshold (or ceiling level) at which the compressor begins to affect the audio passing through it. If the signal coming through the compressor is below this “threshold or ceiling” setting, the compressor has no effect on the audio passing through it. It is only when a signal reaches the level of the Threshold setting (set by the user) that the compressor automatically begins to affect the audio.

To determine to proper Threshold level, it is necessary to watch for the Threshold lights to begin lighting up during the course of an average amount of source material to determine the peaks in the audio signal that exceed the established Threshold settings. Based on the observed incoming signal, you can adjust the threshold control so the compressor begins to compress at the desired peaks in the dynamics. As the Threshold is adjusted, the compression level meter will start to show when the source material is in compression (and how many decibels of compression are being induced).

Attack Time – This control determines how quickly the compressor responds to transients or peaks in the audio levels that exceed the threshold setting. Set to fast attack times, the compressor quickly responds to audio that has triggered the Threshold. Slower attack time settings allow the compressor to respond at a gentler, slower rate. The attack rate is usually stated in units of milliseconds.

Slow attack time settings tend to exaggerate the sound source transients (giving the sound more punch, edge, presence). While fast attack times tend to soften or dull the incoming transients (while reducing overall dynamic range). There are an infinite number of possible attack settings, and much experimentation is needed to determine the best attack settings for the job at hand.

Release Time -This control is the inverse of Attack Time, as it determines how quickly the compressor recovers (or releases control) from a signal that has exceeded the Threshold setting. Fast release times tend to make a compressor pump or breathe, due to the rapid return to an uncompressed state. Slower release times are less noticeable (and more desirable) in most cases.

Slow release times can give a signal a sustained presence (which is a great way to lengthen the sustain of acoustic, electric, and bass guitars. Fast release times are sometimes desirable when trying to hard-limit very short-duration sounds that are peaky and pesky (see hard-limiting below).

Compression Ratio – This one is a little bit more involved, and a little harder to understand (but we’ll give it a shot). This control determines to what degree the audio is reduced when the incoming signal exceeds the threshold setting, and it is expressed as a Ratio (i.e. 1:1, 2:1, 3:1, 4:1, etc.). When a ratio of 10:1 or greater is chosen, the effect is considered to be “Limiting” instead of compression, as this high of a ratio is putting a hard-cap on the dynamics (essentially, starting to square-off the waveform).

Limiting is used when the desired effect is to not allow the dynamics to go beyond a fixed level. Compression, on the other hand, still allows for dynamics to pass, but in a less restrictive way than hard-limiting. If the compressor ratio adjustment is set at a 2:1 ratio, this means that if the input gain rises two decibels above the established threshold, the compressor will reduce the incoming level by only 1 decibel. If the ratio is set to 4:1 ratio, this means that if the incoming level increases by 4 decibels the compressor will only allow for a 1 decibel increase in output. Once the ratio gets as high as 10:1 the incoming level must rise 10 decibels in order for the compressor to increase the output by 1 decibel (again, any ratio above 10:1 is considered hard-limiting).

For most live applications, I find that the more conservative ratios are most desirable for most applications (2:1, 3:1, 4:1). Also, I think the “less is more” model is usually best when it comes to the amount of compression used (with the exception of extremely-dynamic instruments such as drums, lead guitars, and the occasional overly-dynamic singer).

Also, more extreme compressor settings can be applied when the desired result is more of an obvious effect, rather than just a subtle, automatic gain riding. Remember, the higher the compression ratio, the more severe the affect on the incoming audio, and the greater chance of noticeable artifacts (i.e. more squashed sounding, duller, etc.).

Make-up Gain — This control is used to bring the overall level of the compressed signal up (without affecting the dynamics). It’s just a simple and quick way to make-up for the loss of overall level due to the affects of compression.

Happy mixing!

-Michael Grosso, Technical Director/Audio Engineer – Calvary Chapel Fort Lauderdale

Tags: audio, condenser, dynamic, engineer, Microphone, mix, monitor, sound