Multiband compressor: a guide to understanding multiband compression

An often overlooked, but never the less powerful tool, is the multiband compressor. In this article, we’ll learn what a multiband compressor is and how it might help you achieve greater control over dynamic range.

What is a multiband compressor?

A multiband compressor allows you to apply different compression settings to the various frequencies of the audio passing through it. The compressor has multiple bands, each with its own threshold, ratio, attack and release parameters. Many multiband compressors also offer a knee setting and a detection mode for each band.

A multiband compressor usually has 3 to 5 frequency bands. Typical cross over settings may look like this:

Low end: 20 Hz – 100 Hz

Low-mids: 100 Hz – 3 kHz

High-mids: 3 kHz – 10 kHz

High end: 10 kHz – 20 kHz

Many multiband compressors will allow you to define the cross over points of the frequency bands.

The problem with broad band compressors:

Unlike a multiband compressor, a broad band compressor will respond to the occurrence of a breach of its threshold originating from any part of the signal’s frequency spectrum. It will respond by applying gain reduction to the entire frequency spectrum of that signal. Sometimes, this can be problematic. That’s because compressors are generally most responsive to low frequency sounds. As such, lower notes on a bass guitar, lower notes on piano or lower notes performed by a singer, for example, will cause the compressor to apply greater amounts of compression. But the compression is applied to the whole frequency spectrum. This can result in the compressor having an inconsistent response across the frequency spectrum. This can also cause a dulling effect as the higher frequencies of a signal are compressed as a result of the low frequencies breaching the compressor’s threshold.

The solution is a multiband compressor:

With a multiband compressor, a threshold breach will only result in the application of compression to the frequency band in which the breach occurres. This means that you can tailor the compressor to react differently to different frequency bands. This helps to create a more uniform response across the frequency spectrum. This also resolves the dulling effect as compression triggered by low end frequencies is contained in the low end and leaves the higher frequencies unaffected.

In addition to being useful when compressing instruments which produce a broad range of frequencies, this can also be particularly useful when applying compression to a signal which contains multiple instruments, such as the overhead mic recording of a drum kit for example. On such a recording, you would likely have cymbals, kick, snare and toms. In this instance, the kick drum would likely trigger the most compression. With abroad band compressor, the compression would affect all of the frequencies. This could result in the compression affecting the snare and cymbals whenever the kick drum was played. With multiband compression however, you are able to isolate the compression that the kick drum triggers purely to the low end frequency band.

Due to the high level of control that multiband compressors offer over the way that compression is applied to different frequency bands, multiband compression often plays an important role in mastering as well, in which the entire mix is passed through the multiband compressor.

In what ways have you used multiband compression to improve your tracks? Leave your suggestions in the comment section below.