Understanding and using multi-band compressors/limiters

Remember how I said a de-esser was a frequency-specific compressor working only on the sibilant sounds within a track or mix. A multiband compressor follows the same idea, but works across the entire frequency range. By splitting up the compression into multiple parts, or bands, an engineer can focus dynamic control within a specified frequency range, leaving other frequencies uncompressed. Because of these multiple bands of gain control, multiband compressors are especially handy in situations where only certain parts of the signal need dynamics control, like cleaning up the low-end resonance of a signal while leaving the high-frequency content uncompressed, or less compressed, or super- compressing the top end of a vocal to achieve that pop polish without all the harshness that would come from using EQ only.

Mastering engineers will sometime use multiband compression to tighten up elements of a mix that didn't receive enough compression during the mixing stage, like a bass guitar that was left uncompressed, creating unpredictable low-frequency response from note to note. While they might look scary at first glance, a multiband compressor is simply multiple single-band compressors, each working on their own dedicated frequency range. One compressor might be working on the bass notes of a signal, while another works on the mids and high-frequency content. Each band will have its own threshold, ratio, attack, release, and makeup gain, and in most multiband processors, these controls can be linked together for ease of use.

Instead of a single threshold triggering compression, each threshold of a multiband compressor looks for amplitude within its specified band. When the threshold of any band is breached, the compressor will start to attenuate the signal, but only within its specified range of frequencies. Just like a de-esser, the detection in the compression is frequency-specific, giving us total control over when and how the compression is applied. In this sense, you can think of a multiband compressor as a dynamic EQ of sorts. It attenuates a specific frequency band just like an EQ would, but only when certain dynamic conditions are met.

Let's listen to the same example we worked on with our de-esser, only now, using a multiband compressor instead. In this example, only the offending frequency band containing the sibilance has been allowed to compress, while the other bands are made inactive. (Male speaker: There's a girl named Sally who sells seashells.) As you can see, and hopefully hear, multiband compressors and limiters are extremely powerful tools, giving the engineer total control over both the dynamic and frequency response of a signal within the same processor.

But all this power comes with added responsibility. Multiband processors are among the most misused and abused tools in the mixing world. I can't stress enough how degrading multiband compression can be when used incorrectly across an entire mix. While you'll find multiband limiters built into nearly every all-in-one mastering tool, pro mastering engineers tend to use multiband compression only as a last resort and only when going back to the individual tracks of a mix is not possible. Be careful not to flatten your mix out.

I personally find multiband tools most useful when processing individual tracks or small groups, rather than an entire mix. When I do use multiband compression on my entire mix, I often disable most of my bands to focus my work on a specific range of frequencies, like the top end or sub bass. Think of it this way: multiband compression is just another tool in your toolbox of creative ideas, not an automatic solution for mixing or mastering.