Standard compressors used in music production work on what is known as the full-band principle, where the dynamics processing is triggered by a single gain-control element. When the audio signal reaches the threshold, the entire signal level is reduced. Though this method of dynamics processing has it's place, some problems may result when using full-band compression. Within the context of EDM production, a loud kick drum will trigger the gain-reduction and subsequently squash the levels of the kick as well as everything else along with it. That said, when a full-band compressor is acting on a kick drum track, this sort of thing is perfectly fine. However, if full-band compression is used on an entire drum kit, the high-frequency elements such as hi-hats and snares will be compressed along with the kick drum, which will leave your drum kit flat and lifeless.
Multiband compressors are typically available with anywhere from 3 to 5 frequency bands. Though there are often default crossover points for each band, most multiband compressors allow for adjustments to these bands. For example, the first band might cover only bass frequencies from 20hz-100hz, the next band could then cover he mids from 100hz to 2khz, and the last band the highs, from 2khz up to 20khz. A great example of a typical multiband compressor can be found in Logic Pro's 4-band Multipressor. This compressor allows for separate threshold, ratio, attack and release settings for each of the four bands available.
Stereo Buss vs Instrument Tracks
Though multiband compression is most commonly used on the stereo buss, it can also be used on individual instrument tracks as well. Multiband compression is particularly useful on multi-timbral instrument tracks, such as drum groups, in order to tame specific elements of a drum kit without just squashing the entire signal. For example, within a drum track, we could use multiband compression to isolate the kick drum, by setting the band and threshold to allow for gain-reduction to only react to the kick instead of the snare, which would likely trigger a single-band compressor as the snare signal usually sits above the other percussion instruments when used in a style typically found in electronic dance music.
When experimenting with multiband compression for the first time, it is important to remember that there are some unintended consequences inherent in the process. For example, drastically different attack, release and threshold settings, in addition to poorly placed crossover points, can cause serious phase alignment issues in a mix. In order to avoid problems, it is recommended that you use spectral analysis, like those featured on many eq plugins, in order to determine your crossover points. When doing this, focus on prioritizing the elements of a mix which are most crucial to the overall sound. For example, in a techno track, be sure to isolate the kick and bass together in order to assure that the low-end groove remains intact.
Keep in mind that if you are just starting out as a producer and cannot afford high-end hardware processing units, that great results can be achieved via software plugins. In fact, when used properly, software plugins such as multiband compressors can achieve fantastic results, very similar to what would be achieved with expensive hardware. That said, don't be intimidated by seemingly complex layout of most multiband units. Just remember to take it slow, and to treat each band as if it is a separate compressor. Get your crossover points set first, then gradually tweak each band to taste. Before you know it, multiband compression will become a useful tool in crafting a well-balanced mix.
Most dynamics processors, or compressors, act on the full-band principle, affecting the entire audible frequency range of an audio signal, from 20hz up to 20khz. Multiband compressors, however, allow for increased control over the dynamics processing of an audio signal by dividing it into separate bands. By doing this, we can then set a different attack, release, compression ratio, and threshold for each band, and ultimately have much greater control over specific audio frequencies within our compressed signal.