Things you should know and avoid
What is a Multiband Compressor. A multiband compressor is a specific type of audio compressor device by which different compression settings can be applied for different frequency ranges, each one delimited by filter crossover frequencies. There are many applications for such a device but the most common one is to be able to manipulate audio dynamics in target frequency areas. THis would typically be done by inserting a multiband compressor in the master buss. If we were to apply a regular compressor, this would act across the entire frequency spectrum, whereas the multiband compressor will act only at the specific frequency range, which means that the rest of the frequency spectrum would not be affected.
How do we use a Multiband Compressor? The application for a multiband compressor can be of surgical nature, in cases when there is no way to remix and fix certain audio issues, such as for example a problematic frequency range by a given instrument not addressed in the mix. If we were to apply equalization for that specific range, then the output would be affected to all signal levels, and sometimes we only want to control dynamics such as peaks.
The second type of application is to use the multiband compressor as an enhancement tool in the mix. Using the right settings a multiband compressor can significantly add to the mix, providing more punch, and sound definition by managing the signal transients. When used correctly the difference will be subtle but will provide more of a cohesive sound, as something that it is perceived as considered “to be a record sound”. The multiband compressor will also glue the mix together and ensure that everything is under control across the different frequency ranges.
The effect above is typically achieved by defining up to 4 frequency ranges (Low, Mid Low, Mid High and High) to cover the full range and using slow transitions when defining the crossover settings and very low compression ratios for all frequency bands , typically 1.8:1 to 2:1. The threshold will be pulled down so that almost everysignal is compressed very slightly. Gain Reduction should be pretty much the same for all bands. THis depends on the material however in my experience we should be getting no more than 1.5 to 2dB average of gain reduction for each band.
It is extremely important that when setting the compression values for each band, ensure that the band has the right makeup so that when listening that specific band in by-pass mode or with the compressor on, the perceived loudness should be the same. Otherwise there would be a chance that the multiband compressor will be changing the overall frequency spectrum and characteristic of the mix, which is something we don’t want if we intend to use the compressor transparently.
When it comes to compressor settings in a master buss, I typically use a relatively high Attack so that the transients are not affected by the compressor. A good starting point for a mix is between 20 to 30 msec. The Release time must be optimized so that we are not getting a pumping effect as the release time has been set too low or to avoid killing the punch of the mix by setting it too high. I usually run at 50 msec to 100 msec for all practical purposes. This time can be adjusted for any specific band. Low frequency signals should use slightly higher response time than higher frequencies bands, due to the longer waveform nature. At last once you have completed setting up each band, you should listen to the mix with the compressor on and in bypass mode. This check is your best friend in order to confirm that the goal has been achieved.
What we should avoid: The quickest way to ruin your mix is by wrongly using a multiband compressor. The key is going for a subtle effect of the compressor and this will go a long way. Whether it is better to use a single compressor or a multiband compressor is a matter of taste and the intent of the mix. Many times I use both in series, adding the main compressor following the multiband one. The operation of both compressors devices is very subtle so to achieve the overall compression goal in the mix by combining the effect of both. You should experiment to see what actually suits you the best in your mixes.