This blog has been edited from its original format; some references have been changed to reflect Ozone 8.
Compression is one of the most used audio processing techniques in modern recording, mixing, and mastering. It helps improve intelligibility in vocals, tame an overly aggressive snare drum, and restrict dynamic range across a mix so that the listener does not have to keep adjusting the volume on their playback system.
Compression is one of the effects we hear most but, if done correctly, the listener will not even be aware that it’s there. While many mix engineers favor compressors that color their audio, when it comes to mastering, transparency is key. One of the challenges mastering engineers face when it comes to transparent compression is that different parts of the frequency spectrum will require different compression settings to effectively reduce dynamic range. Attack time, or the length of time before the compressor begins reducing gain, is key.
Different Frequencies, Same Compressor Settings
If a recording of a bright guitar and a bass drum is sent through a compressor with a short attack time, the transients of the guitar will be tamed, but the bass drum will lose its punch and feel choked. With the short attack time, the low frequency transient information in the bass drum does not have enough time to make it through before the compressor kicks in. If the attack time is set longer, the bass drum may have the desired punch before the compressor kicks in but the guitar will sound harsh. Its high frequency transients take a far shorter time to get through — the compressor will not compress the guitar much at all.
Regardless of whether a shorter or longer attack time is used, the compressor will favor different sections of the frequency spectrum in the recording and change the relationship between elements in the mix. In order to effectively compress the wide range of frequencies contained in a recording, different attack and release times would be required for different parts of the frequency spectrum. Multiband compression makes this possible.
What is multiband compression?
Multiband compression divides the frequency spectrum into different sections, or bands, so that each has its own unique compression settings. This allows a longer attack time for the low band of that bass drum to punch through, while keeping a shorter attack time in a higher band to keep the guitar in check. By using a multiband compressor, it is possible to closely tailor the compression to the different elements in a mix and compress the recording more transparently than with a standard single-band compressor.
Most multiband compressors feature either 3 or 4 different bands. Since the crossover frequencies define the range of frequencies contained in each band, getting them right can have a big impact on the effectiveness of the compression. Many multiband compressors have a solo function to focus on one band at a time — this can be very helpful when setting the crossover frequencies by revealing exactly what each band contains. Additionally, soloing individual bands can be helpful when setting attack and release times to make sure the transients in each band are shaped as desired.
Keep in mind that the frequency distribution will likely not be even in the recording being mastered, and the threshold — or amplitude at which the compressor begins compressing — for each band may be quite different. Compression ratio however, will often remain rather similar between the bands, or the compression could change the balance of frequency information and instruments in the mix. Most multiband compressor plugins will have different presets to try out, and they’re often a good jumping-off point to test out different compression thresholds and ratios.
As with any kind of audio processing, it is important to listen to exactly what changes in a mix when the processing is active. Bypassing the plugin and A/Bing with the unprocessed audio can be very helpful for this. Multiband compression is one of the most powerful processing techniques currently available to audio engineers and when it is used effectively it can achieve great results. That said, when it is used incorrectly, it can cause more damage than good.
For more information on this subject, mastering engineer and iZotope Education Director Jonathan Wyner has a good discussion on the merits of multiband compression and what to watch out for.
Ozone Multiband Compression Pro Tips
Ozone’s Dynamics module supports anywhere from single-band to four band compression, each band with its own Attack time, Release time, Threshold, Ratio, and Makeup Gain.
Load end-goal-oriented presets to use as a jumping off point or final coat of polish.
By selecting “Learn” in the bottom, right-hand corner of the spectrum analyzer window, Ozone 8 analyzes the incoming audio signal and determines the most transparent place for the crossover frequencies.
The Gain Reduction Meter gives a detailed visual analysis of gain reduction history and can be a good visual aid to help when setting attack and release times for the different bands.
Ozone features an unlimited undo history that makes it easy to do A/B comparisons by recalling previous settings.
Save time with the Linked Band Mode when adjustments need to be made to all bands at once.