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How does multiband compression work?
Multiband compression and dynamic EQ: what’s the difference?
Find the best free and paid multiband compressor plugins.
Compressors are invaluable for shaping the dynamics of audio. They reduce the difference between the highest and lowest amplitudes of a signal. This is achieved by pushing down the peak values when they cross a certain limit. It makes the volume ‘stable’. Stable is good!
Artful compression is the difference between a track sounding like glue and goop. Your mixing and mastering process can benefit from two methods a) using plain old EQ and compression or b) using a multiband compressor.
As if understanding regular compression wasn’t hard enough, we now need to contend with the even more sophisticated multiband realm. It’s a two-edged sword that can either give you the optimal punch or ruin the sonic landscape in a cinch.
The good news is, if you’re already familiar with the basics of compression, then you’ll have no trouble understanding how multiband compression works. You won’t find a plethora of new terms here, but rather a new way of understanding how the basic concepts of compression can be applied to separate frequency bands.
So, let’s explore how multiband compression works alongside some tips on how to use it for mixing and mastering your tracks.
What is Multiband Compression?
A multiband compressor can be thought of as a collection of different compressors, with each one only affecting a certain frequency range of the input audio. The entire frequency range of the signal is divided into sections or ‘bands’ and each band can be compressed using unique parameters.
The output of the multiband compressor is the summation of all the individual compression processes. This is very different from the way regular compressors work. Standard compressors affect the entire signal accordingly because they operate under a set of global parameters.
Using multiband compression provides gain reduction proportional to the input level (like compressors), but at specific frequencies (like EQ), multiband compression can be thought of as a combination of compression and EQ.
This aspect of multiband compressors makes them more flexible. It allows for more detailed, tailored dynamic control than would be possible with a traditional compressor.
Multiband vs Standard Compression?
While standard compressors do a great job at controlling the sound of a single source, like a kick drum or cymbals, they might not be the most desirable tools in situations where the source has a wide range of frequencies, like a full drum bus or even a full mix.
The reason is that transients of different frequencies have different lengths or duration. In general, lower frequencies have longer transients and higher frequencies have shorter transients (as can be inferred by their wavelengths).
If the compressor settings are fast enough to control and tame harsh cymbals on a drum bus, it might turn out to be too fast for the kick and snare transients to pop through. This will result in the kick and snare sounding weak and without a body.
Conversely, if we set the compressor to work for the kick and snare’s transients, it won’t catch the cymbals. They will sound strident and harsh. This is where a multiband compressor plugin can save the day (or your mix, which might be the same thing for an overworked mix engineer).
It provides the ability to split the entire signal into sections of frequency ranges and then configure different compression settings per band. You can process low, mid-range, and top-end ranges completely independent of each other and sum the result back together at the end.
Considering the same scenario of the full drum bus above, we can now set our frequency bands to cover different sections of the kit like the kick subs and thump, the body of the snare and toms, high-mid attack and click, and the highs and air of the cymbals.
We can set a fast attack for our cymbals, and slower attack times for our kick and snare, thus ensuring that each section is processed only as needed. This gives you clean and transparent dynamic control and makes the track sound full and balanced.
Think about full mixes in a mastering scenario. The frequency ranges contain vocals, guitar, bass, and different parts of a drum section. With multiband compression, you can process each section separately for significantly better results.
The ability to tailor compression over a large frequency range makes the multiband compressor popular for bus processing and mastering.
How Does Multiband Compression Work?
As mentioned before, multiband compression divides the entire frequency spectrum into different ‘bands’ or sections. Most multiband compressors feature 3-6 programmable bands, and each band has a dedicated compressor for controlling the dynamics of that section.
No filter is perfect – there will always be some roll-off meaning that frequencies beyond the cutoff will still pass through albeit attenuated. The part where the filters of two adjacent bands intersect is called the crossover section. The slopes of the HPF and LPF of adjacent bands are adjusted to ensure that the amplitude of the signal remains the same across the crossover.
These crossover points can usually be adjusted by the user, but expect some ‘hand holding’ here – meaning that if you adjust the boundaries for one range, the adjacent ranges will adjust to ensure the entire frequency spectrum is still covered. This is at least the case for many multiband compressor plugins.
Inside a multiband compressor, each band has a dedicated compressor attached to it. This means that threshold, attack, release, gain, and ratio parameters can be set independently for each band. Some multiband compressors also feature a ‘range’ option to limit the maximum amount of gain reduction that is allowed for every band.
Any of the bands may also have their compression bypassed to let that frequency range through unaffected. After processing, the output of all the bands is then summed back as the output signal.
How to Use Multiband Compression
As mentioned before, multiband compression is very useful when dynamic control is sought for material with a wide range of frequencies. This makes it useful for bus compression, where groups of instruments and voices are summed together to be processed as a whole.
A synth stack, vocal choir, drum kit, or string section are examples of groups that occupy a wide frequency range. By carefully selecting the ranges of the bands and setting suitable thresholds, attack, and release times for each of them, a multiband compressor can be a very powerful tool to glue the entire group together as a cohesive unit.
While regular compressors are often used in the mix stage for the characteristic color they might impart, at the mastering stage, transparency is key. Multiband compressors are thus well suited for mastering applications since they can control a full song’s spectrum cleanly without artifacts.
They provide a way to shape the dynamics of each range most appropriately, preventing issues like pumping, lack of punch, or harshness. This transparent gain reduction also opens up the way for a louder yet balanced final master.
While multiband compressors are clearly very effective at balancing out a wide range of instruments and frequencies as a whole, they can just as easily be used to focus on problematic frequencies – even if they only occur occasionally.
The threshold is set so that the compressor reacts only when sibilant syllables (‘s’, ‘ch’, ‘f’ sounds) cause the energy of the signal in that frequency band to go over the set threshold. It can also be used to add presence, shape vocals, or even out the mid-range.
A static EQ cut at the problem frequency is not ideal to solve these problems in a vocal track. It will mean cutting out the highs even when there is no sibilance, resulting in dullness.
For Mixing Bass
On the opposite end of the spectrum, multiband compression can be used to control a booming low end. Multiband compressors can really help to make a bass track sound consistent. For example, an electric bass guitar might have a particularly resonant note which makes the low end inflate whenever it’s played.
Setting up a multiband compressor to push down on only that region of the frequency spectrum can thus help clean up and tighten the bass without having to manually automate every occurrence of unwanted resonance.
For the bass, a static EQ cut at the problem frequency won’t work because it takes out the low end from non-problematic notes, resulting in a weak bass sound.
A multiband compressor can thus get rid of problems only when they actually appear. This preserves the overall sonic characteristics of the signal while dynamically carving out undesirable frequencies.
We all are guilty of having read one too many online mixing articles. Reading about the working principle of multiband compression sounds suspiciously similar to another frequency-specific dynamic control process – dynamic EQ.
In many ways, multiband compression and dynamic EQ are similar to each other in concept and function. Both involve choosing particular frequency ranges to compress depending on the signal’s intensity withinin that range. Both have threshold, attack, and release controls to tailor the compression for selected frequencies.
However, there are a few key differences between them. They each approach the selection of frequencies differently and it becomes a matter of frequency vs. dynamic control.
While a multiband compressor takes the foundations of a regular traditional compressor and adds a layer of frequency control over it, dynamic EQ takes its basis from a regular static EQ and adds a layer of dynamic control on top.
User interfaces for Dynamic EQs look largely similar to a regular graphical EQ, with frequency boosts or cuts decided by EQ parameters like frequency center, filter shape (bell, LPF, HPF, etc.), and bandwidth (Q-factor).
What sets them apart from regular graphical EQs is the fact that each EQ band now has a dedicated dynamics section, with controls for threshold, attack, release, and dynamic gain. When the gain of the signal in a particular dynamic band crosses that band’s threshold, the EQ cut occurs.
The gain change in dynamic EQs is linearly related to the input signal crossing the threshold, rather than being dependent on the ratio as in multiband compressors.
Dynamic EQ plugins typically have more bands than multiband compressors. Because these bands have the bandwidth control found in EQs, they can also be set to be much narrower than the crossovers of a multiband compressor might allow.
Thus dynamic EQ can be used for more surgical applications, in contrast to the broader, more ‘musical’ contexts in which multiband compression gets used. The key differences between the two are summarized in the table below.
Differences Between Multiband Compression and Dynamic EQ
Uses crossover filters for frequency selection.
Uses bands with adjustable frequency center, filter shape, and Q-factor.
Typically provides fewer bands
Can provide a large number of bands
Can only perform cuts (unless it has expander capabilities)
Can perform cuts or boosts
Uses ratios to control gain reduction
Gain change is linearly related to input signal over the threshold
Broader, more musical changes
Can provide surgical treatment
Essentially a compressor with EQ elements
Essentially an EQ with compressor elements
The Best Multiband Compression Plugins
To finish off the article and help you get hands-on with multiband compression, here is a quick list of the best free and paid multiband compression plugins to get you started:
OTT by Xfer Records (Free)
ReaXComp by Cockos (Free)
C3 Multi-Band Compressor by slim slow slider (Free)
As hopefully made abundantly clear, using multiband compression cleverly can make all the elements of your mixes work better together.
Combining the strengths of compression and EQ in a single package unlocks a level of tonal and dynamic control that wouldn’t be possible with either of them alone.
Even so, every situation simply does not call for multiband compression. This would be an extremely inefficient use of your computer’s resources because multiband compressors tend to be more demanding than their vanilla counterparts.
Be patient and persistent. It’s a tricky toy to master. As usual, use your ears and learn how multiband compression can be used on different sources. The examples in this article are merely a starting point – it’s expected that you will adapt these techniques to whatever you are working on.
Don’t forget to grab the free plugins above to get started with multiband compression right away!