Track, Bus and Parallel Compression for Mixing Drums

Drums are somewhat defined by the fact that they generate extremely dynamic and transient sound waveforms. As such, dynamics processing tools are fundamental to controlling and enhancing drum recordings in the mix. In many respects, the volume fader is the simplest form of dynamic processing tool, allowing us to manually adjust the amplitude levels of a track. We can either set the fader to one place or adjust it throughout the song. Of course, this is a very manual process that previously could only be conducted in real time as the song plays, and more recently with software automation features. It’s no surprise then that tools were developed to perform some of these volume adjustments automatically or dynamically as the song plays on. Furthermore, using automated dynamic processing tools, we can not only turn individual sounds up and down, but dynamic tools can act so fast that they are capable of reshaping the audio waveforms themselves, enabling many creative possibilities. Here are the steps to Track, Bus and Parallel Compression for Mixing Drums;

1. Compression and limiting

Perhaps the most used processing tool in a mix engineer’s set is the dynamic range compressor. We will describe this in a moment, but first it’s worth describing a more simple version of the dynamic range compressor: the fast acting limiter. A fast acting limiter looks at every single digital sample of the audio waveform and sets a limit on how large the waveform can get. In audio, we generally refer to the loudest possible digital signal level as being at 0 dB (zero decibels). Any signal data greater than 0 dB is technically beyond the capabilities of a digital audio system to convert into accurate sound. So, we can set a limiter’s limit, or threshold, to be somewhere below 0 dB, say −3 dB. In this case, any audio data that is greater than −3 dB will be hard limited to a maximum level of −3 dB and no greater. As you can imagine, this can significantly alter the waveform shape of any sounds that would normally go over the −3 dB limit. Changing the waveform shape with a limiter does alter the sound of the audio, and in scientific terms adds distortion to the sound. But this distortion is not always perceivable, and so it is possible to limit a signal a little without listeners really noticing the distortion artefacts. On the flip side, it is very possible to add excessive limiting to an audio waveform; for example, if the limit were set to −12 dB, depending on the audio material, this could cause a huge amount of audible distortion to be heard on the processed audio. So why would we bother limiting if this causes distortion? Well, we’ve just said that sometimes the distortion introduced is relatively mild and unnoticeable, and if we have limited our waveform to a peak threshold of −3 dB, then we have the opportunity to turn up the volume of the entire audio track by 3 dB (known as make-up gain) without ever risking going over the maximum 0 dB level that our audio device can handle. This is great, with adding a little bit of limiting on just the highest peaks of our audio waveform, we are able to make the signal denser and louder, which can be useful in many mix scenarios.

The compressor is a more complex version of the limiter that allows much more control and creativity in the way it is used. It differs from the fast acting limiter in a number of subtle ways: Firstly, the compressor doesn’t hard limit anything that crosses the threshold; instead it has a compression ratio setting which allows us to define how much a signal is turned down, attenuated, or compressed once it crosses the set threshold. A high ratio, for example 100:1, acts quite like a limiter, because any signal amplitude that crosses the threshold causes the audio to be turned down by 100 times. If we use a lower compression ratio, for example 2:1, this means that waveforms that go over the threshold are turned down by two times (or halved). We therefore have some very fine control over the compressor by choosing how to set the threshold and ratio values with respect to each other. A low threshold with a shallow or low ratio will give some very gentle compression that might be quite unnoticeable, whereas a higher threshold with a steep or high ratio gives a much more drastic alteration of the waveform amplitude. Both are valuable approaches when mixing drums, and sometimes it might be valuable to have two compressors in line on a single channel with different settings applied. Sonically, the harder compression that nears that of limiting allows signals to be turned up in volume quite drastically, and sometimes the distortion created by compressing the peaks of the waveforms can add some subjective brightness and cutting impact to a drum sound. A lower threshold with a shallow ratio is very useful for adding some weight and density to a drum sound without significantly altering the sonic characteristics of the original recording. So both compression approaches can be useful for adding presence and impact to the drums in a mix in subtly different ways.

The compressor also differs from a limiter by the fact that it does not respond immediately to every digital data sample in the audio file. Instead it responds to a moving average of the signal level, which is more in keeping with how our ears and brain respond to or notice volume changes. The threshold is therefore compared against the average waveform signal, and, as such, some aspects of the waveform may cross over the threshold without ever triggering the compressor into action. We have another control here on the compressor which is to set the time window for how the average signal value is calculated. If the average value is calculated over a long period of time, then the compressor is relatively slow to react (compared, for example, to the fast acting limiter) and allows momentary peaks to cross over the compressor with no effect. This is great for drums because it means that with careful setting of the compressor’s attack time, we can decide if we want to compress every aspect of the signal (with a fast attack setting) or if we want to let a little bit of the signal (i.e. the first onset of the drum sound) through without being compressed at all (with a slower compressor attack time). The slower attack time is valuable if you want to let the natural crack of the stick hit on the drumhead through the compressor, before the compression kicks in and boosts the overall body and decay of the drum sound. The same options apply for the compressor returning below the threshold to an inactive state, so we can also set a release time too, to indicate how long the compression effect continues to be applied after the signal has dropped back below the threshold. Again, this is a valuable setting for drums, because it allows us to somewhat control the duration of each drum hit and its decay time, and it can have an influence on how little or how much of the drum’s edge overtone shines through in the decay of the waveform too. The above figure shows two software compressors, each with a number of different settings for adjustment. Every compressor has its own unique way of averaging the signal, of switching in the compression when the threshold is exceeded, and of calculating the amount of compression to apply, and each has a unique sonic signature of its own too.

There are a huge number of ways to manipulate the basic compression settings (threshold, ratio, attack, and release) when mixing drums. Given all the subtly different compressor designs, it’s therefore worthwhile to experiment with many compressor types and all of the associated settings to develop your own understanding of how drum sounds can be manipulated with compression. Put a compressor on the kick drum channel and experiment with adjusting the attack and release times and the threshold and ratio values. Get an idea of how these settings affect the sound of the drum and how you might use these to enhance or control aspects of the sound in a mix. With the attack and release times, you should also be able to find a setting that allows the drums to pump or “breathe” in and out at a sympathetic rate in relation to the tempo of the song, the result of which can sound tight and impactful. But what works great or is needed in one mix might be totally inappropriate in another project. On the whole, we use compression on drums to level out volume variations in a performance, to add volume, density, weight, or presence, to soften or sharpen the attack and to shorten or lengthen the decay, and often to add some subtle distortion or sonic character to a drum recording too. So, should you compress every track? Compression is certainly not essential for many instruments and mix approaches, and it is very possible to get excited by the sound that compression brings to a single instrument sound, and hence consider applying it to everything. But when more and more instruments are compressed, the full-mix sound of the track can start to lose its overall energy and dynamic interest, and compression brings distortion as a by product too, which can start to become noticeable. So you should be careful when adding track compression, to be sure that it is being used to enhance particular aspects of the sound you are treating and giving an improvement to the overall mix, not just the track you are considering in isolation.

2. Compressing drum sub-mixes

So far, we’ve talked about adding compression to a single track, i.e. to the kick, snare, or toms. But we can add compression to drums sub-mix busses too. We might do this if we want to apply one dynamic control equally to a number of instruments in one simple process. Or we might add compression to the drum sub-mix as part of a staged compression chain, for example, so the kick drum is compressed once on its own channel (perhaps with a low threshold and low ratio) and then compressed again as part of the drum sub-mix (perhaps this time with a relatively high threshold and a high ratio). It can also be valuable to compress the cymbal recordings too, either individually or more often on a sub-mix of the cymbals. Adding some subtle compression to the cymbals brings a crispness and brightness to their sound and allows their quieter decay tails to be brought up in volume to a more noticeable and charismatic level. Some mix engineers like to hard compress room mic recordings too, and subtly blend in a hyper-compressed element that gives some enhanced character of the recording space that was used.

3. How to use parallel compression on drums

It is also possible (and recommended) to add compression in a parallel form, and many engineers are often interested in understanding how to put parallel compression on drums. Parallel compression refers to the case where the compressor is positioned on its own auxiliary effects bus and is then driven through the DAW send busses. Parallel compression allows some quite drastic compression to be applied, but to be blended in more subtly alongside the direct tracks and sub-mixes. The result is that parallel compression applied to drums allows the overall volume and density of the drums to be increased from the ground up, without significantly increasing the peak level of the drum tracks. With parallel compression on drums, it’s sometimes advantageous to have one parallel compressor for the kick and one for the snare, and another for all of the other drums or instruments in the mix. When feeding a snare or a kick track into a parallel compressor, these very dynamic sounds impact the compressor settings as they are heard, so they can often cause an unwanted pumping effect if using a single parallel compressor for multiple instruments (of course, this might be wanted also for some styles of music!). If the kick and snare have their own parallel compression channels, then this can be very effective for blending strength and density into the mix without the risk of causing noticeable distortion. If you are mixing metal music, then it’s not uncommon to send a parallel compression channel to a second parallel auxiliary with compression applied too, to achieve a two- or three-tier parallel compression process that adds more and more impact to the drums with each extra stage. Of course, it’s possible to go too far and result in everything sounding like a muddy distorted mess, so a creative judgement needs to be applied with each decision to add extra compression. The Logic Pro screenshot below shows an in-line compressor inserted on each of the Kick Drum, Snare, Floor Tom and Rack Tom tracks. It also shows further compression effects are applied to both the DRUMS and CYMBALS sub-mixes (on busses 1 and 2, respectively) and parallel compression set up on bus 10.

You might also want to consider using an envelope shaper tool, such as the SPL Transient Designer or Waves Smack Attack plugins, which have their own unique algorithms for manipulating the dynamic profiles of drum recordings. Either way, it’s well worth experimenting with all these types of compression and look for an overall mix setup that takes your initial drum recordings to a new level of power, crispness, and impact, and even making them larger than in real life!

If you want to know more about the underlying science of drumheads and drum sound, and learn more creative approaches to drum sound and drum tuning, check out the free iDrumTune ‘Drum Sound and Drum Tuning’ course at www.idrumtune.com/learn

Author Professor Rob Toulson is an established musician, sound engineer and music producer who works across a number of different music genres. He is also an expert in musical acoustics and inventor of the iDrumTune Pro mobile app, which can be downloaded from the App Store links below: