When compressing low-end, it’s best to consider which instrument or group you’re trying to control - for example, a bass guitar is typically compressed with an optical compressor with a soft knee. Alternatively, a kick drum will typically sound better with a hard knee and faster attack and release.
Let’s start off by compressing instruments that mainly occupy the low-frequency range.
Although how you compress your bass guitar may differ, typically, listeners enjoy a bass that’s full and smooth sounding with a fair amount of dynamic control. For that reason, I’ll use an optical compressor - I’ll set the attack to 10ms and use a longer automatic release.
A soft-knee, 4:1 ratio, and 5 to 10dB of compression will work well. Let’s listen to how this controls bass guitar.
Like the previous chapter, how you approach a kick drum will be unique, but emphasizing mild distortion on a kick with a short attack, moderate release, and a hard knee is a popular route to take. Hitting a hard knee with a higher ratio will create a 'smack' sound.
A super short attack and release will emphasize a high-frequency click sound, which can also work well on kicks. Let’s listen and notice how we achieve a more percussive sound.
Last up for individual instruments that we’re going to look at, it’s worth asking if compressing a bass synth makes sense.
Typically, a bass synth is already going to have a fair amount of dynamic control due to the oscillators that create these instruments having relatively consistent dynamics. Additionally, more modern bass synths that utilize samples often pull from already processed audio, making more compression somewhat redundant.
With that in mind, it may not be necessary to compress your bass synth unless you’re trying to achieve a particular timbre, but even then it might make more sense to use saturation or a transient shaper. The main reason would be to use it for ducking, but we’ll cover that in more detail later on.
Let’s listen to a bass synth without and then with compression, and let me know in the comments if your think the compression was needed.
If we’re working on a full mix or maybe a bus, and we want to extract as much detail as possible, a low-level compressor is a good option. We can use it directly on the signal, or, set up a parallel send with a linear phase EQ.
With the EQ we can isolate the lows, and then use the upward compressor to ensure that only low-frequencies are getting this increase in low-level detail.
Let’s listen and notice how this increases detail in the low-frequencies.
Although dynamic EQ may not be considered compression, this trick is a good one so I thought I’d still include it.
I’ll place the EQ on my bass guitar or synth, and then select my kick drum as the external side-chain. After observing where they overlap, I’ll create a band, and make it dynamic with a negative range, then I’ll use the external-side chain as the band’s trigger.
Now whenever the kick hits, this specific frequency of the bass is attenuated - creating some needed room for the 2.
Let’s listen and notice how they become more distinct.
Similar to our last chapter, we can insert a downward compressor on our bass guitar or synth, and set the external side chain as the kick. After ensuring that the kick is triggering the compression, we can control how the bass is attenuated using the typical compressor settings.
Like the previous chapter, this will create some room for both - although may become noticeable or create pumping at higher levels.
Let’s listen and notice how this creates room for the kick drum.
One thing to keep in mind when compressing low frequencies is how certain settings will cause distortion. If the attack and/or release is shorter than the length of an individual waveform you’ll get distortion, which we saw in chapter 2 can be a good or bad thing.
If we divide 1000 / Hz. for example, 1000/20Hz, we see we get 1 waveform every 50ms - meaning an attack or release shorter than this will cause distortion.
This is more noticeable on a full mix so let’s take a listen to it, and consider when this effect could be useful.
If you’re mastering a track and want to compress your low frequencies, it might make sense to attenuate the lows from the side image first. To do this I’ll use a mid-side EQ, and create a high-pass on the sides to make the lows mono.
Otherwise, compression and subsequent amplification of the lows may result in a muddy sound or one with a lot of phase cancellation or temporal masking.
Let’s listen to compression on the lows with and without this EQ in place.
If you’re mastering and don’t have access to stems or individual instruments then a multi-band compressor is the best option for controlling the dynamics of low-frequencies. Everything we’ve discussed previously, like distortion, and how different compressor types affect the timbre still applies here.
I usually like to isolate the kick and bass up to right before the snare, but depending on the bass, you might need to set this range a little higher.
Let’s listen and notice how this offers a lot of control when mastering.
Split EQ is a really interesting processor that separates the signal into sustain and transients, meaning when can use it to expand or compress a signal. If I reduce the transients of the lows and increase the sustain, we’ll cause compression - this becomes more useful over small bandwidths.
Let’s listen and notice how this controls dynamics, but offers a unique timbre to the low-end.
Saturation is a combination of soft-knee compression and harmonic distortion, meaning we’re controlling dynamics from peaks down, as well as from the noise floor up by adding low-level signal. The heavier the saturation, the greater the dynamic control, but the harmonics can greatly change the timbre.
Let’s listen to saturation on the low frequencies, and consider how this can complement or replace typical compression.