The simplest way to prevent drum mix compression robbing your kick of low end is to use your compressor’s side‑chain high‑pass filter.
I’ve noticed that quite a few engineers who work with bands like driving their drum submix into fairly heavy compression, but whenever I try doing this it seems like I end up losing the low‑end weight from my kick drum. Why is this happening — and is there anything I can do to avoid it?
Anon via email
Mike Senior, mixing expert replies: There are a couple of reasons why a bus compressor can rob your drum mix of low‑end weight, and I’ll deal with them one at a time.
Probably the most common cause is that, on the whole, compressors are more sensitive to low frequencies than high frequencies. This is why, for example, vocal compression often overemphasises sibilant consonants — the lower‑frequency sung notes trigger more gain‑reduction than the higher‑frequency esses do. With mixed drum signals, this frequency bias means a strong kick will usually trigger more gain reduction than the other kit instruments, because it has more low‑spectrum energy. If you’ve set up your compressor to release its gain reduction reasonably fast, then the kick will effectively be turned down more than snare and tom hits, giving the impression that your drum kit as a whole is losing low‑end weight.
It might seem pretty counterintuitive, but one thing that can remedy this problem is high‑pass filtering the kick‑drum channel.
It might seem pretty counterintuitive, but one thing that can remedy this problem is high‑pass filtering the kick‑drum channel. Yes, you read that right: cutting the kick’s low end! Specifically, you should cut only the very lowest frequencies, and keep the filter’s cutoff frequency low enough that you scarcely hear any difference to the soloed kick‑drum channel. The reason this can improve matters is that many project‑studio drum submixes have flabby subsonic frequencies lurking underneath the kick drum. These frequencies won’t be contributing anything subjectively useful to the kick‑drum sound at all, and may even be too low for you to hear if you’re listening on headphones or typical small project‑studio monitors. Crucially, though, these subs may still merrily trigger excessive gain reduction from your compressor. By filtering them out, your kick appears to the compressor to have less low‑frequency energy, so it triggers less gain reduction per kick hit, leaving your drum submix sounding weightier.
You have to be careful when high‑pass filtering any drum channel, though, as the time‑smearing and resonant effects of the EQ can potentially soften your attack transients. So do keep your ears peeled for that side‑effect. If you encounter this problem, there’s another method of reducing the compressor’s kick‑drum gain‑reduction if your compressor includes side‑chain EQ functionality: just cut some low end from the compressor’s detector signal rather than the drums themselves. This effectively makes the compressor less sensitive to low frequencies, so it won’t respond as strongly to the kick‑drum hits.
The second reason a compressor can cause a drum mix to lose low end is that you’re using a very fast‑acting compression setting — the rapid gain reduction change triggered by each kick hit can put an appreciable kink in the hit’s waveform, rather than just shaping its level envelope. In this situation, the slower‑moving low‑frequency waveform cycles will inevitably be more heavily waveform‑mangled than the faster‑moving high‑frequency ones, so the result is that the low frequencies get distorted (effectively converting some of their energy into upper‑spectrum harmonics). Probably the best protection against this is simply being aware that it can happen. After all, it’s pretty simple to increase your compressor’s attack time a little once you’ve realised it’s the source of the problem. Of course, lengthening the attack time may lessen the compressor’s gain reduction action across the board a little, and you may have to adjust your threshold and ratio settings to compensate if that’s a concern.