Sound Design Workshop: Bottom's Up

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Use a highpass filter to clean up the bass end of a track by triggering the filter envelope from a kick drum sidechain.

Sidechain compression is an industry staple for cleaning up the bottom end of bass tracks. It uses a kick drum as the sidechain input to a compressor applied to the bass channels, which reduces bass peaks without affecting the perceived level or weight of the bass.

Sidechain compression is also used in a lot of dance music to provide a regular and rhythmic ducking effect on pads and sustained sounds. But there are more uses for sidechaining than these two examples. With today's DAWs and effects plug-ins, you can get a little more creative.

The High Road

To clean up the bass without ducking, use a highpass filter on the bass track and use the kick on the sidechain input to trigger the filter envelope. Set the cut-off frequency as low as possible and use a fast attack (10 ms or so) and a long decay (about 500 ms), and then adjust the envelope depth by ear. This will reduce the low frequencies of the bass when the sidechain triggers (see Fig. 1 and Web Clip 1) — an effect similar to ducking that can sometimes feel more subtle.

If your noise gate offers positive gain reduction, use this as an alternative to a compressor for that ducking-pad sound. Use the kick as the sidechain input, set the reduction level to a positive value (+8 dB is a good starting point) and then set a fast attack (10 ms or so) and a medium release (about 150 ms). Next, adjust the threshold level until you get the required depth of effect (see Web Clip 2). Depending on the sound of the kick drum, you may need to adjust the sidechain input filters (highpass and lowpass) to get the desired effect.

Out of the Gate

You can often tighten up the timing of a bass part by using a kick-triggered noise gate on the bass channel. The triggering of the noise gate by the kick gives an incredible tightness to the rhythm section. However, you can use a milder version of this technique to add some rhythmic interest to sustained parts. When a noise gate is applied to a pad and triggered by a kick, the noise gate drops the level of the pad by only a couple of decibels when the kick dies away.

Set the reduction level (called Range on the Sonalksis noise gate I used in this example) to reduce the level by only a few decibels and dial in a fast attack and a slightly longer release (around 300 ms). The kick drum triggering will emphasize the pad in a way that ties in rhythmically with the kick pattern without obviously chopping it (see Web Clip 3).

Beyond the Call

You can use similar techniques to tidy up other elements of your mix. If you have a good balance in most of your song but it gets a little cluttered in some places, try using sidechain compression instead of the usual tricks to create some space.

If you have an extra element in some places — say, a guitar present only in the chorus — that is fighting with a part that is present all the way through (strings, for example), then put a compressor on the strings and use the guitar as the sidechain input. Use a slightly quicker release than in the other examples to avoid any obvious ducking and go easy on the gain reduction. That will often make enough room without fundamentally altering the balance of the rest of the track (see Web Clip 4). This technique can also help to make space for vocals when they are struggling to be heard in a mix.

Unless you are deliberately aiming for a very noticeable effect, use these techniques gently. Apply them to a few different sources rather than using one overall compressor or dynamics process on the whole mix. The results will usually be far better.

Simon Langford is a professional songwriter, producer and remixer who has worked for some of the biggest names in pop music, including Rihanna, Britney Spears, Kelly Clarkson and many more.