In The Studio
Back from the concert tour now and the band want you to mix their new album (wow, you really made a good impression on them, didn't you?). The first song is a heavy rock, funk, electronica dance track (this is an experimental band) that the record company want to release as their first single. On listening to the drum tracks you notice there's a lot of spill on the individual tracks - the kick track has picked up the snare, the snare track has picked up the hi-hat etc. This will mean that any processing on those instruments will be difficult, if not impossible. Let's start with the kick track.

In the above waveform display we can see the loud kick drum hits as large peaks and the quiet snare hits as small peaks. If we were to apply compression to this track, a common practice on kick drum, then the snare hits would become louder. This will cause problems when we want to mix down. So by applying a noise gate to the kick drum track in the same manner that we used in the live example, we can eliminate the snare hits completely resulting in a clean kick drum track that can be further processed.
Now let's look at the snare track.

Here we can see that in between the actual snare hits (the large peaks) there are a lot of other sounds. These comprise of toms, hi-hats and other cymbal hits. Again, if we were to apply compression to this track, all those other sounds would become louder. But this snare part also contains some ghost hits (quiet snare hits) that we want to keep so if we just applied a noise gate to the whole track, those ghost snare hits would be lost too.

This is where we need to apply frequency conscious gating and this is how it's done.

1. Route the snare track to a sub group and insert the noise gate on that group. 2. Also, send the original snare track's signal into the side chain input of the noise gate. 3. Then listen to the side chain input monitor - usually selected with a button on the gate itself - and bring down the low pass filter until the high frequency cymbals cannot be heard. What you should be hearing now is a rather dull sounding snare. 4. Then turn off the side chain monitor and adjust the threshold until you can only hear the snare, ghost notes too.

This won't completely clean up the snare track, some tom hits will still sneak through for example, but it will probably clean 90% of it.
Below is a snapshot of the audio mixer in Logic, which shows the settings I used on the gate for this example. Notice the high and low pass filter settings; the gate is being triggered with audio from the snare track between the range of 610Hz to 830Hz thereby ensuring that high frequency cymbals or low frequency toms won't trigger the gate.

Cleaning up recorded tracks in this manner is quite common practice in studios. Again, it's usually applied to acoustic instruments but unlike live applications it can also be applied to recorded vocal tracks (see the first example in paragraph 3). However, I do not recommended that noise gates are used during the actual recording process for one simple reason - if the threshold is not correct then you won't capture all of the performance. It's better to get everything down on tape, warts 'n' all, and then apply gating afterwards.