Quick Summary

In this article Julian demonstrates a clever trick which eliminates bleed between mics using nothing more than a stock compressor plugin.

Going Deeper

I’ve long been a fan of the SPL de-esser. It’s easy to use and it sounds clean and is less affected by the artefacts which can trouble conventional designs. The reason this de-esser is interesting is that it works in a different way to standard de-essers.

A normal de-esser is a compressor which has a filter applied to the side chain detector to make it sensitive only to the frequencies at which the S occurs. This frequency-specific compression is the basis of the way most de-essers work but the SPL is different because it works through phase cancellation.

The fact that two identical signals cancel to silence when one is polarity-inverted and mixed with the other is one of those aspects of audio which is unbelievably useful. This approach can be exploited to eliminate spill in drum recordings.

This technique works on the basis that only the difference between two signals will make itself heard when two otherwise identical audio tracks are combined with the polarity of one inverted. If you have ever pressed the Delta button in a compressor you'll be familiar with hearing only the part of the signal which is having its gain reduced by the compressor. And it's exactly this method which is used to eliminate spill in drum recordings.

The most common spill I find myself wanting to control in a multi mic drum recording is bleed from the hi hat onto the top snare mic. The proximity of these two kit pieces makes this kind of spill inevitable and often the off-axis spill into a cardioid mic such as an SM57 can be very coloured and sound quite nasty.

How To Eliminate Spill From A Top Snare Mic

• Duplicate your top snare track

• On the duplicate track add a compressor, choose the fastest one you have.

• Set the ratio as high as it will go.

• Set the attack and release to their fastest setting.

• Lower the threshold until you are catching as much as possible of the snare hits.

• If you are catching the kick drum use the side chain filter and exclude it. You only want snare compression.

• Invert the polarity of the duplicate track.

• Mixed together at identical levels the bleed on these two tracks will cancel to silence and you will be left with only snare hits.

• You can high pass filter the duplicate to keep the cancellation only at the top end if you prefer, it can sound cleaner.

• The processed snare can lose brightness, use EQ to restore if necessary.

How Does It Work?

It seems like magic but if you keep in mind that when nulling two tracks using phase cancellation, only the difference between them will be heard. In the case of this trick, the only difference is the compression applied to the snare hits. The fact that in the original track this was a gain reduction makes no difference in this case, Difference is difference and is audible when the original and the compressed, polarity inverted tracks are combined.

This is very similar to the trick people use to eliminate spill when playing a monitor mix via a loudspeaker to a large ensemble when tracking. For example if tracking a choir using headphones for a monitor mix would be impractical What can be done instead is to record with playback via speakers and to re-record another pass with the playback but with the choir keeping silent. If these two passes are combined with a polarity inversion on the second pass, the spill will cancel.

This trick, or a variant of it which also includes the use of a high pass filter to target only the top end, was the Find Of The Week for Gareth Nuttall, a studio owner and recording engineer who was a guest on our podcast ‘Fix It In The Mix. Check out the podcast, and how Gareth uses this trick below.