In this clip from our Bob Horn Mixing Episode, Bob separates out a rim click sound from his snare track in order to process it differently from the snare hits. It’s extremely common to process distinct sounds differently, but how does Bob do it? First, he cut and moved the sections of the snare track with rim clicks on it onto their own track. After separating the rim clicks from the snare hits, he found that there was some kick in his rim click track. He then gates the kick out of the rim click track, and EQ’s out the low end to reduce the kick. Bob only touched upon this subject briefly, so today we are going to go into more detail on how to isolate sounds for mixing.

Why Isolate Sounds
As audio engineers, we frequently isolate sounds, and there are various reasons we might not want a sound somewhere in our mix. For example, noise reduction, whether it be a wind noise, room noise, or something else, is a huge part of audio for film. In music, we commonly get bleed between instruments, just like Bob experiences with his snare and kick. But this doesn’t only happen with percussion. We could track instruments separately and sacrifice a certain organic quality, but the decision is often made to let a band play together so that they can play off each other and produce a more cohesive track. In this sort of situation, we often want as much isolation as possible so that we can process and improve each instrument in its own unique way, and to its full potential. Isolation is also useful to ensure that the incorrect type of processing isn’t applied to a bleed instrument, thus lessening the impact of the processing that you actually want to apply to that instrument on its correct track. As Bob says about his rim click track: “So when it’s done, you have very little of that kick left. So I don’t have to worry about it phasing with my natural kick. Because I want to be able to get this, if necessary, as loud as possible, without changing the kick sound.”

Isolation While Tracking
Sound isolation while tracking is the first step in the signal chain, and is not to be ignored. Besides tracking instruments separately, you can make your job easier down the line by trying a handful of tricks to isolate sound more.

Of course, you primarily want to position the mic so that you will get a good clean signal from the target instrument. But also try strategically positioning the microphone so that the instruments likely to bleed into the mic are at an angle that’s least likely to be picked up in the mic. Understand your microphone’s polar pattern, and direct the least sensitive region of the mic towards the instrument you’re avoiding (if possible). For more information on how to do this, check out our article on polar patterns here. You can also try adding sound dividers, or gobos, in the room to isolate instruments. These are especially useful if you have drums or a bass that you would like in a large room for the acoustics, but would like some isolation. Yet another trick, although more rarely used, is to lift anything that might cause sympathetic vibrations in the flooring off the floor. For example, to avoid bleed from a guitar amp, you could place padding between the amp and the floor. It’s a similar concept to why people use isolation pads below studio monitors.

Isolation with EQ
Of course, we often have to compromise on isolation in order to get a good vibe for the musicians. This is when we pull out our digital toolbox to further isolate a sound. If you’re lucky, the offending audio will be a different frequency spectrum than the desired audio. In these cases, you can use EQ to remove all or most of the unwanted sound. In Bob’s case, the kick lives in such a low frequency spectrum, and the rim clicks exist in a higher frequency spectrum, so he was able to use EQ to his advantage a bit.

Isolation with Gates
Gates mute the audio signal whenever its level falls below a set threshold. Since bleed and unwanted noise are generally lower in volume than the desired audio, a gate can be useful to cut that noise. The key is to set the threshold so that the wanted noise is above the threshold, and the unwanted noise is below it. The gate will then mute the unwanted audio (the kick drum in this case), and allow the wanted audio to pass through (the rim click).

Gates have similar controls to a compressor. Besides the threshold, there are attack and release variables that control how quickly the gate opens and shuts. Using your ear, and setting an appropriate attack and release time will keep your audio sounding natural. It will ensure that the sound fades to silence smoothly, and avoid abrupt clicks from the gate kicking into gear.

Sometimes the audio can cause the gate to kick in and out very quickly, causing a weird and unnatural sound, called chattering. Setting the hold time can fix this issue. The hold time dictates the minimum amount of time that the gate can stay open. It keeps the gate from kicking in and out quickly because once activated, the gate must stay open for a set period of time. Usually a gate will have a hold time of some sort, whether or not there is a hold time parameter that you can control. Another parameter that prevents chattering is hysteresis. The hysteresis lets you set two different thresholds, raising the threshold for the gate open above that of the gate close threshold. Hysteresis is another parameter that is often built into a gate, even if there is no parameter that you can control. Hysteresis is often not labelled as such. For example, in the plugin below, you can see the separate “Gate Open” and “Gate Close” parameters, and corresponding faders. When you set two different values for these parameters, you are controlling hysteresis. Depending on the gate, there are other variables you can control that are mostly useful for making the affected audio sound more natural. For example, range settings can let you attenuate the signal below the threshold instead of muting it, making the transition into gated audio less abrupt.

One draw back with gates, however, is that they will allow the unwanted audio to pass if it’s playing simultaneously with the louder, wanted audio. Sometimes that isn’t a problem though, since the desired audio will often mask the unwanted sounds.

Isolation with Compression
Bob uses sidechain compression to reduce the kick sound in his rim click track. To do this, start with the track for the audio that is bleeding into the track you ultimately want to clean up. In Bob’s case, this is the kick track. You then use a send to send the audio for that track (the kick) out a bus. This bus number is then keyed into a compression plugin on the track you want to clean up, or the rim click track in this case. Then the parameters are set for the compression so that it properly reduces the unwanted audio signal. Bob set the compressor to effectively compresses the rim click track whenever the kick plays, reducing the amount of kick in the rim click track. Bob probably used a fast attack here because he doesn’t want the attack of the kick drum to get past the compressor, and therefore heard on the rim track. For more info on sidechaining, check out our article on side chain compression.

Isolation with Noise Reduction Software
Another technique is to simply use noise reduction software to eliminate the unwanted noise. Now, these plugins aren’t perfect, and you can easily remove too much of the audio that you actually want to keep. But if used correctly, they are powerful tools that can be very effective. Generally, to use a noise reduction plugin, you highlight and play the section of audio that contains the unwanted noise. Then hit a button in the plugin that gets the “noise print” of this unwanted audio. Once the plugin has this noise print, you can set parameters to adjust how much audio is removed. Some good noise reduction plugins include iZotope’s RX series, Sonnox’s Oxford DeNoiser, and Waves’ W43.

So that’s it! You can find the rest of the Bob Horn mixing episode here, and For more mixing advice from Bob Horn and other seasoned audio engineers/mixers/producers become a member at Pro Studio Live!

Questions? Comments? As always, please share in the comments below!