What's that all about?
Behind this ugly term hides a phenomenon that is relatively simple to understand. When you record several instruments at the same time, it's not unusual for a mic aimed at registering the sound of one instrument to also capture the sound produced by others. These unwanted signals are referred as "bleed" (aka "spill" or "leakage)." Bleed was pretty common back in the day when bands used to record "live," but nowadays overdubbing is a far more common practice in most music styles. However, you shouldn't take for granted that you are safe from this phenomenon. For one thing, mic bleed is impossible to avoid when recording drum tracks.
But that's not the only case, far from it, actually. For instance, it's not uncommon for mics that are very sensitive — like condenser mics — to capture the playback signal from the musician's headphones when recording, especially if the cans are not of the closed-back variety. Another common problem in home studio productions is the tracking of instruments in rooms with dubious acoustic qualities. This results in recordings contaminated by the room sound. This could also be considered a form of bleed.
To make a long story short, nothing is safe from this phenomenon. But why is it so annoying, and to what extent can it damage your mix? That's what we are about to find out...
The blind spot
Mic bleed poses problems during mixdown for several reasons. The first one is pretty obvious: If you put a mic in front of each instrument, it's because you want to record their signals discretely. If the instruments bleed to other mics than those intented to record them, it will at least partially defeat the purpose of individual miking. Take, for instance, a snare track that has some of the hi-hat in it and vice versa. It can be really hard to find a balance between these two elements because increasing one results in the other being amplified as well. Not to mention the issues that can arise when placing an instrument in the stereo field.
Furthermore, bleed can cause phase problems due to comb filtering, which I wrote about in another series. In fact, spill betwen mics can make sound lack definition or punch, and it can even result in the timbre of an instrument being altered.
Another common problem is the intrusion of bleed in the signal's processing chain, which can prove catastrophic. For instance, if the kick track has enough leakage from the snare, the compressor of the kick might be mistakenly triggered, causing a real mess.
Finally, when an instrument isn't being played, mic bleed can result in unwanted noise. That's usually the case when you track the drums together with the bass. Whenever there are softer passages where the drums are very quiet or non-existent, the bass can make some of the drum elements vibrate, which can result in unwanted sounds ending up in the take.
As you can see, mic bleed isn't your friend. The best antidote against it is an adequate placement of the mics and optimal acoustic conditions. However, its nasty effects can be minimized during mixdown with the use of noise gates, as we will see in the next installment.