Recording an instrument and a voice together
Every microphone hears everything in the room; that is bleed, and it is why two tracks fight each other until the geometry is right.
You set up two microphones for a lesson. One on the instrument, one on you. Sensible, and the right instinct.
Then you sit down to edit and find that turning the instrument down turns you down with it, and bringing your voice up drags the instrument along. The two are locked together, and it makes no sense, because they are on separate tracks.
They are locked because a microphone is not a camera. It does not point at one thing and exclude everything else. The microphone on your instrument also hears you talking, and the microphone on you also hears the instrument. That is bleed, and it is the most common reason a two microphone lesson cannot be balanced afterwards.
Why does turning one down not work?
Because you are not turning down one sound. You are turning down two copies of two sounds.
The instrument exists on both tracks: a close, strong version on its own microphone and a weaker, more distant version on yours. Pull the instrument track down and the weak copy on the voice track stays exactly where it was, so the instrument does not get quieter so much as it changes character. It loses the close, clear version and keeps the roomy one. The balance moves and the tone moves with it, in a direction nobody asked for.
There is a second problem underneath. Two copies of one sound arriving at slightly different moments partly cancel each other, which makes the result thinner as well as murkier. That fault has its own post; bleed is what puts you in a position to suffer from it.
How do you reduce bleed without a bigger room?
You cannot eliminate it, and anybody who says otherwise has not tried it in a room with a piano in it. You can reduce it enough that the two tracks become independently useful, and all three methods are free.
Point them away from each other
A cardioid microphone hears least from directly behind. If the microphone on your instrument has its back turned towards you, and the microphone on you has its back turned towards the instrument, each one is rejecting the other's source with the quietest part of its own hearing.
Two microphones facing each other across a room is the worst arrangement available. It is also the most instinctive one, which is unfortunate.
Use distance and angle, not volume
Bleed is a ratio between what a microphone is meant to hear and what it is not. Moving a microphone closer to its own source improves that ratio without touching anything else, because the wanted sound gets louder while the unwanted one stays exactly where it was.
Turning the recording level up does the opposite. It lifts both equally, changes nothing between them, and costs you headroom you may need later. The fix is always a step, not a knob.
Record the empty room
Before you start, or the moment you finish, record a few seconds of the room with nobody playing and nobody talking. Do not skip it because it feels like recording nothing. It is the only accurate description of the noise in that space, and it is what lets the noise be taken out later without part of you leaving with it.
What does a proper redesign look like?
On one music education partnership, the audio problem was diagnosed from the delivered files alone. Nobody was in the room. The placement was audible in the recordings: what each microphone was hearing that it should not have been, and what that was doing to the balance between the instrument and the speech.
That reading was then confirmed with the partner rather than acted on, because a diagnosis made from files can be wrong about a room, and the person standing in it knows things the files do not. Only then was the placement redesigned, so that the instrument and the speech stopped bleeding into each other.
Two things followed. The room tone became cleanable, because there was finally a version of the noise not welded to a voice. And the ceiling on the series went up, because every later decision about balance was now available rather than already settled.
None of that involved better microphones. It was the same microphones, in different places.
Does this mean one microphone is better?
Sometimes, yes, and it is worth saying before you buy a second.
If your instrument and your voice stay in roughly the same proportion, and you never have to raise your voice over the playing, one well placed microphone between the two sources will give you a recording that sounds like a person and an instrument in a room. That is a legitimate result, and it is one decision instead of five.
Two microphones earn their trouble when the balance genuinely has to change: a lesson where the talking must stay clear over the playing, or a series that will be recut at different lengths later by somebody who is not you. If neither of those is true, keep the setup time and spend it on the playing.
What can you change before the next lesson?
Turn both microphones so that neither is looking at the other's source. Then move each one closer to what it is supposed to be hearing, until it sits nearer than feels polite.
Then record two minutes of playing and talking exactly as you normally would, and listen to each track completely alone. If you can hear the instrument clearly on the voice track, the placement is still doing the blending for you, and it will keep doing it until it is moved.
That listen is the whole diagnosis and it costs one take. What it tells you is not whether your equipment is good enough, which is the question most people ask first. It tells you which of the two sounds in your room you have already agreed to stop controlling.
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