I went to see a musical in London and they had two singers singing at the same time into each other's faces but there wasn't any bleed or colouration in the sound. But when I watch news shows that have two presenters I can hear the Mic bleed with each presenter being heard in the other presenters microphone. How do theatre sound engineers avoid this?
Musical theatre, more often than not, uses head-worn lavalier microphones that are positioned very close to the mouth. This gives a much greater signal-to-noise ratio for the Front-of-House mixer to work with.
These mics are often flesh-coloured so that they are almost impossible to see.
Lavalier mounting in musical theatre is a craft that requires an immense amount of skill to get right, both in positioning the mic correctly from a technical standpoint and also from an aesthetic standpoint so that they cannot be seen.
Popular places to mount lavaliers are on the forehead in the hairline and around the ear and resting on the cheek. Some productions will use a small wire boom that is mounted behind the ear, allowing the mic to be positioned just next to the mouth.
All of these very close positions will give excellent signal to noise ratio. You will still get bleed, but it will be much lower in the signal compared with the direct sound coming from the mouth.
Conversely, TV presenters use lavaliers mounted on clothing, such as suits or shirts. The distance from the mouth is much greater and consequently, the amount of spill in the signal is much greater.
A forehead mic, though, does actually have bleed. You're singing right into somebodies face, and right into the mic. The person facing you might actually have a more direct (and possibly closer) route to the microphone than the person wearing the microphone. A cheek mic, on the otherhand, is about 15cm closer to the mouth, and is much more direct to the person it is intended on picking up.