Spill (audio)

Spill (also known as bleed and leakage) is the occurrence in sound recording (particularly in close miking) and live sound mixing whereby sound is picked up by a microphone from a source other than that which is intended. Spill is usually seen as a problem, and various steps are taken to avoid it or reduce it. In some styles of music, such as orchestral music, jazz, and blues, it is more likely to be accepted or even seen as desirable.
Occurrence.
Spill occurs when sound is detected by a microphone not intended to pick it up (for example, the vocals being detected by the microphone for the guitar). Spill is often undesirable in popular music recording, as the combined signals during the mix process can cause phase cancellation and may cause difficulty in processing individual tracks. It also causes difficulty in overdubbing, where the spill from the sound being replaced may still be audible on other channels. For sound reinforcement in live shows, mic bleed can make it hard for the sound engineer to control the levels of the different instruments and vocals onstage. For example, if an electric guitarist's loud amplifier is bleeding into the drum and vocal mics, it may be hard for the sound engineer to reduce the volume of the guitar in the onstage mix.
Spill can also introduce sounds which are not desired as part of the recording or live sound mix, such as the sound of a squeaking piano pedal, the clacking of keys on a bassoon, or the rustling of papers on a public speaker's podium.
Spill is sometimes accepted or even desirable, particularly in classical music recordings, as it can create a natural sound between instruments. A guide to orchestral recordings notes that an ″...advantage in using a ribbon mic on the brass is that...there will be a slight pickup of the strings on 
those mics which gives you a nice depth of field on the strings due to mic bleed (i.e., strings 
bleeding into the brass mics on the other side of the stage)″. For some classical recordings, the spacious sound of mic bleed is simulated. The ″SPACE programming module... uses delay and phasing to emulate positioning and microphone bleed within a heavily multi-miked sound stage that has come to define that [orchestral] film score sound we are all used to hearing″.
This is also true for drum recording and productions that need a "live" feel. Whereas pop songs are often recorded one track at a time, for jazz and blues and other improvisation-based music, it is often desirable to have the band perform together, because this creates a better "feel" and more "swing", and because the musicians typically "feed" off each other's ideas in real time. For example, in a jazz tune, the "comping" musicians will alter their improvised accompaniment in response to the solo lines played by the saxophone player; as well, in some cases, the comping musicians will introduce melodic or rhythmic ideas which are picked up by the solo improviser.
In Jamaican reggae and dub, mic bleed is purposely used in recordings.
Spill is often experienced with vocal recording, when the accompaniment is monitored through speakers or open-backed headphones. It may also be a problem in studios without talkback facilities.
Avoiding spill.
Recording engineers and live sound engineers aim to avoid spill by:
Spill is also avoided by using a 3:1 distance rule of thumb, which states that for each unit of distance between a sound source and its microphone, other microphones should be placed at least three times as far.
Other methods of minimising spill include the use of noise gates.
Examples.
Spill is evident on The Beatles' song "Yesterday", where Paul McCartney overdubbed his lead vocal. He had originally recorded acoustic guitar and vocals together on different tracks, though the spill of vocals onto the acoustic guitar track gave an effect similar to double tracking.
Spill can also be heard on the vocal track of Christina Aguilera's "Beautiful". Engineer Dave Pensado said that although the vocal track contained spill from Aguilera's headphones, the "bleed is honest", which suited the song as it was "about being beautiful and honest in every way".
The effect can also be observed when operating a vinyl record player in the same room as the loudspeakers. Depending on the construction of the player the sound may acoustically couple into the record player's dust cover or other mechanical parts and cause a feedback loop into the magnetic cartridge.