Clean audio is nearly always strived for in any sound recording or reinforcement but is often difficult to achieve due to microphone bleed/spill; extraneous and ambient noise, and other noise factors.
What is microphone bleed/spill? When close-miking any particular sound source, microphone bleed/spill refers to all extraneous noise other than the intended sound source. Common microphone bleed situations include headphones near vocal mics and the sound of adjacent drums entering the mics intended for other drums in a drum kit.
In this article, we’ll look at common scenarios where microphone bleed/spill is an issue; whether ridding of it is necessary, and methods of reducing bleed/spill if need be.
What Is Microphone Bleed/Spill?
Microphone bleed/spill is best described as the sound a microphone picks up from non-intended sound sources. That is, any sound source that the microphone is not primarily set up to capture.
Microphone bleed/spill does not refer to other types of potential noise in a mic signal such as electromagnetic interference, handling noise and self-noise.
To learn more about all the types of microphone noise and how to effectively reduce them, check out my article 15 Ways To Effectively Reduce Microphone Noise.
Rather, microphone bleed is the “noise” from other sound sources that enters a microphone designated to capture a single sound source.
Room microphones and ambient microphones, for example, do not really have microphone bleed since they are set up to capture all the sounds in an environment.
Spot mics, however, are subjected to microphone bleed in non-soundproof environments and/or when there are other sound sources in the environment.
The most common situations for microphone bleed to occur are:
- Headphone bleed in a vocal mic during overdubs.
- Drums/cymbals bleed from a drum kit in individual drum/cymbal mics.
- Bleed from other instruments in a specified instruments’ mic in a live studio room or on a stage.
Headphone bleed is probably the most common form of microphone bleed in studios around the world.
As the name suggests, headphone bleed happens when sound escapes from headphones and enters a microphone. This is a common occurrence in vocal microphone when the vocalist is overdubbing their vocal tracks over the recorded music.
Of course, singing along to the actual recording or, at the very least, a reference track is essential. It allows the vocalist to sing in rhythm and key, which is necessary for the intended results (most of the time).
The issue of headphone bleed arises when one or more of the following happens:
- The headphones naturally expel too much of the headphone signal.
- The headphones are turned up too loud.
- The microphone gain is too high.
So it could be as simple as bad headphone design. Some headphones allow sound to escape quite easily. A nice pair of professional-grade closed-back headphones are typically best for minimizing headphone bleed.
But even with high-quality headphones, headphone bleed can still be an issue.
This is particularly true if the singer requires a loud headphone mix to sing along to.
Alternatively, this happens when the singer is quiet and thus the microphone requires more gain. This makes the mic more sensitive to environmental sound (like the sound emanating from the headphones).
We’ll discuss methods to fix this in the next section of this article.
Drum Kit Bleed
This one is inevitable. Miking a drum kit is difficult work. This is especially true when close-miking each individual drum.
The typical drum kit has both drums and cymbals positioned in very close proximity to one another. On top of that, these percussion instruments are very loud and project sound waves omnidirectionally.
So when it comes time to mic up individual drums in a drum kit, we should expect quite a bit of bleed. For example, the mic intended to pick up the snare drum can be positioned as close as possible to the snare but still pick up the hi-hat very loudly and the rest of the kit with significant signal strength.
So the snare mic is subjected to drum kit bleed from all the other drums and cymbals in the kit.
Close-miking each drum in a large drum kit proves to be challenging because of this bleeding. Phase issues arise when a mic intended for say, the floor tom, picks up the attack of the snare and this mic’s signal is then mixed with the snare drum mic.
To minimize phasing issues, many engineers simplify drum miking to 4 microphones (kick, snare and two overheads) or even less in some instances.
To learn more about phase and microphones, check out my article Microphone Polarity & Phase: How They Affect Mic Signals.
When miking individual tom drums, gates are often used to mute the mic signal unless the tom drum has been hit (and the mic signal strength thus exceeds a certain threshold).
Finally, we have instrument bleed which is very similar to drum kit bleed but deserves its own category. This is because I like to look at a drum kit as being a single musical instrument with different components.
Therefore, if the drum kit is in the same room as, say, a bass guitar (and bass guitar cabinet), then instrument bleed would be the sound of the bass guitar in any of the drum kit mics or the sound of the drums in any of the bass cabinet mics.
Instrument bleed is common in recordings but is usually not a big deal. If the instruments are positioned far enough apart, the bleed can actually help make the mix sound more natural.
In studios with iso-booths, instrument bleed can be avoided by simply putting each instrument in its own soundproof room. This method, however, sometimes ruins the musicality that comes from having all the musicians and their instruments in the same acoustic space.
Methods To Reduce Microphone Bleed/Spill
So we’ve discussed the definition of microphone bleed/spill along with some of the common situations where mic bleed happens. Now let’s talk about ways in which we can reduce unwanted microphone bleed:
- Turn down and EQ monitoring levels in headphones (and even in the loudspeakers).
- Use closed-back headphones.
- Position instruments and sound sources strategically
- Position directional microphones strategically.
- Use gates and/or editing to your advantage.
Turn Down And EQ Headphone Monitoring Levels
With headphone bleed, the most obvious answer is to turn down the headphone send and/or volume. This will effectively reduce the sound pressure level of the sound emanating from the headphones and, therefore, reduce the level of the headphone’s output in the microphone signal.
Alternatively, we could EQ the headphone send so that the overall level of the headphone monitoring can be brought down without overly affecting the perceived volume.
This typically means high-pass filtering the headphone send so that there is less bottom-end in the headphone’s sound. Usually, this will not overly affect the singer’s performance but it may noticeably reduce the headphone bleed in the mic signal.
Try to also dip the EQ of the headphone send/signal where the microphone’s frequency response is particularly sensitive. For example, a mic with a presence boost will be extra-sensitive in the 3-6 kHz range so a coinciding dip in the EQ of the headphone send could help to minimize the amount of mic bleed without overly affecting the headphone signal.
For a related article on microphone and the presence range, check out my article What Does “Presence” Mean In Terms Of Microphones?
Note that too much EQing will lead to bad results. The singer still needs to hear a good mix in order to deliver a proper performance. Use these techniques sparingly and stick to level adjustments unless you feel EQ is needed!
Use Closed-Back Headphones
Professional-grade closed-back headphones are designed to keep external sound out of the listener’s ears but also work well to keep the sound of the headphone’s output within the body of the headphones.
Therefore, choosing a nice pair of closed-back headphones will typically yield better results than a nice pair of open-back headphones when it comes to microphone bleed.
My top recommendation for “low-bleed” studio-grade closed-back headphones is the Beyerdynamic DT-100 (link to check the price on Amazon).
These aren’t the most comfortable pair of headphones and they certainly don’t sound the punchiest. However, for sound isolation and reducing microphone bleed, the DT-100s are a bargain!
For a cheap at-home hack, you can even use earbuds with can style hearing protectors as you would use in a loud workshop.
Position Instruments And Sound Sources Strategically
Avoiding microphone bleed in an acoustic environment that is host to many instruments/sound sources is tough. It’s best to position microphones (and therefore instruments) as far apart from one another as possible.
Another studio strategy, if space is available, is to position instruments in their own iso-booths and rely on headphone mixes for proper monitoring. However, as mentioned previously, this often interferes with creative expression and the performance between musicians.
With that being said, electric instruments can often be routed through the walls of the studio and amplified in iso-booths while the musicians play the instruments in the live room (with the acoustic drum kit, for example).
Using go-bos is yet another strategy. A go-bo is a movable acoustic isolation panel that can be positioned between instruments within the same room while still allowing eye contact between musicians. For example, a guitarist can stand away from his or her amplifier while the amp is effectively isolated in a corner with a go-bo. Headphone mixes may be required when using this strategy.
Position Directional Microphones Strategically
As previously mentioned, room microphones do not really suffer from “microphone bleed” because they are positioned to capture all the sounds and sonic character of the acoustic space.
Rather it is the mics that are dedicated to individual sound sources that are subject to mic bleed because they are positioned to capture one primary sound source or instrument and every other sound is secondary.
For the best isolation of a sound source, directional microphones are recommended. Unidirectional cardioid mics are generally the best bet since they have a rear null point or sensitivity and are therefore easy to “point” in the best direction (toward the sound source).
To learn more about cardioid microphones, check out my article What Is A Cardioid Microphone? (Polar Pattern + Mic Examples).
So along with the strategic placement of the instruments and sound sources, we should also take care to place our microphones strategically.
When close-miking any sound source, our primary concern should be to capture the best sound of the source in question. However, we should also be thinking of how other instruments in the room will be affecting the signal of these close mics.
If possible, try pointing the close-miked unidirectional mics away from loud and/or close sound sources.
Use Gates And/Or Editing
Applying gates or manually muting/unmuting recorded audio can work wonders in eliminating microphone bleed. This method isn’t overly effective on main instruments but works well on elements that are only sounded once in a while during a performance.
The best example I can think of for this method is the close-miked tom drums of a drum kit. I personally use this trick on tom drums in almost all genres of music.
Gate the tom drum mics so that the mic signal is passed when the tom drum is struck. Have the threshold and release set so that the mic signal is attenuated shortly after the tom drum is hit.
This way, the tom drum mics are “activated” only when the tom drums are hit. The microphone bleed in those mics is practically eliminated.
The same goes for manually editing the tom drum hits as part of the drum editing process. This way, we have more control over when the tom drum mics are heard and unheard.
How do you mic a snare? There are many strategies when it comes to miking a snare. The quick answer would be to close-mic the snare to avoid too much bleed from other drums in the kit. Point the mic at the centre of the top of the snare and out of the way of the drum sticks. Also, consider miking the bottom of the snare with another mic with the phase-flipped.
To learn about my recommended snare drum microphones, check out My New Microphone’s Best Snare Drum Microphones.
What is the best bass drum microphone? In my opinion, the best bass drum mic is the Shure Beta 52 (link to check the price on Amazon). This supercardioid large-diaphragm dynamic mic has an incredible low end to capture the boom of the kick drum; a dip in the mids to clear out bleed, and a presence boost to capture the beater attack.
To learn more about my recommended kick drum microphones, check out My New Microphone’s Best Kick Drum Microphones.