Microphones Phase Reversal

[wolfman005]

So a bit of sound wave theory/thought from someone who dabbles in audio enough to get himself in trouble...

Lately I have been reading about the Grateful Dead's "Wall of Sound" and how they used 2 out of phase microphones to deal with feedback.

I also happened across some threads about lectern mics and how you can use 2 our of phase mics to reduce feedback.

My question is, how concrete of a feedback elimination solution is this? Because I don't see it being used much when I tag along with a buddy who does band gigs, and at one of the theaters I work at, I don't see it used either. If it's really as simple as swapping pins 2 and 3 (via the push of a button or a miswired cable) and all it requires is a second mic placed within a few inches of the first mic, than it seems like it could benefit a lot of situations. Obviously not all of the situations would allow a second mic to be placed properly, but for a lot of the feedback issues I encounter it seems like it would be a really easy way to reduce feedback.

Any reasons why I don't see this in the wild or have I just not done any gigs big enough to have the budget for extra mics yet?

 

[TJCornish]

Noise cancelling multiple microphone schemes are used every day - your cell phone, conference phone at work, etc. The effectiveness depends on placement of the mics, the desirable source, and the noise you are trying to cancel. It's not generally used in a live setting because it's more effort to set up extra equipment, and there are generally better ways to setup the sytem - i.e. don't put it behind the performers facing into the mics.

 

[wolfman005]

What you said makes sense. When I first read about it it just seemed like a relatively easy to use solution for feedback.

 

[FMEng]

It didn't catch on because it is difficult to use in most live situations. In order for the noise cancelling mic to work, it has to be kept very close to the sound source to keep a high ratio of the desired signal reaching the primary mic capsule, compared to the cancellation capsule. The singer's lips have to practically touch the primary mic, so it takes training, practice and familiarity to work successfully. If the performer doesn't adhere to that, the result will be severe comb filtering or outright cancellation.

A few years ago, Crown tried to revive the concept in a mic they called a "Differoid." The CM-300 and CM-310 were handheld, noise cancelling mics that were discontinued. The CM-311 head worn mic lives on, branded as an AKG when the Borg assimilated them.

On a semi-related note, the TOA AM-1 array mic appears to be a revolutionary product for podiums and tables. The catch is that it costs $2500.

 

[venuetech]

It didn't catch on because it is difficult to use in most live situations. In order for the noise cancelling mic to work, it has to be kept very close to the sound source to keep a high ratio of the desired signal reaching the primary mic capsule, compared to the cancellation capsule. The singer's lips have to practically touch the primary mic, so it takes training, practice and familiarity to work successfully. If the performer doesn't adhere to that, the result will be severe comb filtering or outright cancellation.

A few years ago, Crown tried to revive the concept in a mic they called a "Differoid." The CM-300 and CM-310 were handheld, noise cancelling mics that were discontinued. The CM-311 head worn mic lives on, branded as an AKG when the Borg assimilated them.

On a semi-related note, the TOA AM-1 array mic appears to be a revolutionary product for podiums and tables. The catch is that it costs $2500.

Is this how Lip mic's do their noise Cancelation?

 

[FMEng]

Is this how Lip mic's do their noise Cancelation?

It is essentially how a lip mic, like the Coles 4104 works. The difference is the cancellation is done acoustically instead of electronically. They have a metal bar that rests on the face to help keep the placement consistent. BBC radio field reporters used them a lot. They are still made.

 

[JD]

For the trick to work, one mic must be further from the source. If the person is 6 inches from mic "A" and 12 inches from mic "B", the person's voice will be much hotter on one mic so the dominate signal will come through. Conversely, the monitor may be 6 to 10 feet away and therefore produce the same exact signal on each mic therefore canceling out. The real trick is to use mics that are as close to identical as possible. Also, if they are too far apart, you will produce an audible notch in the mic's spectrum. Takes a bit of playing with but really works well.

 

[cmckeeman]

Just one note, since feedback is a wavelength related phenomena sometimes swapping the phase of a single mic will help eliminate feedback, it isn't a miracle solution but it can sometimes give you those few extra dB before feedback.