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Mike Mulcahy

Phase Interference questions

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Hi,

My general understanding of this phenomenon is that mics equally spaced apart in relation to a sound source, if mixed together, result in a degraded audio signal. For example, here is Audio-Technica's explanation: http://www.audio-technica.com/cms/site/f54c2e7c578b36f7/index.html

My questions:

1. If the two mics consist of a lav (1' from actor's mouth) and a directional mic on a boom that moves to 1' (very close miking) would phase cancellation occur? I am assuming it would...

2. If one iso'd such mics, would they also produce cancellation if later mixed or layered in relation to each other?

 

Thanks in advance,

 

Mike

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1. Cancellation would occur when the sources are 180 degrees out of phase, which is unlikely. What is likely however, is that they will be slightly out of phase, which will give that slightly "hollow" sound.

2. Yes, but if the post mixer wants to mix the two, they can slide the second track (or use a delay) to align it with the first.  

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Of course "cancellations", sometimes referred to as "comb filtering", can occur with a single mic from waves simply reflecting off a hard surface (walls, floors, ceilings, desk, etc.)  and entering the microphone's capsule slightly delayed from the direct sound.  Fortunately for us, this perceived 'hollowness", provided it is not extreme, may be quite appropriate for the scene.  In fact in a classical music recording, the natural reverb of the room (which is nothing more than waves reflecting off of hard surfaces in the room and entering the microphone delayed) is considered quite desirable, provided it is not excessive. Certainly some location mixers, when tracking a "wide shot" where lavs must be used, like to mix in some of a boom mic, well outside of the frame, even though it may be unusable by itself, because in doing so it adds back some of these "reflections" to the mix and provides a more nature sounding track that is otherwise often lost when using only a lav.  In real life we hear these reflections all the time.  When using a microphone they may be more noticeable, and as a result we often seek to minimize them, but in our business we would never want to completely eliminate them.  Doing so would yield a very dry and unnatural sounding mix.

Tom

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Just to clear up one potential misunderstanding here. Phase cancellations happen when a source is picked up by two mics at *different* distances. If a voice, say, is picked up by two microphones that are exactly the same distance from the voice, then no cancellation would occur. This ignores the stuff Tom brought up about reflective surfaces. And, as Johnny pointed out, for complete cancellation to occur, you'd need one mic out of polarity with the other. 

In most real world situations, you'll have two mics that are NOT the same distance from the source, and thus, you'll get some comb filtering, giving you the "hollow" sound mentioned above. And as either the voice moves in relation to the two mics, or one of the mics moves, the sound will change due to differences in phase interactions. 

Yes, if you ISO either of the mics, you won't hear it. It only happens when the signals are mixed together. 

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Hi,

 

Righto. I've been looking carefully at a film I am working on that incorporates interview footage, with a lav and a boom for the talent. The boom track seems to be about 5 subframes (in pro tools) later than the lav track.

I'm not "sure" I can hear that difference - soloing each track and then listening to them both - but I've been moving the boom track to align with the lav (as much for practice as for effect).

 

Thx,

 

M

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Since relative mics/mouth positions constantly change, mixing both mics is rarely a good idea as it will result in moving comb filter frequencies- an ugly sound at best.  This can't be corrected by moving one track relative to the other in the timeline.  Typically, for various reasons, one source will be more appropriate and the other can be feathered in for problems (clothing, distortion, off mic, etc.)   On rare occasions I've delegated the two tracks to different frequency bands and been able to use them together.  A good quality boom mic and similar sounding lav make it that much easier.

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