phase cancellation and natural reverb

Schaddaddy

Schaddaddy

New member
What is the relationship between naturual reverb of a room and phase cancellation? I've read articles where people said reflection of sound waves on the wall in to the mic can cause some phase cancellation. Does that mean putting mics around the room to capture the natural reverb of the room cause phase cancellation?
 
Schaddaddy said:
What is the relationship between naturual reverb of a room and phase cancellation? I've read articles where people said reflection of sound waves on the wall in to the mic can cause some phase cancellation. Does that mean putting mics around the room to capture the natural reverb of the room cause phase cancellation?
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Hi there,

Phase cancellation isn't really a massive concern with stereo miking or with room ambience mics because phase differences are really what make stereo, or ambient, recordings sound the way they do.
Don't get me wrong, you'll still want to move the mics around to find the best sounding positions, but your job isn't to completely avoid cancellation.

It's more of a concern when you're miking several instruments with several mics in close proximity to eachother, or when you're miking a source with more than one mic in close proximity to the source.
If you had two mics on a guitar cabinet, you might want to avoid cancellation by keeping the distances the same, see?

Polarity is a different thing all together and the "phase switch", which is really a polarity inversion, should be used when you're miking one source from two sides, like top and bottom of a snare, or front and back of an open back cabinet.

Hope that's useful.
 
Problematic phase cancellation happens when you have your drum overheads too close to your basement ceiling, or you little combo amp sitting flat on the floor with the mic in front of it, or your vocal mic too close to the wall. That is, all the time. It absolutely is worth having some understanding if for no other reason than to waste less time "looking for the right spot" by ruling out some of the wrong ones via logic.

But it has nothing to do with what we usually call reverb. It's pretty much always the first reflections that are the big problem, and usually worse when the distance that reflection takes to the mic isn't a whole lot longer than the distance the direct sound takes. Once it's bounced around the room a few times it will no longer correlate closely enough with the direct signal (because the direct has moved on to something else, unless you're recording pure sine waves) to have a "phasing" effect.
 
Don't forget that for phase interaction to be audible the two waves to have about the same amplitude (within 6dB). Sound's power falls off by 6dB per doubling of distance. First reflections may have enough power to interact, but after that the sound is generally too low for the phase to matter. Of course this all depends on the placement of the source and mic relative to boundaries, and the nature of those boundaries.

Yes, placing mics around the room will result in some kind of phase interaction. It will depend on where you put them and how you mix them.
 
To add, first or prominent close reflections you're asking about can be simple and steady state reflections. If they're both short enough and strong enough- they are heard as 'connected' part of the original (within the Haas zone time wise), along with the strong comb filtered notches that go along with that.
Reflections associated with room ambiance- although there can be a lot of crossover between these two and can strongly color from a small room, imply both multiple and random reflections. It's the randomness (and lower levels hopefully) that average out the notching and peaking for a more useful effect.
 
Schwarzenyaeger said:
Holy s., is this true? I've read so much about phase stuffs and this never came up.
Wow.
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That is exactly what that '3:1 thing is all about -all be it expressed as relative distance to attenuate between two equally loud sources only.
Keeping any (short- within the Haas zone particularly) delay below about - I thought it was -9'ish whatever, the combing' isn't as deep and isn't heard so much as destructive.
Same same as your 'depth control on a phase shifter effect.
 
Yep, 3:1 distance works out to about 9db, and that is the rule of thumb because then the quieter one can only contribute about 1db either way the final output, and 1db is generally considered to be "barely audible".
 
ashcat_lt said:
Yep, 3:1 distance works out to about 9db, and that is the rule of thumb because then the quieter one can only contribute about 1db either way the final output, and 1db is generally considered to be "barely audible".
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There you go. Didn't know how it actually worked out. Likely, there's plenty of room for fudge' depending on the source in question. For a vocal, yeah we might want to keep it pretty pure' in that respect.
But guitar cab tracks, who's to say what the tone 'ought to be'. Drums in particular- we include phase coloring all the time (almost can't avoided other than an XY pair and kick maybe)
Notice how on most 'how do I check for my drum multi-mic phase', it typically boils down to polarity for the thickest low end on the drums, but from the mid range on up, for the most part, you can only juggle partially in and out of phase combinations.
 
With a coincident pair of overheads you can get things pretty much in phase. Using ideas like 3:1 etc. you can largely isolate your close mics from the bleed, getting it low enough to disregard. I spend 3 minutes to time align close mics to the overheads and it's essentially free of audible phase interactions.
 
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