Comb filtering isn't just caused by reflections. You'll get comb filtering even if the recording was made in an anechoic chamber.

That's because two microphones at different distances are always going to catch some frequencies out of phase. The comb filtering occurs not in the room but later, when you mix the two tracks together.

That's why I keep emphasizing that I am speaking about a circumstance where you have aligned the tracks before you observe how they mix together.

It's also why I mentioned the ideal of a single point source rather than the practical reality of encountering a 113 square inch speaker cone and some cabinet resonance. 

Just thinking out loud.

best regards,
mike

I think I tend to agree with Mike on this one. Anechoic chamber means no reflections at all Dave so what Mike is saying is that after aligning up the two tracks there would probably be no comb filtering effects at all.

But why are we even talking about anechoic chambers at all. They are not realistic recording environments so there is not much point in talking about them. What we are dealing with is real world situations in managing multiple microphones picking up the same sound in a room of some sort. How best to get them sounding good. It comes down to your ears and judgements in the end.

The reason I brought it up is that:

1) It was an idea that flashed through my mind and I was asking for confirmation of the idea... and I'm still open to correction.

2) I suspected that if we agreed that a single or primary source can be aligned than we might be able to agree that the SPL from the reflections etc. may, or may not be, a relatively insignificant portion of the whole. This possible realization might be useful when we consider where and when we find that reflections etc. cause inordinate amounts of undesirable comb filtering.

Consider these examples. If you mic a single 12" speaker guitar cabinet with 1 mic 18" from the cone and a second mic 6 feet from the cone, the results will be very different if the amp is placed out in a sweet spot of a 40' x 70' room than if you have the amp and mic squeezed in to 10' x 11' room.

In other words it's not necessarily the difference in distance between the source and the two mics but rather a difference in distance from the source and the early (please recall that the "early" reflections have the greatest amplitude.) reflections that causes the practical issues.


That's why I am speaking about this... because I think maintaining some context may be helpful to understanding. :-)


As an aside... there are circumstances where you can work out doors and find that the reflections are entirely insignificant... they are simply too low in SPL. Beach sand and an ocean make it all just go away. :-)


best regards,
mike

I think you're right. It would seem that moving a track forward in time by 1ms would have exactly the same effect as moving the microphone a foot closer to the source -- ignoring room reflections, of course. And assuming that the two microphones are aimed squarely at the source from the same angles.


You'd have to adjust the timing in such a way as to duplicate the effect of having the microphones at identical distances from the source. On a guitar cabinet this would seem to defeat the purpose of using two mics. 


On drums, it might well be impossible because of the complex relationships between multiple microphones. There can be no overhead correction that's appropriate for all the individual instruments. Aligning the overheads to match the snare mic would not give the proper alignment for kick, tom or hi-hat microphones.