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Old 13th August 2019
Lives for gear

Now at this test, I pointed the mic at the speakers, and not the ceiling - should I have done that instead?
There is a difference when the mic is angled, as you already discovered. Intuitively, you'd think that an omni mic would show no difference, but so many things in acoustics are not intuitive, and this is yet another of those things!

A simple (but incomplete/"sort of but not quite") explanation is this: A perfect omni mic would detect only the pressure variations around it, equally in all directions... But the pressure variations are NOT really equal in all directions! First, there's the "ambient" pressure of all sounds in the room, often referred to as the "reverberant field"... and in a perfect room it would arrive at the mic with equal probability from all angles, and that's what you are actually trying to measure here. However, it's a fools errand, because in a small room there is no such random field! It does not exist. It's not random. That only happens in a very large room. Small rooms don't have true reverberant fields... but that's sort of splitting hairs, and doesn't really matter, because what you REALLY want to measure, is whatever is actually happening in the room, regardless of whether or not is is a truly random, reverberant field: it is what it is, and you want to measure it, and you'd think that sticking a true omni mic in that field would capture it perfectly, because it's just pressure changes arriving from all over, and omni mics don't favor any particular direction, so it shouldn't matter which way you point the mic... except that it DOES matter, because... in addition to the reverberant field that is hitting your mic from all angles sort-of evenly, there's also the DIRECT pressure wave being emitted by the speaker, which travels as a spherical wavefront. If your mic is pointing towards that wavefront, so that it hits the mic capsule "head on", then the mic picks up the direct pressure wave strongly. On the other hand, if your mic is pointing at a steep angle such that the pressure wave hits it side-on, not head on, then the capsule doesn't "see" that pressure wave the same. In summary, the mic always picks up the ROOM sound the same, regardless of which way it is pointing, because that sound field is arriving from all angles the same. But the direct sound from the SPEAKER arrives from only one single direction, and if you point the mic in that direction, you pick it up better than if the mic is pointing tangential to the wavefront.

Short story: if you point the mic at the speakers, you are measuring more of the speakers, less of the room. If you point the mic tangentially, you are measuring more of the room, less of the speakers.

This isn't about whether or not the mix is truly omni: it's about the speakers vs. the room.

This is why you often see the recommendation to aim the mic at the ceiling if you want room measurements, or directly at the speaker if you want to measure the speaker response and plan to gate out the room response ("window" it). The problem comes when you want to measure both speakers individually, and also together: Where do you point the mic then? Do you flip int back and forth, pointing at the left speaker to measure that, then re-pointing to the right or that, then in the middle for both? Or do you just leave it in the middle and not get true on-axis? Hmm.... And iff you do decide to flip the mic around all over the place, how do you ensure that it gets back to the exact same spot for every subsequent measurement? Hmmm...

Personally, since I'm usually measuring rooms, not speakers, I angle the mic upwards at about 60-70° degrees, rather than vertical, and this angle is for other reasons that are too complex to go into right now. But that does tend to give the best actual measurement of the room. Assuming that you are trying to treat your room and want to know what it is doing, I'd make the same suggestion here: angle your mic upwards about 70°.

Yeah, not a very good explanation, I know, and the purest acoustician will be rightfully shouting "Not true! That's not really the way it works! It's much more complex!", but I think it's a useful mental picture to help understand the issue.

- Stuart -