On 2011-07-23, Robert Greene wrote:
I feel a little diffident in commenting on this in the presence of so many experts on the Soundfield mike in theory as well as in practice, but unless I am misunderstanding how it works, there are VERY serious problems of other kinds with using it at the kinds of distances (fractions of a meter less than 1/2 , much less often enough) where proximity effect becomes really major.
That is true as well. I seem to remember the original impetus for the reactivity and sound intensity talks a few years back eventually proved to be not so much about sound fields, but about the poor response of the classical SoundField design towards surrounding soundfields which had higher order components. Rejection of directional aliasing from the higher order components of the local soundfield, that is. Which is precisely which progressively mounts up when you bring a small source up close, and/or have a closeby reflective geometry (such as Angelo did in his car work, then).
It proved that SoundFields have no problem with out-of-phase zeroth and first order components, but in fact responded remarkably accurately to them; the only excluded explanation was directional aliasing rejection. Which in retrospect should be rather self-evident, shouldn't it? :)
Namely, as I understand it, the way the B format signals are built is predicated upon the distances among the four capsules being quite small compared to the distance of the source, for the following reason:
That, but not only that. They're also physically extensive, with an innate directivity of their own.
Compensation is needed for the fact that the capsules are on the faces of a tetrahedron, not coincident and all at the center.
Yes, and this is based on them having the cardioid characteristic, plus being approximately coincident. That is only an approximation in total, especially at the higher frequencies. Thus, the overall mic won't totally reject higher harmonics. Again, we've gone over similar considerations with Filippo in the past, just considering spherical clouds of omnis. I think we even got into topology, with the difference between just spherical clouds, and clouds on top of an acoustically opaque sphere. ;)
So it seems to me(and I am prepared to be all wrong!) that the Soundfield mike could not be expected to work at all well except when the source is quite far away--a matter of meters, not inches.
Long story short, I suspect very much the same, and I think there's already good empirical showing for the problems that follow, even on this very list.
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