On Sat, Jul 23, 2011 at 05:45:48PM -0700, Robert Greene wrote: > To make sense of this jargon, suppose a source is on the line that is > equistant from three of the capsules. Then its distance to those three > will always be the same, and if the source is reasonably far away the > distance to the fourth capsule will be a constnat. This comes from the > Pythagorean theorem limit case in effect: at large distances , the > difference between A to S and B to S is equal to the length of the > projection of the line from A to B onto the line from A to S (or B to S > these being parallel in the limit case). > > If one does NOT have such large distance to the source, the variation of > distances to the capsules will be extreme and also complicated. > Just think of how the distances to the four face centers of the > tetrahedron will vary in odd ways when the source is close by! > > 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. At close distances, there will be wild phase differentials among > the four mike capsule outputs of a kind that depends on the distance > of the source from the center of the mike--something which the mike > does not "know" so that it cannot be compensated for. > > Am I all wet here?
Just a little :-) See also my previous post which hints at this as well. For a classic soundfield mic (using directional capsules), the 'velocity' signals (X,Y,Z) are formed mostly by using the directiviy of the capsules. Imagine they are really coincident. In that case it's just a matter of combining the four signals in such proportions that the sum of the omni components in each of them is zero and the fig-8 ones combine in the right direction. This is still the dominant mechanism if the capsules are not really coincident. There will be a contribution from the finite distance as well, but this will be quite small at low F. It would require a 'Blumlein shuffler' to be of the same order of magnitude as the contribution from the directivity of the capsules themselves. This is what SF mics using omnis on a sphere do - it requires significant gain on difference signals at LF, and without that gain X,Y,Z would be of very low magnitude at LF. In a normal SF mic the effect could become significant if the distance between the capsules is a non-trivial fraction of the source distance AND of the wavelength, so not really at low F. Ciao, -- FA _______________________________________________ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound
