I'd like to expand just a bit on what Dave said. The narrowing of the pattern of microphones at high frequencies is equivalent to the addition of higher order spherical harmonics into the directionality. I recently went through the exercise of decomposing the pattern of a 1" capsule into its spherical harmonics and it took up to 10th order to do a good approximation at 16 kHz. If one were to derive either an omni (monopole) or a Figure 8 (dipole) by adding or subtracting capsules then half the higher order harmonics would remain, resulting in a polar pattern that differs greatly from what was desired. This is true even assuming that you could make the capsules coincident, which you can't. A mental model of a soundfield microphone at HF is of four beams pointing out into space from the locations of each of the capsules.
The non-coincidence is of course a separate effect. If we were to use perfect, point-sized capsules then they could conceivably have perfect cardioid patterns. But the spacing effects are still there. I've measured most of the available soundfield microphones to determine the value of r. It's a little difficult because the center of the array isn't available, but one can measure from the center of one diaphragm to another and get r from that. If the capsules are cylinders of length l and diameter d, then r = l +.2887d SF MkIV and MkV1.47 cm (from literature) SF SPS2002.71 cm (from measurement AGM MR1 and MR22.27 cm (from measurement) Tetramic1.77 cm (from measurement) Note that r for the SPS200 is almost twice the value for the MkIV type design. Long capsules make things worse! I've built prototypes here with r = .7 cm, but none of those are ready for use. Finally, Aaron Heller and I presented two papers at the 133rd AES convention that deal with some of these matters, in particular the diffuse-field response. They are: Calibration of Soundfield Microphones using the Diffuse-Field Response http://www.aes.org/tmpFiles/elib/20130429/16453.pdf A second-order soundfield microphone with improved polar pattern shape http://www.aes.org/tmpFiles/elib/20130429/16470.pdf I hope that the illustrations in these papers will make clearer what we've been talking about. Either Aaron or I will be happy to send a copy to anyone who is interested. Eric Benjamin ----- Original Message ---- From: Dave Malham <[email protected]> To: Surround Sound discussion group <[email protected]> Sent: Mon, April 29, 2013 9:28:32 AM Subject: Re: [Sursound] what mics do you use? Ok, you have two problems with large capsules. Firstly there's the standard one of the basic directionality going off. The directional patterns of any capsules degrades as the frequency goes up, due to interference effects, and this happens at lower frequencies with larger capsules. Secondly if you are deriving B format signals (or anything similar) from a capsule array, the wider the separation the lower the frequency at which the derivation fails which is why the tetramic produces such good patterns to such high frequencies compared with the actual Soundfield. However, the larger capsules of the Soundfield are a lot quieter and "nicer' simply because they are based on better quality and larger diaphragm capsules - so, you pays your money and makes your choice. Dave On 29 April 2013 15:56, Ronald C.F. Antony <[email protected]> wrote: > On 29 Apr 2013, at 02:33, Dave Malham <[email protected]> wrote: > > > but the 30+ > > mm size will seriously mess with the high frequency response f any > derived > > horizontal only B format. > > > Could you please elaborate on the expected effects from the larger > capsules? > Trying to figure out if that would result in something one can live with, > or something that turns it useless. > -------------- next part -------------- > A non-text attachment was scrubbed... > Name: smime.p7s > Type: application/pkcs7-signature > Size: 4853 bytes > Desc: not available > URL: < >https://mail.music.vt.edu/mailman/private/sursound/attachments/20130429/7898aef7/attachment.bin >n > > > _______________________________________________ > Sursound mailing list > [email protected] > https://mail.music.vt.edu/mailman/listinfo/sursound > -- As of 1st October 2012, I have retired from the University, so this disclaimer is redundant.... These are my own views and may or may not be shared by my employer Dave Malham Ex-Music Research Centre Department of Music The University of York Heslington York YO10 5DD UK 'Ambisonics - Component Imaging for Audio' -------------- next part -------------- An HTML attachment was scrubbed... URL: <https://mail.music.vt.edu/mailman/private/sursound/attachments/20130429/7ca662bc/attachment.html> _______________________________________________ Sursound mailing list [email protected] https://mail.music.vt.edu/mailman/listinfo/sursound _______________________________________________ Sursound mailing list [email protected] https://mail.music.vt.edu/mailman/listinfo/sursound
