Eric's second-order soundfield microphone looks very good for recording and down mixing to other microphone patterns. I mostly use the tetramic that I have to produce crossed 8 recordings.
Looks like Erik have found a very good way to capture signals for using as source for virtual mic recordings. There is now of course a need for a software in the likeness of the of VVmic :-) If we can find or select for microphone elements that are reasonably well matched, do anyone have an idea of how necessary individual calibration would be, as we now have for the tetramic? I am really thinking of the possibility of DIY construction of the second order microphone as it looks like the design have very good inherent designed capabilities, or am I wrong? Can a skeleton be created for printing by a 3D printer if we can choose an electret element that is good enough ? I do wonder if it is possible to have a low signal matrix between the microphones and the recorder to create a more ideal first order mic, or just decrease the needed recorded channels to 6, this to match a 6 channel Tascam DR-680 :-). From me you see a very clear sign of the "want it now at a low cost" syndrome as we see in a lot of places nowadays :-) I really enjoyed reading the paper. Best Regards Bo-Erik Sandholm -----Original Message----- From: sursound-boun...@music.vt.edu [mailto:sursound-boun...@music.vt.edu] On Behalf Of Eric Benjamin Sent: den 29 april 2013 22:08 To: Surround Sound discussion group Subject: Re: [Sursound] what mics do you use? 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 ---- _______________________________________________ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound
