The radius of the tetrahedral array determines the frequency at which the B-format polar patterns start to breakdown. The formula given by Gerzon is c/(pi*r), where c is the speed of sound and r is the radius of the array. Depending of the design, the acoustic radius is about 10% larger than the physical radius, because the sound has to diffract around the structures. So, in round numbers 10/r kHz, with r in cm. In a Soundfield mic, the physical radius 1.47cm, so around 6.8 kHz. The Octava is over 4 cm, so less than 2.5kHz. Note that very small capsules tend to be noisy, so there is a tradeoff between noise and integrity of the patterns at high frequencies.
In many of the 3D printed designs, the array is not open enough and the interior space behind the capsules becomes a resonant chamber. This causes peaks, dips and phase shifts in the response of the individual capsules that are difficult to correct and affect the resulting patterns. There is also the general geometry of the microphone body that tells you how much care went into the design in terms of acoustic shadowing, reflections, and diffraction. The large flat surface on the top of the preamp enclosure in the Octava does not look good to me. Part of the magic of a tetrahedral microphone is that the free- and diffuse-field responses track each other. To achieve this, it is important that the directivities of the four capsules are well matched . Calibration can compensate for this to some degree, but the better the capsules match, the better the result will be. The only way to do this is have a large collection of capsules, measure them individually, pick sets of four, and then calibrate the entire array. I know that Core Sound does this (and Calrec did this). I don't know about other companies. In general, I am suspicious of any tetrahedral mic that uses generic A-to-B conversion, with no individual calibration.  M. A. Gerzon, "The Design of Precisely Coincident Microphone Arrays for Stereo and Surround Sound," 50th AES Convention Preprints, London, no. 20, 1975.  A. J. Heller and E. M. Benjamin, "Calibration of Soundfield Microphones using the Diffuse-Field Response," 133rd AES Convention Preprints, San Francisco, no. 7811, 2012. On Sun, Mar 11, 2018 at 3:13 PM, Peter P. <peterpar...@fastmail.com> wrote: > > * Len Moskowitz <lenmoskow...@optonline.net> [2018-03-11 18:48]: > > Gerard Lardner wrote: > > > > > Fons Adriaensen in Italy calibrated my Oktava. I believe Richard Lee in > > > Australia might still offer a calibration service, though he appears to > > > be less active on the internet these days, and I think Core Sound in the > > > USA also will do it - they used to say it on their website, but I > > > haven't checked lately. > > > > We could, but in general we can confidently state that Oktava doesn't > > understand how to build a first-order ambisonic microphone, and the cost and > > effort to calibrate it is not worthwhile. > > Thank you for your opinion Len. I am tempted to ask 'why' but let me > ask instead what are the most difficult things to get right when > building a first-order microphone. > > best, P > _______________________________________________ > Sursound mailing list > Sursound@music.vt.edu > https://mail.music.vt.edu/mailman/listinfo/sursound - unsubscribe here, edit account or options, view archives and so on. -------------- next part -------------- An HTML attachment was scrubbed... URL: <https://mail.music.vt.edu/mailman/private/sursound/attachments/20180312/16e9f315/attachment.html> _______________________________________________ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound - unsubscribe here, edit account or options, view archives and so on.