When the test oscillator is switched on, the levels at all four B-format outputs should be the same and the meters should all read 0 dB (modulo the interruptions for channel identification). However that doesn't tell you much about the calibration of the capsule signals to B-format circuitry, and as Dave and Jorn point out, that's the critical thing in a Soundfield mic.
The test tones are inserted into the record amps (the amps that feed the B-format outputs) and are provided to align the record and replay levels on your recorder and identify the channels. There are gain trims on the record amps and playback amps, as well as the level meters. Setting those should be a straight forward exercise with an RMS-reading voltmeter. As for calibrating the capsule to B-format circuitry, you need a reference mic (e.g., B&K 4134) and a way to do anechoic or quasianechoic measurements. It essentially comes down to nulling the responses of X, Y, and Z at 90 deg, and matching them at 45 degrees. However life is never that simple -- the measurements always have some artifacts, the polar patterns of the capsules vary a bit with frequency, and the response of the upward-facing capsules is a bit different from the downward-facing ones due to diffraction and shadowing from the support structure and housing, so some good taste and experience is needed. -- Aaron Heller ([email protected]) Menlo Park, CA US On Sat, Sep 12, 2015 at 6:09 AM, Dave Malham <[email protected]> wrote: > > Absolutely right, Jörn. The difficulty with Soundfield type microphones is > that the 1st order B format signals are generated from differences between > capsules and these become very small as the frequency goes down which means > more and more make-up gain is needed which means, in turn, that any > differences between capsules become exaggerated. Capsules need to be > matched as accurately as possible over frequency and need to stay that way. > The MkIII workshop manual has (at least) five pages of instructions on the > alignment of the microphone to B format process and tt needs access to a > good anechoic chamber. Calrec used a smallish anechoic chamber for the > mid/high region and a plane wave tube for the low frequencies, if I > remember right. That was all done in the analog domain with preset trimmer > pots - I'm sure there are people (Richard? Fons? Eric?) on this list who > can comment better than me on the more advanced techniques available (and > used) in this digital age. > > Dave > > > On 11 September 2015 at 23:21, Jörn Nettingsmeier < > [email protected]> wrote: > > > On 09/11/2015 01:21 PM, James Anthony Enda Bates wrote: > > > >> Hi everyone, > >> I have a quick question about calibrating a Soundfield MKV. At the > >> moment > >> the test tone oscillators are coming in with the W channel about 3.5 dB > >> lower than X,Y, or Z (these three channels are within about 0.5 dB of each > >> other). > >> > > > > It's late and I've had a long day, but isn't that the expected W gain for > > classical Ambisonics? > > > > So obviously the best thing would be to return the unit to get it > >> calibrated as it's probably due for one; and this has been noted before on > >> this list; > >> > >> http://sursound.music.vt.narkive.com/UtLGYHeC/sursound-level-alignment-of-soundfield-microphones > >> > >> However, my question is, if we know what the gain mismatches are based on > >> the test tone output, is it sufficient to just apply gain adjustments to > >> the b-format recordings based upon these Test Tone signal levels? Or is > >> there a possibility that the test tone channel levels, and microphone > >> channel levels could differ? > >> > > > > The problem is not the B-format output. B-format is very robust wrt small > > gain mismatches - say your Y is 3 dB down, all it does is make the image a > > little less wide. > > > > IIUC, the problem is in the individual capsule gains - if one capsule is > > off by 3 dB (which would be a catastrophic mismatch), _all_ output > > components will be totally warped. > > > > The capsule EQs might have similarly severe consequences, leading to weird > > direction-dependant coloration. > > > > > > > > > > -- > > Jörn Nettingsmeier > > Lortzingstr. 11, 45128 Essen, Tel. +49 177 7937487 > > > > Meister für Veranstaltungstechnik (Bühne/Studio) > > Tonmeister VDT > > > > http://stackingdwarves.net > > > > _______________________________________________ > > Sursound mailing list > > [email protected] > > https://mail.music.vt.edu/mailman/listinfo/sursound - unsubscribe here, > > edit account or options, view archives and so on. > > > > > > -- > > As of 1st October 2012, I have retired from the University. > > These are my own views and may or may not be shared by the University > > Dave Malham > Honorary Fellow, Department of Music > The University of York > 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/20150912/20f2d3f8/attachment.html > > _______________________________________________ > Sursound mailing list > [email protected] > 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... 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