Right you are, Dan. The reason I like that calibration format is
because I am anticipating a much wider dynamic range in the field and
don't want to have to re-adjust and calculate (later) what was done.
I think your point about a 64dBA might be better (that thought hadn't
occurred to me but it certainly makes sense).
My lab is pretty acoustically stable so I'm not too worried about
level readings too much. But I will take a look at the pink noise
range and see how the calibration flies with a 2k top end. Anyway,
these types of calibrations (similar to what we did with Rudy for the
Nat. Park Service Sequoia study) are only accurate, at best, to
within 2 - 3dB, so it ain't exact science.
As for the 722 set point readings, I agree they don't represent
actual dB readings but they are reproducable and stable so that once
you obtain a reading and know the setting readout, if the input pot
has somehow changed position during the course of travel, it can be
accurately reset. That's very cool. If only the Sony DATs and the
Narga IVss had that capability.
The other nice thing about the Sennheisers is that each of my MS
systems (I had three, now two) calibrated exactly the same when
tested. Not a 1/10 of a dB variation between sets of mics. That, to
me, was very impressive and unexpected.
Bernie
>Bernie, you wrote in response to my asking how you were calibrating rigs,
>
>>The Sennheiser MKH30/40 combo calibrates nicely to w/in =3D/-2dB, Dan.
>>On the 722 set the input to -30dB and calibrate a pink noise tone to
>>that level measured at 65dBA at the capsule of the M mic.
>
>I'm not clear from that description what the procedure is; is it this:
>
>Play pink noise on monitor speakers. Adjust the playback level to
>65dBA by reading a Sound Level Meter at a point we'll call the
>measuring point. Substitute the field mic M-S array with the M mic at
>the measuring point. Adjust the recorder's record gain for -30dBFS on
>the record level meter.--?
>
>At this record gain, you could record up to 95dB SPL before peaking
>out. A quiet ambience of say 30dB SPL would be 35dB below 65, so it
>would record at -30 + -35 =3D -65dBFS. That seems a bit low to me, but
>I guess with great gear it's ok. I'm in the habit of recording quiet
>ambiences about 25dB hotter, calibrating 65dBA =3D -5 dBFS, but then I
>have to turn down when something loud is happening.
>
>Perhaps we should use 64dBA as a reference level, to make it an even
>30dB down from the standard mic calibration level of 94dB.
>
>I also band-limit my pink noise to 200Hz-2KHz for calibration. That's
>the practice used in calibrating home theater systems. It reduces the
>variability from room acoustics and speaker peaks. You may be
>measuring dBA, but the recorder is metering full-range, and the
>recorder's gain might be set too low if there's a low frequency peak
>being excited in the room.
>
>You may have noticed that the Sound Devices 722 displays input gain
>momentarily when you adjust the pots. Unfortunately it isn't an
>accurate gain reading, but it does give you reproducible set points
>that you can come back to accurately.
>
>-Dan Dugan
>
>
>"Microphones are not ears,
>Loudspeakers are not birds,
>A listening room is not nature."
>Klas Strandberg
>Yahoo! Groups Links
>
>
>
>
--
Wild Sanctuary
P. O. Box 536
Glen Ellen, CA 95442
t. 707-996-6677
f. 707-996-0280
http://www.wildsanctuary.com
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