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
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