At 8:31 AM +0000 5/9/07, Raimund Specht wrote:
>Dan Dugan wrote:
>
>> Calibrating the input to the exact same gain is useful, but it
>> doesn't give you a way to match the sensitivities of different
>> mics--I think that has to be done acoustically. Since mics can't be
>> fitted into a sound level meter calibrator, a tone at a measured
>> distance seems to be all that remains.
>
>Yep. The electrical calibration method that I explained would of
>course only make sense when one used the same microphone in exactly
>the same arrangement (a setup in that only the recorder is being
>exchanged). I think that this is what Rob D. is actually doing.
>
>If one also wants to compare different microphones, things would get
>even more complicated. I think that one would need a professional
>anechoic test chamber to get reliable results. Otherwise, the room
>acoustics (reflections from other directions) would introduce errors
>that are caused by the different pick-up patterns of the various
>microphones.
>
>Regards,
>Raimund
About two years ago I used my 8 channel RME audio card and three
mp2's to record 6 mics simultaneously. I ran a 1K tone to match the
output levels from the mics and placed them 20+ feet in the air to
record middle of the night urban presence. The differences in the
frequencies produced by the mics at the lower end were profound. Then
I took the mkh-40 and ran a 1K Hz signal through it and matched the
output of all of the mp2 channels to probably within .5dB. I
connected the 6 mics again and compared the sound files they created.
The huge differences in low-end response made me realize how narrow
the standard sensitivity rating method is. I recall that my MBHO mics
rated at 12mV/Pa produced a sound file with 3dB more saturation than
the file produced by my mkh40 rated at 25mV/Pa. Similarly for the
NT1-A's-- they are rated at 25mV/Pa but they are very sensitive under
30 Hz. At 500Hz the NT1-A's are about 6dB less sensitive than my
mkh40-- also rated at 25mV/Pa.
Based on my assumptions about what can make ambience recordings
unique, I think it would be ideal to include the effects of
reverberations in mic and pre performance tests. Stereo imaging is
pretty fundamental as well. A challenge with stereo is that any one
mic array will benefit some mics and detract from others. I keep
imagining a very large, outdoor lab where one can enjoy very low
background levels and engage meaningful distances from sources and
the ground. Rob D.
>
>
>
>
>"Microphones are not ears,
>Loudspeakers are not birds,
>A listening room is not nature."
>Klas Strandberg
>Yahoo! Groups Links
>
>
>
--
Rob Danielson
Peck School of the Arts
Department of Film
University of Wisconsin- Milwaukee
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