Raimund Specht wrote:
>I agree that the short ticks of a clock are probably a bit too
>variable for an exact gain matching of the different recorders.
>
>However, even a 1 kHz sine signal played through a speaker in a more
>or less reverberant room might cause other problems. The delayed
>short-term echoes will produce interferences (superposition) that can
>also lead to relatively high measurement errors.
True; but all that's needed is a way to match the sensitivity of two
mics, for example. If the geometry is kept the same it ought to work.
Quarter-wavelength of 1K is six inches?
>An even better calibration method would be to bypass the acoustic path
>and to use an electrical reference signal, fed directly into the
>microphone input. In this way, it is also possible to determine the
>absolute input sensitivity (the input clipping level or "gain") of the
>recorder. This is what I did in my tests. I played a synthetic 1 kHz
>sine signal through the head phone output of my (battery-operated)
>Edirol R-09. I measured the rms voltage of that test signal by using a
>common multimeter (435 mVrms =3D -5.03 dBu in my case). I then connected
>the R-09 output with the microphone input of the recorder by using a
>simple voltage divider (two resistors, 100000 and 150 Ohms, providing
>an attenuation of -56,5 dB; the 150 ohms resistor connected to the XLR
>pins 2 and 3). This electrically recorded test signal creates a
>certain dBFS level (for instance -12dBFS) in the recorded .wav files,
>which can be used to normalize the recordings across the various
>recorders.
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.
>Nevertheless, I would claim that even a subjective clock tick
>calibration method by ear is probably sufficient for our purposes.
>Yes, it is true that there would be some uncertainty in the range of 1
>or 2 dB. However, such small noise floor differences should be
>irrelevant when using the equipment under real-world conditions in the
>field. For instance, if you managed to half the distance between the
>microphone and the subject, you would improve the signal-to-noise
>ratio by 6 dB.
I agree that one or two dB is getting to the insignificant level, but
I'd want to get measurements as close or closer than that, so I'll
know what gear differences are significant and which are not.
>Therefore, I do believe that the influence of the recorder and even
>the microphone is small compared to the specific circumstances
>(including personal experience, luck and patience) in the field.
Yep, being in the right place at the right time and pushing the right
button* trumps all gear considerations. Mic specs come next, tho.
-Dan Dugan
* a few days ago I set up a 4-channel array with two MD recorders to
record a dawn chorus at Cathedral Grove in Muir Woods. I was pleased
that I started my recording just 5 minutes before the birds. When I
came back after 1:10 and did my tail synch click, I learned that when
I had put the front channels MD back in the pocket of my vest (on a
stand), I had inadvertently put it back into pause. 11-second
recording. D'oh!
The rear channels were nice.
-dD-
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