Very cool Bruce. Perhaps with some technique suggestions from our
other test bench experienced folks we could at least come-up with a
fairly accurate method to get _relative_ results like you've done
between the 722 and 671 for all the recorders we could mass in one
spot. Your 125 vs 102 sounds more like what people have been
describing. Rob D.
At 10:01 PM -0600 4/4/06, Bruce Wilson wrote:
>Sorry if this bores any of you, I know it's getting rather technical.
>
>Here's how it's done. You need a 150 ohm resistor, a sound editing program
>with good dB scales or analysis abilities like Audition, and you must know
>your mic preamp gain.
>
>Place the resistor across pins 2 and 3 of your preamp/recorder (turn off
>phantom power). Turn the mic trim (level controls) all the way up (so they
>are not attenuating the signal at all) and record the faint noise. If you
>don't have the resistor then use a wire or leave it open, all the
>measurements will be close, but only 150 ohms is the standard.
>
>Pull the recording into the audio editor. If you have a way to limit the
>bandwidth to 20-20k exactly, do so. If you can A-weight the audio you might
>do that too. I built a 23-20k FFT filter to do this, flat-topped in the
>passband. Play the noise, and watch the meters to estimate the level of the
>noise.
>
>To determine the EIN, you must add the noise just measured to the gain of
>all stages prior to recording. The result will be in dB, but I don't know if
>it's possible to put the dB's on a specific reference, like dBu. To do that
>you'd need a very good rms voltmeter to measure the output of the preamp
>stage, then you could put the noise measurement on an absolute scale.
>
>On my new 722 (woo hoo!) using a wire (not a resistor) I get a peak reading
>of -55 dB FS. In the analysis menu I get an average of -66.7 RMS. If I trust
>the 70 dB gain figure quoted in the 722 literature, then the EIN is -125 dB
>peak, and -136.7 dB average. This is compared to the thermal noise of a wire
>at room temperature of -145 dBu or so. Close to the quoted figures of -131
>dBu for the unweighted 50 ohm EIN.
>
>For my older 670, the noise measurement (using the wire) is -54 dB
>peak, -59.3 dB average rms. Using the 48 dB gain value calculated form the
>1.2 mV MIC to 300 mV LINE difference in the specs, the EIN is -102 dB
>peak, -107.3 dB average. This is quite a bit above my calculated value
>of -121 dBu.
>
>If you don't know the preamp gain then you'd need to measure it by inserting
>an audio tone of known rms voltage into the mic input, record at full volume
>(zero attenuation) and observe where it ends up during playback. You could
>then calculate the gain. You could then calculate the EIN in reference dB
>units, like dBu or dBV.
>
>I need to go get a 150 ohm resistor now.
>
>Bruce Wilson KF7K
>http://science.uvsc.edu/wilson
>
>
>
>
>"Microphones are not ears,
>Loudspeakers are not birds,
>A listening room is not nature."
>Klas Strandberg
>Yahoo! Groups Links
>
>
>
>
--
Rob Danielson
Film Department
University of Wisconsin-Milwaukee
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