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I'm not familiar with the topic of sound. I have ultrasonic audio recordings sampled at 250kHz (16bit) in WAV format. The equipment used to record them is linked here: http://www.avisoft.com/usg/usg116Hn.htm

I've use the matlab programming language to plot the audio signal. While the X axis is just going to be the sample number (divide this by the sampling rate to get time), I don't quite know what the units for the amplitude are.

Typical values (which can be positive or negative) have very high precision and are of the order of 10^-4 or 10^-5. An example amplitude value is:

-3.051757812500000e-05 (at least according to Matlab)

Anyone have any idea of what units I am dealing with? I did not use the equipment myself, my supervisor did. When I asked him what units the amplitude were, he advised I look at the link, but I am not too certain I understand.

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    $\begingroup$ is there a gain knob between the transducer (what we call a "microphone" in less-than-ultra sound) and the A/D converter? do you have specs on the transducer (like look at the "Calibration Chart" in this B&K mic spec sheet). if you're in SI units, your ultrasound is in pascals and the transducer will convert to volts and the A/D converter will convert volts into dimensionless numbers which is what you will see in MATLAB. that is the only way i know to get a handle on the actual units of the actual data. $\endgroup$
    – robert bristow-johnson
    May 13, 2015 at 19:09
  • $\begingroup$ @robert bristow-johnson I know of the recording equipment but I've not actually used the equipment. I've only been provided with the audio recordings. The only thing I know about the equipment is from the link posted. I've read some research papers about some rats ultrasound calls producing 65-80dB, and I wanted to see what the decibel of my data would be. But I don't quite see how I can obtain this. $\endgroup$
    – user3120554
    May 13, 2015 at 19:21
  • $\begingroup$ yeah, i found one page that relates Pa to mV and the page you mention relates sensitivity to dBu (about 775 mV if i recall). but it doesn't say anything about what is "full scale" (which likely is +1 or -1 in your MATLAB file if you're reading in a .wav). $\endgroup$
    – robert bristow-johnson
    May 13, 2015 at 19:28
  • $\begingroup$ another thing, even assuming a reference level, like 0 dB is 20 micropascals (common in acoustics), saying your rat produces 65 dB (relative to 20 micropascals) means nothing without knowing the distance between rat and transducer. there's that inverse-square law to think about. $\endgroup$
    – robert bristow-johnson
    May 13, 2015 at 19:37

1 Answer 1

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Given that the gain of the system and/or ADC seems to be unknown, the scale of the sample values can only be determined from samples of a calibrated source of known value at a known distance from the transducer. With a variable enough gain knob (or AGC) the scale of the samples could be anything.

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