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calibration for sound pressure level measurement

Hey! I want to measure wind induced ambient noise levels in dB in forest habitats with AudioMoth to analyze the distribution of frequencies (I am mainly interested in the spectrum 500Hz - 4kHz). Do I need an external microphone, compatible with a common sound pressure calibrator (eg. 94dB at 1kHz) or is it possible to calibrate the built-in microphone, as well? What else do I have to consider?

Thanks for your support,

Vincent

8 answers0 replies

8 Comments

Andy

Apr 21, 2021

Hi Silvy,

I'll try my best to answer these for you.

(i) the sensitivity of the acoustic transducer, which defines how much voltage is generated per unit of sound pressure.

The MEMS mic on AudioMoth 1.1.0 has a sensitivity of -18 dBV/Pa, this is measured when used together with the MEMS output amplifier set to a gain of 10. Here is the MEMS mic data sheet for your reference.

(ii) the amount of additional voltage gain applied by the pre-amplifier.

At the mid gain setting this is 15, below are the gain values of the other gain settings corresponding to the radio buttons in the AudioMoth Configuration App.

Gain

0 Min 4.33

1 7

2 Med 15

3 25.1

4 Max 30

(iii) the ADC input voltage which corresponds to the maximum amplitude that can be represented in the audio files.

On AudioMoth 1.1.0 the full 16-bit 0 to 65535 amplitude range corresponds to 0V to 3.3V.

I hope this helps,

Andy

Sam Lapp

Oct 25

Replying to

Thanks for the clarification, this helps a lot. It took me a WHILE to work this out haha, but now it's all aligned and making sense. It took me a bit to realize the 10x MEMs gain is included in the -18dBV/Pa figure provided by the spec sheet. In particular, the implication is that the multiplier for V/Pascals of the mic without MEMS internal gain is 0.01259, which when added to the internal gain gives 0.1259 V/Pa (-18 dBV/Pa) out of the MEMs for Audiomoth 1.1.0 with MEMS gain at 10x, or 0.1385 V/Pa (-17.2 V/Pa) out of the MEMs for Audiomoth 1.2.0.

Now, the predicted sensitivity matches empirical observations. Woo, I've been meaning to do this for a long time so thanks for the assistance. I updated the calculator sheet for future reference

Alex Rogers

Oct 25

Replying to

Great. I think the terminology in this space is super-confusing. A simple amplifier boosts the voltage amplitude by a fixed ratio of say 10, and we're happy to say it has a voltage gain of 10. However, when we use dB we have somehow switched to talking about the increase in the power derived from the increase in the voltage. Even when we say do say 'voltage gain', when we are using dB, we actually still mean 'power gain' just calculated using the voltage ratio.

Sam Lapp

Oct 25

Replying to

I agree, dB units are always confusing (so much so that I have a cheat sheet for myself for every time I forget what's going on) so relying on voltage multipliers makes sense. It's good to see alignment between the theoretical and empirical measurements though!

silvyvankuijk

Apr 16, 2021

Hi Vincent and Alex! Vincent, I have been trying to calibrate recordings too and want to use the PAMGuide package from Merchant et al (2015)*. You may want to check this out too. I'll post the full citation below.

Alex, to use the calibration function in the PAMGuide package, I'd need a few pieces of information. As per Merchant et al: "To retrieve the original measurement of sound pressure from the audio files, it is necessary to know: (i) the sensitivity of the acoustic transducer, which defines how much voltage is generated per unit of sound pressure; (ii) the amount of voltage gain (if any) applied by the pre-amplifier; and (iii) the ADC input voltage which corresponds to the maximum amplitude that can be represented in the audio files.

I've been having a hard time finding these specs for version 1.1.0. Would you happen to know these, or where I can find them? Thanks for such awesome devices, I love working with them!

Best, Silvy

*Merchant, N. D., Fristrup, K. M., Johnson, M. P., Tyack, P. L., Witt, M. J., Blondel, P., & Parks, S. E. (2015). Measuring acoustic habitats. Methods in Ecology and Evolution, 6(3), 257-265.

Alex Rogers

Apr 11, 2021

Hi, I think this would work fine but you would probably need to calibrate the whole recording chain - microphone, case, and recording settings - and then compare the resulting WAV files against your calibrated meter. Alex