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In discussions with colleagues I've learned that the sensitivity of a recorder can vary over the duration of a deployment.

I'm not sure why or how this happens, but how can you evaluate if it has occurred? (without going back to the field +altering equipment design).

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    Are there any known reasons why it happens? Jul 25 at 20:04
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    @angie_zorka the 'why' is a whole different question!!
    – Shannon
    Jul 26 at 0:09
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    @ASiimonis, what type of deployment duration you are talking about.
    – WMXZ
    Jul 26 at 7:11
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    @Shannon, the why matters, if sensitivity changes due to bio fouling then answer is different than if changes are due to electronics changes.
    – WMXZ
    Jul 26 at 7:11
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    I do not mean to say that the 'why' does not matter, but that the OP is asking a different question. First we need to identify a problem, and then when it is identified, the next question is to ask how to address it. The OP is asking 'How can you evaluate that this has happened".... So, the "why" is a different question (not less important in the least!). I personally think it is better to separate these two questions, and perhaps the OP or someone else might consider asking that as a second and independent question, and then linking the two.
    – Shannon
    Jul 26 at 14:44

3 Answers 3

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The only way to know if calibration has changes is to calibrate the full system before and after deployment. This can be done for a single frequency, e.g. 2 kHz and has not to be done for the whole frequency range.

Edit: this can be done even in air: have a time with same amplitude be played to system before and after deployment keeping same geometry.

Edit2: this applies also to hydrophones, even if sensitivity is not as high as in water, but for the OP (changes in sensitivity) only the difference matter.

Edit3: Obviously, using a single tone only implies that changes in sensitivity are not spectral but effect whole spectrum in the same way, Otherwise multiple tones with different frequencies are required.

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  • My original response was based on wrong reading of OP, so I changed it
    – WMXZ
    Jul 26 at 18:56
  • Do you mean "can be done even in air" specifically for a hydrophone?
    – selene
    Jul 26 at 19:56
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Answer is for hydrophones as I think A.Simonis has more hydrophones than microphones (but probably applies to microphones as well)

The normal approach to this is to have a known precision sound source with a suitable coupler to the hydrophone (e.g. a pistonphone https://www.bksv.com/en/transducers/acoustic/calibrators/pistonphone) and then play a tone into the hydrophone before deployment and after deployment (pre- and post-calibration).

Note that to get good results the hydrophone needs to be at "deployment temperature" for both calibrations, so dip into the water for some time prior to pre-calibration (In my experience, the sensitivity drift for short deployments have always been smaller than 1 dB, so not very significant, compared to other parameters).

With heavy biofouling, post-calibration becomes tricky, as a coupler device might interfere with the bio-fouling layer and not give a correct calibration (in this case you might need a more elaborate pre- and post-calibration scheme involving calibrated transducers in a controlled water body).

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Edit: this answer tackles the topic of what could be done if the sensitivity change needed to be quantified, based on reading the question state at that point!

Not an aquatic person here, but this is a gener problem for sound/vibrations in any medium I'm guessing. Also, full disclosure, I have no direct experience with doing this.

Would it be possible to have a regular playback every few days/weeks/etc. ? Here of course two issues arise: 1) is this going to hinder the primary objective of the experiment (how invasive is the playback?) and 2) how much variation do you expect in the playback itself (internal noise, weather related etc.)?

If 2) is expected to be relatively stable, then perhaps a regular ping of some sort could work - but it shouldn't conflict with 1) in the first place...

I guess that most of the variation is typically expected from the 'mechanical' parts of the sensor (membranes, cavities, etc). If the concern is that the sensitivity variation is coming at the ADC stage - a fixed electronic signal needs to be fed into the recorder part of the chain occasionally...

If sensor sensitivity is stable, you'd expect constant RMS/peak levels of the playback.

These solutions are definitely not a 'real-time' one of course, but rather post-hoc ones. The real-time solution, if one exists would be pretty neat!!!

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  • your answer is in the line of what I was original suggesting, but based on "but how can you evaluate IF it has occurred" (emphasis mine) I ended up understanding to simply assert that sensitivity HAD changed and not HOW sensitivity changed during deployment.
    – WMXZ
    Jul 27 at 12:16
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    Using calibration signals before and after a deployment seems like a great plan in the future, but I wonder how one might evaluate this potential issue after the data was collected. I wonder about comparing ambient noise levels to measured wind speed, but that seems troublesome due to the presence of other noise sources.
    – ASimonis
    Jul 27 at 17:45
  • Ah, :|. Now i get what was meant. I have tried my luck in editing the Q and A to reflect this.
    – Thejasvi
    Jul 27 at 17:57

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