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A colleague asked the room this the other day and nobody could confidently answer this... How long, after an acoustic sensor has been calibrated, is its calibration curve trustworthy for?

There was consensus that devices should be recalibrated after they have been used in the field / deployed at depth, possibly knocked about, etc., but what if a calibrated device has been sitting for years in a protective box on a shelf? Has anybody any idea on this?

If there is a change in the calibration curve for a device that has been sitting/unused for years, what is this due to? Thanks!

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  • $\begingroup$ clarification: When I say "calibration", I am only interested in the sensitivity of a generic hydro/micro-phone. $\endgroup$
    – Chloe
    Nov 25, 2022 at 12:52
  • $\begingroup$ could you specify, if hydrophone is with or without pre-amplifier? $\endgroup$
    – WMXZ
    Nov 28, 2022 at 11:29
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    $\begingroup$ @WMXZ, I am asking this question generally without a specific hydrophone in mind. I'm intrigued though - how would you expect the presence of a pre-amplifier to impact the calibration curve over time? $\endgroup$
    – Chloe
    Nov 29, 2022 at 15:02

2 Answers 2

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when bidding for projects our hydrophones and pistonphones are generally required to have been calibrated (certified to IEC 60565) within the previous 24 months, preferably 12 months. But if were is any indication within this period, from pistonphone tests before / after recording sessions, that the hydrophone has drifted out of spec, it would need recalibrating.

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Intro

In general, there are two calibration procedures used with measurement equipment (I come from an acoustics background but this is more generic to metrology than a specific acoustics field).

Temporary calibration

One is to correct possible drifts resulting from temporary environmental changes. For example, when one uses a sensor array to perform Direction-of-Arrival estimation the speed of sound is needed. Alas, the speed of sound is dependent on temperature, which may exhibit spatial and temporal variations (sometimes quite large). For this reason, one has to calibrate their setup before taking a measurement at a specific location and time. This will account for those temporary changes (compared to the laboratory environment where the initial calibration was done).

This "type" of calibration should be performed when the environmental conditions change (for any reason this may happen).

Long-term drifts calibration

The second kind of calibration is done to correct/account for long-term drifts in the parts of the equipment, whether they are mechanical, electrical or of any other type. For example, capacitors may exhibit capacitance drift with time (for various reasons which I am not going to state here). These effects should be corrected or accounted for. Usually, this type of calibration is done from the manufacturer, or some specialised engineer/lab-technician, which in the capacitor example, some values in an embedded algorithm could be changed or the capacitor itself be replaced.

This kind of calibration should be done whenever the manufacturer suggests. It is rather typical for precision measurement equipment to be about every one or two years. For example, I have encountered a Phosphorous Oscilloscope that declared it should be recalibrated once per year by the manufacturer.

Whether this is indeed needed and what the consequences of not calibrating the equipment will be (drifts and the amount of them) strongly depends on the equipment.

Summary

To directly answer your question, calibration should be performed every time prior to taking measurements (one calibration process before a batch of measurements is common practice), unless your equipment does not allow for it (in my field, there are calibrators for sound level metres that should be used before performing measurements on site).

I believe it was also clear that even though your hydropnone/microphone was protected in a nice protective case, it most probably needs recalibration for long-term drifts. As already mentioned, this is very equipment dependent.

Additional remarks

Please note that whether your intended use case is in need of a calibrated piece of equipment is a whole different story. You may be working with relative values and don't care at all about the actual/absolute values of the quantities you are measuring. In this case you may avoid the calibration process altogether.

But, again, this is also equipment dependent. As an extreme example, think of an Analogue-to-Digital Converter that has huge drift at some Phased Locked Loop component and exhibits large jittering values. This can screw the phase (and frequency) content of your measurements which may, or may not, be detrimental for your algorithms, or whatever your use case is.

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  • $\begingroup$ OK, thank you... but the question remains unanswered. I already know that a calibrated hydrophone that has been sitting on a shelf "most probably needs recalibration" and am wondering how frequently the sensitivity of an untouched generic hydrophone should be re-evaluated. $\endgroup$
    – Chloe
    Nov 25, 2022 at 12:51
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    $\begingroup$ As briefly stated in the answer, this depends on the equipment and for the "right" answer you should most probably consult the manufacturer of the hydrophone. Without even knowledge of the manufacturer and/or model I don't think it is advisable to reach any generic conclusions. $\endgroup$
    – ZaellixA
    Nov 25, 2022 at 13:58
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    $\begingroup$ Assuming this is the scenario you are dealing with (hydrophones sitting around for a few years) it would be really helpful if you report back after calibrating. Obviously, this would be a very small sample size but it may (or may not) raise sufficient red flags in the community to consider this. Plus, if it's soundtraps it would be good to have an RMS calibration in addition to the p-to-p calibration provided by the manufacturer. $\endgroup$ Mar 3, 2023 at 20:40

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