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I've always wondered how 'stable' microphone frequency response and sensitivities are? In particular, I'm not talking about the high-end (GRAS/B&K/others) calibration microphones, but rather 'normal' mics that we use on a daily basis.

This paper shows sensitivity varies over years of around +/- 0.02-0.003 dB/year - which is for high-end calibration mics at the NIST. I'm wondering if any of you have references/experiences to share about calibration stability of microphones - and which types you see 'hold their calibrations' for longer?

question inspiration: inspired by @Marinebioacoustics Aarhus Univ 's answer saying hydrophones typically hold their calibrations if nothing happens to them. Have always wondered how often one should calibrate mics!

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This is a question that is not asked often enough in my view - outside of the marine acoustic sector anyway...

A good paper looking at this is Turgeon & Wilgenburg (2017): DOI:10.5751/ACE-00958-120109, which shows that field use over 1-2 seasons (of Wildlife Acoustics mics) resulted in significant declines in sensitivity:

Field use had a statistically significant(Kruskal-Wallis test χ 2 = 130.44, p < 0.0001) effect on microphone sensitivity (Fig. 4), and post hoc Mann-Whitney U tests between the [microphone age] groups show that all groups differ significantly from one another. The median sensitivity of microphones deployed for one season was 1.0 dBV lower --- than new microphones -- Similarly, the median sensitivity of microphones deployed for two or more seasons was 1.9 dBV less sensitive than new microphones

There is a real need for issues around microphone management and calibration to be recognised outside of marine work, e.g. for bat and bird surveys, and for at least seasonal or annual checks to be made of equipment. We do this on an annual basis in my company, and while all our mics passed muster this year, we threw away six last year due to sensitivity being too degraded.

I'd be keen to hear anyone else's experiences or practices in this regard.

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We have used cheap knowles microphones for bat echolocation studies and they can get calibrated often and they have rendered the same sensitivity at least over 3 years. The main issue with microphones is that their diaphragm gets scratched or damaged and that can lead to 3-6 dB offsets that can go undetected for a long time unless regular calibration. I suggest to calibrate before and after each experiment as a minimum. In some setups we calibrate before each trial by sending out a standard sweep to the mic array from a speaker at a known distance.

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Echoing the answer above in that it's ideal to calibrate before and after experiments/deployments.

Often we are deploying underwater recorders at depths of several hundred meters, and if something hits it (and damages the ceramic in the hydrophone), we wouldn't know about it.

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