Answers.Body: <h2>Intro</h2>
  <p>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).</p>
  <h2>Temporary calibration</h2>
  <p>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).</p>
  <p>This &quot;type&quot; of calibration should be performed when the environmental conditions change (for any reason this may happen).</p>
  <h2>Long-term drifts calibration</h2>
  <p>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.</p>
  <p>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.</p>
  <p>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.</p>
  <h2>Summary</h2>
  <p>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).</p>
  <p>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.</p>
  <h2>Additional remarks</h2>
  <p>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.</p>
  <p>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.</p>
  
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2022-11-25T12:51:38.687Z
Answers.Text: 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.
Answers.UserDisplayName: Chloe
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2022-11-25T13:58:27.610Z
Answers.Text: 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.
Answers.UserDisplayName: ZaellixA
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2023-03-03T20:40:36.093Z
Answers.Text: 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.
Answers.UserDisplayName: K.J. Palmer
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Answers.CreationDate: 2022-11-24T18:14:35.573Z
Answers.LastActivityDate: 2022-11-25T01:28:07.350Z
Answers.LastEditDate: 2022-11-25T01:28:07.350Z
Answers.UserDisplayName: ZaellixA
Answers.Body: <p>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.</p>
  
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Answers.CreationDate: 2022-11-28T11:15:15.633Z
Answers.LastActivityDate: 2022-11-28T11:15:15.633Z
Answers.UserDisplayName: ChrisP
Body: <p>A colleague asked the room this the other day and nobody could confidently answer this... <strong>How long, after an acoustic sensor has been calibrated, is its calibration curve trustworthy for?</strong></p>
  <p>There was consensus that devices should be recalibrated after they have been used in the field / deployed at depth, possibly knocked about, <em>etc</em>., but what if a calibrated device has been sitting for years in a protective box on a shelf? Has anybody any idea on this?</p>
  <p>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!</p>
  
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2022-11-25T12:52:21.570Z
Comments.Text: clarification: When I say "calibration", I am only interested in the sensitivity of a generic hydro/micro-phone.
Comments.UserDisplayName: Chloe
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2022-11-28T11:29:19.597Z
Comments.Text: could you specify, if hydrophone is with or without pre-amplifier?
Comments.UserDisplayName: WMXZ
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2022-11-29T15:02:56.590Z
Comments.Text: @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?
Comments.UserDisplayName: Chloe
Tags: <calibration><microphone><hydrophone>
Title: How long do hydrophone (/microphone) calibrations last for?
ThreadText: # How long do hydrophone (/microphone) calibrations last for?
  
  (Asked by: username_0 on 2022-11-24)
  
  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!
  
  **Tags:** calibration, microphone, hydrophone
  
  > **Comments:**
  > **username_0**: clarification: When I say "calibration", I am only interested in the sensitivity of a generic hydro/micro-phone.  
  > **username_1**: could you specify, if hydrophone is with or without pre-amplifier?  
  > **username_0**: @username_1, 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?  
  
  ---
  
  ## 2 answers
  
  ### Answer by username_4 on 2022-11-28. Score: 6 | Accepted answer
  
  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.
  
  
  ---
  
  ### Answer by username_2 on 2022-11-24. Score: 6
  
  ## 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.
  
  > **Comments:**
  > **username_0**: 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.  
  > **username_2**: 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.  
  > **username_3**: 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.  
  
  ---
  
  