Answers.Body: <p>In a free field, the attenuation is 20 log<sub>10</sub>(d) where d is the distance between the source and the receiver. This means that relative positioning errors will have more impact on the calibration as the recorder to be calibrated gets closer to the source. You can evaluate the uncertainty u on the measurement of distance and try to select d so that 20 log<sub>10</sub>((d+u)/(d-u)) is low enough.</p>
  <p>But there are certainly other causes for uncertainty in your calibration setup and procedure: is your sound source small enough to avoid directivity effects? What is the signal to noise ratio during the calibration? How do you control the output of your sound source? Do you bring your recorders to the lab to calibrate them or do you calibrate them in the field?</p>
  <p>A 3 dB difference between two successive calibrations seems quite large. In acoustic measurements standards, a common rule is that a measurement should be discarded when the drift between the calibration done at the beginning and the one done at the end is above 0.5 dB. But one should bear in mind that automatic recorders are not designed to measure sound pressure. So this requirement is probably too strict in your case.</p>
  
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Answers.CreationDate: 2022-10-10T08:02:20.710Z
Answers.Text: Thanks for the answer, I am doing calibrations in the lab, in an echo-attenuated room. At the moment I use a sound level meter and a recorder and a calibrating microphone at the same time, 1m in front of a loudspeaker. But somehow I have a lot of variability between two measurements and I can't get everything to stay reproducible. Maybe my loudspeaker is too loud and I still pick up some reflexion from the ground? What would be a "good" level for a calibration? I get sometimes measurements that are as good as 1 dB apart and sometimes as bad as +/-5 dB, and I can't understand why as of now.
Answers.UserDisplayName: lframond
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Answers.CreationDate: 2022-10-11T10:46:57.430Z
Answers.Text: You will get a reflection from the ground whatever the playback level. Let's forget the device you want to calibrate for a moment. Did you check the stability of your sound source? What happens when you place the sound level meter in front of the source and do repeated acquisitions without moving the sound level meter (SLM)? If your levels do not change, what happens when you (1) place your SLM, (2) measure the sound level, (3) remove the SLM several times in a row? What signal do you use for calibration at what sound level typically? Do you calibrate your SLM with a 1kHz calibrator?
Answers.UserDisplayName: Guillaume Dutilleux
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Answers.CreationDate: 2022-10-17T07:08:43.400Z
Answers.Text: The sound source is very stable when I don't move the SPL meter, (within 0.2 dB), and when I move the SPL meter it is not too bad either (within 1 dB). I had used a source level of 48 dB @ 1 m @ 1 kHz in my echo free room which had an ambient level of 32 dB, but the signal at different frequencies ranged between 40 and 53 dB due to the frequency response of the loudspeaker. I did not calibrate the SPL meter, next time I'll do it
Answers.UserDisplayName: lframond
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Answers.CreationDate: 2023-06-08T18:58:00.047Z
Answers.Text: Those levels seem rather low. Vunerable to small sounds, influencing the result. Might not even notice them yourself. I believe the reference SPL for sensitivity measurements is 94 dB SPL. So I would try going from 48 to at least 68 dB, but maybe closer to 94 dB SPL as long as the distortion of speaker is not problematic.
Answers.UserDisplayName: Jon Nordby
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Answers.CreationDate: 2022-10-01T05:55:46.653Z
Answers.LastActivityDate: 2022-10-04T23:15:23.897Z
Answers.LastEditDate: 2022-10-04T23:15:23.897Z
Answers.UserDisplayName: Guillaume Dutilleux
Body: <p>I would like to get the ambient noise levels of a habitat that I have been monitoring for some time (monitoring birds, 0-16 kHz). For this, I want to calibrate my recorders and get their sensitivity and frequency response and then use my calibrations on the recordings to get to the noise levels.</p>
  <p>However, I still didn't manage to come up with a workflow that would enable me to reproduce the sensitivity measurements very well. Does  someone have an idea/solution to help me with :</p>
  <ul>
  <li>making sure that the recorders are all exactly at the same spot every time and facing the same direction (until now, attaching them to a tripod always gave rise to some variation in the direction the microphone was facing, even with a lot of care)</li>
  <li>reduce as much as possible the reflections from tripods around the microphone: what would be an appropriate distance between the loudspeaker and the microphone for audible sound?</li>
  <li>analysis: what would be an acceptable measurement error? let's say that I have a 3dB difference between two sensitivity measurements of the same microphone two weeks apart, is it fine? or should I aim for smaller differences?</li>
  </ul>
  <p>Finally, is my workflow OK? or are long-term recordings too variable (due to e.g. temperature variation, battery level, etc.) to extract anything reliable?</p>
  <p>Any experience/tips and tricks to do good and reliable microphone calibration are welcome!</p>
  
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2022-09-28T06:53:49.440Z
Comments.Text: "what would be an appropriate distance between the loudspeaker and the microphone for audible sound": do you mean a *sound source* instead of a *loudspeaker*? I understand from your first paragraph that you are recording natural sounds in the field, aren't you?
Comments.UserDisplayName: Noil
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2022-10-10T07:55:06.617Z
Comments.Text: I am recording natural sound in the field but I came back to the lab for the calibration
Comments.UserDisplayName: lframond
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2023-05-09T22:59:14.253Z
Comments.Text: An important question. I am a bit late in commenting, but there is a promising design for this task offered by Dan Mennitt in his 2019 paper, 'An adaptive coupler for calibration of arbitrarily shaped microphones.' I have seen him demonstrate the tool in person and it was compelling to witness, but it may be difficult to implement without the accompanying code.
Comments.UserDisplayName: Halyn Betchkal
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2023-06-08T18:53:44.593Z
Comments.Text: How exactly are you measuring the sensitivity? You said you use 1khz playback. How long measurement period? Do you do multiple measurements and discard outliers?
Comments.UserDisplayName: Jon Nordby
Tags: <calibration><audiomoth><spl><field-recordings>
Title: Calibrate automatic recorders: advice for reliable / reproducible workflow?
ThreadText: # Calibrate automatic recorders: advice for reliable / reproducible workflow?
  
  (Asked by: username_0 on 2022-09-22)
  
  I would like to get the ambient noise levels of a habitat that I have been monitoring for some time (monitoring birds, 0-16 kHz). For this, I want to calibrate my recorders and get their sensitivity and frequency response and then use my calibrations on the recordings to get to the noise levels.
  
  However, I still didn't manage to come up with a workflow that would enable me to reproduce the sensitivity measurements very well. Does someone have an idea/solution to help me with :
  
  * making sure that the recorders are all exactly at the same spot every time and facing the same direction (until now, attaching them to a tripod always gave rise to some variation in the direction the microphone was facing, even with a lot of care)
  * reduce as much as possible the reflections from tripods around the microphone: what would be an appropriate distance between the loudspeaker and the microphone for audible sound?
  * analysis: what would be an acceptable measurement error? let's say that I have a 3dB difference between two sensitivity measurements of the same microphone two weeks apart, is it fine? or should I aim for smaller differences?
  
  Finally, is my workflow OK? or are long-term recordings too variable (due to e.g. temperature variation, battery level, etc.) to extract anything reliable?
  
  Any experience/tips and tricks to do good and reliable microphone calibration are welcome!
  
  **Tags:** calibration, audiomoth, spl, field-recordings
  
  > **Comments:**
  > **username_1**: "what would be an appropriate distance between the loudspeaker and the microphone for audible sound": do you mean a *sound source* instead of a *loudspeaker*? I understand from your first paragraph that you are recording natural sounds in the field, aren't you?  
  > **username_0**: I am recording natural sound in the field but I came back to the lab for the calibration  
  > **username_2**: An important question. I am a bit late in commenting, but there is a promising design for this task offered by Dan Mennitt in his 2019 paper, 'An adaptive coupler for calibration of arbitrarily shaped microphones.' I have seen him demonstrate the tool in person and it was compelling to witness, but it may be difficult to implement without the accompanying code.  
  > **username_3**: How exactly are you measuring the sensitivity? You said you use 1khz playback. How long measurement period? Do you do multiple measurements and discard outliers?  
  
  ---
  
  ## 1 answers
  
  ### Answer by username_4 on 2022-10-01. Score: 2
  
  In a free field, the attenuation is 20 log10(d) where d is the distance between the source and the receiver. This means that relative positioning errors will have more impact on the calibration as the recorder to be calibrated gets closer to the source. You can evaluate the uncertainty u on the measurement of distance and try to select d so that 20 log10((d+u)/(d-u)) is low enough.
  
  But there are certainly other causes for uncertainty in your calibration setup and procedure: is your sound source small enough to avoid directivity effects? What is the signal to noise ratio during the calibration? How do you control the output of your sound source? Do you bring your recorders to the lab to calibrate them or do you calibrate them in the field?
  
  A 3 dB difference between two successive calibrations seems quite large. In acoustic measurements standards, a common rule is that a measurement should be discarded when the drift between the calibration done at the beginning and the one done at the end is above 0.5 dB. But one should bear in mind that automatic recorders are not designed to measure sound pressure. So this requirement is probably too strict in your case.
  
  > **Comments:**
  > **username_0**: Thanks for the answer, I am doing calibrations in the lab, in an echo-attenuated room. At the moment I use a sound level meter and a recorder and a calibrating microphone at the same time, 1m in front of a loudspeaker. But somehow I have a lot of variability between two measurements and I can't get everything to stay reproducible. Maybe my loudspeaker is too loud and I still pick up some reflexion from the ground? What would be a "good" level for a calibration? I get sometimes measurements that are as good as 1 dB apart and sometimes as bad as +/-5 dB, and I can't understand why as of now.  
  > **username_4**: You will get a reflection from the ground whatever the playback level. Let's forget the device you want to calibrate for a moment. Did you check the stability of your sound source? What happens when you place the sound level meter in front of the source and do repeated acquisitions without moving the sound level meter (SLM)? If your levels do not change, what happens when you (1) place your SLM, (2) measure the sound level, (3) remove the SLM several times in a row? What signal do you use for calibration at what sound level typically? Do you calibrate your SLM with a 1kHz calibrator?  
  > **username_0**: The sound source is very stable when I don't move the SPL meter, (within 0.2 dB), and when I move the SPL meter it is not too bad either (within 1 dB). I had used a source level of 48 dB @ 1 m @ 1 kHz in my echo free room which had an ambient level of 32 dB, but the signal at different frequencies ranged between 40 and 53 dB due to the frequency response of the loudspeaker. I did not calibrate the SPL meter, next time I'll do it  
  > **username_3**: Those levels seem rather low. Vunerable to small sounds, influencing the result. Might not even notice them yourself. I believe the reference SPL for sensitivity measurements is 94 dB SPL. So I would try going from 48 to at least 68 dB, but maybe closer to 94 dB SPL as long as the distortion of speaker is not problematic.  
  
  ---
  
  