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Author Topic: Measurement Microphone Maximum SPL  (Read 484 times)

Russell Ault

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Measurement Microphone Maximum SPL
« on: September 19, 2019, 12:35:34 am »

I'm assuming this is a simple question with a complicated answer:

How is the maximum SPL handling of a measurement microphone determined? I'm assuming it has something to so with a test tone and measuring harmonic distortion, but is there a metric that says how loud the harmonics can get before you declare the microphone "out of gas"?

I ask this question both in the interests of being able to verify published specs, and to determine SPL handling for cheaper mics for which published specs aren't available (ECM8000 comes to mind).

Thoughts?

Thanks!

-Russ
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Frank Koenig

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Re: Measurement Microphone Maximum SPL
« Reply #1 on: September 19, 2019, 11:05:53 pm »

I'm assuming this is a simple question with a complicated answer:

How is the maximum SPL handling of a measurement microphone determined? I'm assuming it has something to so with a test tone and measuring harmonic distortion, but is there a metric that says how loud the harmonics can get before you declare the microphone "out of gas"?

I ask this question both in the interests of being able to verify published specs, and to determine SPL handling for cheaper mics for which published specs aren't available (ECM8000 comes to mind).

Thoughts?

Thanks!

-Russ


I did a quick search on "microphone maximum spl standard". I found this:

https://www.dpamicrophones.com/mic-university/how-to-read-microphone-specifications

from DPA, a pretty reputable outfit, and they say they spec max SPL at 1% THD.

Page 139 here:

https://tandfbis.s3.amazonaws.com/fp-usermedia/uploadedFiles/Books/Book_Media/Audio/The_Microphone_Book_Ch07.pdf

has more. Upshot is .5% at 1 kHz.

There likely is an actual standard from AES, ANSI, or the like but I don't have it.

--Frank
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Russell Ault

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Re: Measurement Microphone Maximum SPL
« Reply #2 on: September 20, 2019, 12:57:43 am »


I did a quick search on "microphone maximum spl standard". I found this:

https://www.dpamicrophones.com/mic-university/how-to-read-microphone-specifications

from DPA, a pretty reputable outfit, and they say they spec max SPL at 1% THD.

Page 139 here:

https://tandfbis.s3.amazonaws.com/fp-usermedia/uploadedFiles/Books/Book_Media/Audio/The_Microphone_Book_Ch07.pdf

has more. Upshot is .5% at 1 kHz.

There likely is an actual standard from AES, ANSI, or the like but I don't have it.

--Frank

Those were the two numbers I had been seeing, too. Neumann has an interesting description of why the 0.5% vs 1% discrepancy exhists, which basically comes down to audiblity in different applications. This leads me to the question: in scenarios where audibility of THD isn't a concern, how much THD can be tolerated before it starts throwing measurements off? I feel like the answer is probably more than 1%, but I'm not sure.

Thanks!

-Russ
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Barry Singleton

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Re: Measurement Microphone Maximum SPL
« Reply #3 on: September 20, 2019, 01:21:48 am »

  Both my TEF-04s and Earthworks M30s? I think? Run out of voltage swing and flat line peak just under 125dB SPL.

  Its would be an easy cheat to make something look really smooth by putting the mics on the voltage rails, look! dead flat!

  Barry.
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Frank Koenig

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Re: Measurement Microphone Maximum SPL
« Reply #4 on: September 20, 2019, 10:46:02 am »

This leads me to the question: in scenarios where audibility of THD isn't a concern, how much THD can be tolerated before it starts throwing measurements off?

If all you are measuring is frequency response I suspect you are correct that a bit of distortion is not critical. The trouble is that the amount and nature of the error it causes is likely to depend on the nature of the distortion, the test signal, and the detection method. As you suggest, it's complicated. If you want to measure distortion in the system under test then, of course, you need a clean mic.

If mics behave like most physical systems the nonlinearity builds quickly after a certain amplitude threshold is reached which is why I'm guessing .5 to 1% THD is a good redline, in addition to being the threshold of audibility in most situations. Simply generating high SPL with low distortion for testing the mics presents its own problems. As the second above reference indicates, for pressure mics a pistonphone can be used but all bets are off for gradient mics. At very high SPL I assume the acoustic nonlinearity of the air will interfere.

I'm curious what you are measuring and why you need to do it at very high SPL. I do speaker frequency response measurements at modest SPLs to stay well within the linear range of all elements and use copious signal averaging If I'm in a noisy environment.
N^.5 is your friend  :)

--Frank
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Mark Wilkinson

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Re: Measurement Microphone Maximum SPL
« Reply #5 on: September 20, 2019, 02:04:31 pm »

If all you are measuring is frequency response I suspect you are correct that a bit of distortion is not critical. The trouble is that the amount and nature of the error it causes is likely to depend on the nature of the distortion, the test signal, and the detection method. As you suggest, it's complicated. If you want to measure distortion in the system under test then, of course, you need a clean mic.

If mics behave like most physical systems the nonlinearity builds quickly after a certain amplitude threshold is reached which is why I'm guessing .5 to 1% THD is a good redline, in addition to being the threshold of audibility in most situations. Simply generating high SPL with low distortion for testing the mics presents its own problems. As the second above reference indicates, for pressure mics a pistonphone can be used but all bets are off for gradient mics. At very high SPL I assume the acoustic nonlinearity of the air will interfere.

I'm curious what you are measuring and why you need to do it at very high SPL. I do speaker frequency response measurements at modest SPLs to stay well within the linear range of all elements and use copious signal averaging If I'm in a noisy environment.
N^.5 is your friend  :)

--Frank

This link http://www.isemcon.com/datasheets/EMX7150-US-r04.pdf
shows a THD vs SPL graph for their mic, and also a Class 1 comparison (no wonder Class 1 is so expensive)
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Frank Koenig

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Re: Measurement Microphone Maximum SPL
« Reply #6 on: September 21, 2019, 06:18:52 pm »

This link http://www.isemcon.com/datasheets/EMX7150-US-r04.pdf
shows a THD vs SPL graph for their mic, and also a Class 1 comparison (no wonder Class 1 is so expensive)

That's a very thorough data sheet. At ~$350 not a bad price for the mic, either.

In the little application note at the end they discuss free-field vs diffuse-field, pointing out that most measurements are made in an at least somewhat diffuse field. My question is if you make a pseudo-anechoic measurement, by windowing out all reflections, does that constitute a true free-field measurement so far as the mic is concerned? I think the answer is yes, as the reflected contributions add linearly to the direct sound and, after windowing, it is as though they never happened since the derivation of the impulse response from the pseudo-noise or sine-sweep response is a linear process. Thoughts?

--Frank
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Russell Ault

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Re: Measurement Microphone Maximum SPL
« Reply #7 on: September 22, 2019, 11:49:11 pm »

[...] I'm curious what you are measuring and why you need to do it at very high SPL. [...]

I'm trying to get out ahead of the seemingly inevitable regulation of loudness in our industry by getting some experience with SPL monitoring. Of course, since no one is currently asking for me to do this (or needing to verify my results) I can't justify spending big dollars on dedicated equipment, so I'm trying to see what I can do with what I already have.

My measurement mic of choice historically has been the ECM8000 for its ability to do the job without me having to worry about it getting damaged (funny how the cheap stuff never gets run over by the inattentive usher), but it seems like its SPL handling is both unpublished and not brilliant. Basically, I'm trying to figure out if, for my purposes, I can get away with using what I've got, or if I should go out and buy something with a published (and reasonably high) max SPL spec.

Thanks!

-Russ
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Chris Grimshaw

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Re: Measurement Microphone Maximum SPL
« Reply #8 on: September 24, 2019, 11:43:21 am »

IIRC, the ECM8000 is fine up to around 120dB.

You don't want to be measuring at those SPLs.
If you want to measure at those SPLs, move the mic further away, and wear ear defenders.

1% distortion swamps what my speakers do (I've had below 0.1%, albeit at low levels), so I make sure the mic isn't anywhere near its limits.

I once needed to measure very loud SPLs, and couldn't move the mic further away (1m maximum SPL testing), so swapped in a Beyer MC930 with the -15dB pad engaged. A quick comparison between that and the measurement mic (at lower levels) meant I could re-calibrate for the different mic, and then push towards the 140dB limit of that mic.

Chris
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Re: Measurement Microphone Maximum SPL
« Reply #8 on: September 24, 2019, 11:43:21 am »


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