# Interest in a low-frequency mic calibration jig?



## Anechoic (Jan 16, 2009)

Hey all,

Based on the posts I read on this forum as well as the requests I get for my mic calibration services, I know that many of the posters here are very concerned about the low-frequency performance of their microphones and sound level meters.

I know that there is interest in calibrated mics, but I was wondering if there might also be an interest in a low-frequency calibration jig that folks can use to calibrate mics on their own?

The jig I'm thinking about would be similar to the one I use for low-freq mic calibrations, it's basically s small PVC chamber with a speaker inside. I would provide you with a cal curve for the speaker, you stick your mic into the appropriate hole and use your favorite measurement program (as long as you can use a cal curve) to measure the response of your mic from <5Hz up to 500Hz or so. 

I'm thinking this would be priced in the $75-$100 range. And of course I'd probably have some intro priving for HTS members . I would include a cal curve with the unit, one free calibration and maybe ~$25 per cal check after that.

There are a lot of problems I would have to think through to make this viable for a consumer market and I would have to parts fabricated at a machine shop to make this work which is why this is purely in the "thinking about it" stage. But knowing if people were or were not interested in this would make the thinking go a lot easier.

So please, let me know your thoughts.


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## JimP (May 18, 2006)

I think you wind up with a situation similar to an episode of Monk. He has a level. He also has a calibrated level to check his other level. To be sure that is calibration level is correct, he sends it in to be recalibrated. Most of us just want to check frequencies and decay times and not open up a new area to be obsessive over. Actually, we'd rather live in ignorace as to how much our 3 year old testing mic has drifted than have the calibration rechecked. You may do well to promote regular retesting of our mics.

EDIT to add.....O.K., my obsessive behavior has me wonder about these mics drifting. Do they?


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## chadcummings (Apr 26, 2008)

I agree; with the home theater market 1db of difference is not enough to justify me spending the extra money.

Radioshack is plenty accurate for what I need.


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## Anechoic (Jan 16, 2009)

JimP said:


> I think you wind up with a situation similar to an episode of Monk. He has a level. He also has a calibrated level to check his other level. To be sure that is calibration level is correct, he sends it in to be recalibrated. Most of us just want to check frequencies and decay times and not open up a new area to be obsessive over. Actually, we'd rather live in ignorace as to how much our 3 year old testing mic has drifted than have the calibration rechecked. You may do well to promote regular retesting of our mics.
> 
> EDIT to add.....O.K., my obsessive behavior has me wonder about these mics drifting. Do they?


Anyone who's a fan of Monk can't be wrong 

I started thinking about it some more and while some of the issues I anticipated can be overcome, I'm not sure they can be overcome cheaply enough to make this a via product. So consider the question withdrawn. I'm just looking for ways to address DIY needs (and make money doing it!)


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## sparky77 (Feb 22, 2008)

This whole concept gets me thinking about what is relative to what. If you have a mic calibrated to one company, just how much is the cal file going to differ to the next company. AND, exactly where does the reference come from, and based on WHO's theory.......


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## Anechoic (Jan 16, 2009)

That's the easy part - you just make everything along the chain eventually traceable to NIST and ANSI standards where everything is well defined.

The hard part is keeping things consistent over time.


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## Dennis H (Apr 2, 2007)

Those interested in building their own pressure chamber (only costs a few bucks) can download this article from the ARTA page. Ideally, you'd want a calibrated mic to compare to but, IF you can get everything perfectly sealed, the pressure will stay the same as frequency changes as long as you stay well below the resonance frequency of the chamber.

http://www.fesb.hr/~mateljan/arta/AppNotes/AP5_MikroMeasChamber-Rev03Eng.pdf

There are some other interesting articles there related to measurements.

http://www.fesb.hr/~mateljan/arta/support.htm


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## jimfrank (Oct 30, 2009)

It sounds interesting to me. Would it be valuable to build a test model, use two or three already calibrated mikes in it, and compare the values you get to the published specs? Then you'd know if the test jig was going to be reliably reproducible. (Try saying that three times fast.... )


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