# Mic Calibration: Thinking Out Loud



## chris319 (Dec 16, 2008)

"The man who wears two watches does not know what time it is."

I own two ECM-8000s. Both have been calibrated by Herb Singleton's Cross Spectrum Labs. Thinking there would be some benefit to having 1/12 octave calibration (which it turns out I no longer need) I also had both of these mics calibrated by another lab which does mic calibration to 1/12 octave.

According to the transitive property in algebra, if a = b and b = c then a = c. Imagine "b" as a calibration lab's reference mic and "a" and "c" as my ECM8000s. Using REW I measured both of my mics, applying each of the two lab's calibration data to the measurements. Using REW's overlay feature I found the curves were nearly identical in the range from about 200 Hz to 3,000 Hz. Outside of this range there is some deviation between the curves in the Cross Spectrum calibrations and a LOT of deviation between the curves in the alternate lab's calibrations. As illustrated above, if both mics are calibrated to the same standard there should, in theory, be no deviation between the two mics' curves once the calibration data is applied. I think this serves to demonstrate that there is no perfection in acoustical measurements. Both labs are reputable, both use high-quality measurement equipment and the proprietors of both know what they are doing. I don't think there is an error in either lab's methodology. The dilemma I face is: Which lab's measurement is "more correct"? I have no way of knowing this.

In pondering this dilemma I took an unorthodox tack. I took the calibration data for each mic and _averaged_ the calibration values reported by each lab for each mic. The resulting curves were applied in REW and sure enough, the frequency response curves are now much, much closer between the two mics, particularly in the high frequencies. I am pretty happy about this.

My aim is to characterize the frequency responses of the mics I own (not speakers and rooms). For this I need a truly flat reference mic. I have more mics than I have funds to have them all measured by outside labs, so I have set up a test station which gives results that are "close enough for me". I had been tinkering with the idea of purchasing an expensive "holy grail" reference mic but given the results I've obtained thus far by averaging this doesn't seem necessary. If I can find another lab or labs which handles the ECM8000 I will send them my mics for calibration and average the results into my calibration files.

I am quite pleased with the way this project has turned out. I now have a much better grasp on what my mics can do. I can't say enough good things about REW. John has been responsive to suggestions and the program does just what I need it to do.


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## robbo266317 (Sep 22, 2008)

Mic response can vary enormously if the two mic's are not in the exact same spot when you do the measurements. Were you doing near field (close to the driver) or in room measurement?


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## chris319 (Dec 16, 2008)

The mics are positioned 24" from the sound source per a paper found on the Neumann web site.

The mics are connected via an XLR connector attached to the end of a gooseneck. All I do is unplug one mic and plug in the other. As the gooseneck isn't repositioned, I estimate the mics are within a fraction of a millimeter of each other.


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## robbo266317 (Sep 22, 2008)

Hmmm... 
Well I guess you have the mic positioning covered! :scratch:
I guess you will just have to use the average like you are doing.


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## chris319 (Dec 16, 2008)

I would like to find one or two more labs to calibrate the ECM8000s. Alternatively, I could go with an Earthworks and take their calibration data on faith, then have it calibrated by a panoply of other labs.


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## chris319 (Dec 16, 2008)

I had a theory which turned out to be invalid. Never mind. :sn:


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## chris319 (Dec 16, 2008)

I emailed three different mic calibration labs, asking about the Behringer ECM8000. One returned my email, two others didn't. I called the ones that didn't. In both cases I got guys with European accents who had never even heard of the ECM8000. One guy was confused about 48V phantom powering, and his lab only measures up to 16 kHz anyway.

Oh well.


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## Anechoic (Jan 16, 2009)

chris319 said:


> I emailed three different mic calibration labs, asking about the Behringer ECM8000. One returned my email, two others didn't. I called the ones that didn't. In both cases I got guys with European accents who had never even heard of the ECM8000. One guy was confused about 48V phantom powering, and his lab only measures up to 16 kHz anyway.
> 
> Oh well.


The other folks I know who handle ECM8000s are:

Joe Demarinis at WinchesterLab 

Kim Girardin ([email protected])

Hi-Fi-Selbstbau 

If you know of others, please let me know, I have a list of folks on my site.

Could you post the cal curves? I'd be curious to see them.

Also, I'm not sure what you mean when you say "some deviation between the curves in the Cross Spectrum calibrations and a LOT of deviation between the curves in the alternate lab's calibrations" - are you talking about comparing our two mics?


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## Anechoic (Jan 16, 2009)

chris319 said:


> "The man who wears two watches does not know what time it is."


Similarly, the man who has two sound level meters does not know how loud it is!  (although this is an actual principle we use in doing noise control measurements - more data is not necessarily better).



> Using REW's overlay feature I found the curves were nearly identical in the range from about 200 Hz to 3,000 Hz. Outside of this range there is some deviation between the curves in the Cross Spectrum calibrations and a LOT of deviation between the curves in the alternate lab's calibrations.


And there's a reason for this - the range from about 200 Hz to a few kHz is actually pretty easy to measure since you only have to worry about the direct sound field. Below 200 Hz you have to worry about room effects (if you're measuring indoors) and ambient noise (if you're measuring outdoors to minimize room effects). Above a few kHz you have to worry about room reflections (unless you gate the measurements) as well as diffraction effects caused by the microphone body and the mounting method/stand.

NVLAP-accredited labs attempt to work around these difficulties by calibrating the mics using electrostatic actuators rather than using an acoustical method, but they still need to apply correction factors to account for the acoustical effects which increases the error. I get my reference mics calibrated at Scantek in Maryland. Their NVLAP documentation describes the measurement errors for the electrostatic method (p 5). The minimum error (+/- 0.18 dB) is in the range of 100 Hz to 1.25 kHz, with the errors increasing below and above that. This increased error is also reflected in the ANSI and IEC standards for measurement microphones where the allowable errors in the low and high ranges increase by several decibels.



> The dilemma I face is: Which lab's measurement is "more correct"? I have no way of knowing this.


Welcome to my world. 

Let me show you two frequency response curves generated for one of my Type 2 measurement mics, one by the factory (top), and the other by Scantek (bottom):










The curves were made 2 years apart, but the mic was in storage so it wasn't being used. These are curves generated by two ANSI/IEC traceable labs and although they look similar (in general) they aren't the same - the factory curve has a 1.25 dB hump at 5 kHz while the Scantek curve shows the hump around 8.5 kHz. Both curves pass the Type 2 spec so it's not a concern on my part, but these are calibration measurements performed under controlled meteorological conditions with hundreds of thousands of dollars in precision lab quality (Type 0) equipment. If those two labs can't agree to within 1.5 dB with an electrostatic method, it's not totally surprising that the spread between calibrations performed with an acoustical method would be greater.

As I've mentioned before I do make sure the results I get with my setup are consistent with the results a reference lab gets, but that's really the best I can do.



> In pondering this dilemma I took an unorthodox tack. I took the calibration data for each mic and _averaged_ the calibration values reported by each lab for each mic.


This sounds perfectly reasonable. 



> I am quite pleased with the way this project has turned out. I now have a much better grasp on what my mics can do.


Good. Now you can teach me.


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## chris319 (Dec 16, 2008)

> I'm not sure what you mean when you say "some deviation between the curves in the Cross Spectrum calibrations and a LOT of deviation between the curves in the alternate lab's calibrations" - are you talking about comparing our two mics?


I own two ECM8000s, both of which have been calibrated by Cross Spectrum _and_ by Kim G. In a perfect world, two perfectly calibrated mics should match exactly after applying the calibration curves. In reality they don't match exactly, but the two mics are much closer to each other when the Cross Spectrum curves are applied than when Kim's curves are applied. That's no reflection on Kim, that's just the way it is. However, if I average the curves, the two mics match much more closely. Either of these mics can be used as my reference mic, but I need confidence that they are indeed flat. Does that make sense?



> these are calibration measurements performed under controlled meteorological conditions with hundreds of thousands of dollars in precision lab quality (Type 0) equipment


That statement is very revealing. It says that even with fancy, expensive precision equipment and controlled conditions, even the big guys run into variance.



> Now you can teach me.


Hehe I don't know about that. I can tell you that over the weekend I got the opportunity to measure a buddy's three vintage RCA 74B's. What was remarkable was how uniform the responses of the three mics were.

http://www.coutant.org/rca74b/index.html


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## chris319 (Dec 16, 2008)

Two ECM8000s: "Green" and "Red". The first graph shows the difference between "Green" and "Red" as calibrated by KG. All I did was point the mics (one at a time) at my speaker and apply the mic calibration data and an impulse response window. There is no correction to make these flat. The second graph shows the mics as calibrated by Cross Spectrum. They are closer. The third shows a composite mic calibration file obtained by averaging the KG and Cross Spectrum files for each mic. The two mics are closer still. In a perfect world the two calibrated mics would match exactly.


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## RollsRoyce (Apr 20, 2006)

Perhaps some of the difference lies in repeatability-the ability of an instrument to achieve near-identical results over several different measurements of the same device under test. The measurements could be separated by hours or days. It is an important specification with colorimetry equipment, but I don't remember seeing such a number quoted for the mics we commonly use with REW.


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