Cross-Spectrum Microphone Calibration Service - USA

**Anechoic** · **03-04-09, 12:24 AM**

I've heard it repeated by extremely trustworthy sources (including people who have written some of the books on my shelf and have written ANSI/IEC microphone standards) that it is indeed the case, but have never seen a detailed explanation as to how/why.

The condenser mic is a fundamentlally different transducer than a loudspeaker driver- the loudspeaker is a linear motor that can be modeled by 2-D array of point sources on a plane while a condenser mic is a capacitor of varying charge and their mathematical models exhibit very different behavior (for example the response of the condenser mic is independent of frequency below the diaphragm resonance). It is my understanding that dynamic mics and ribbon mics, which basically act as loudspeakers in reverse, are minimum phase but condensers are not.

**Anechoic** · **03-04-09, 02:03 PM**

For anyone interested in purchasing a calibrated ECM8000 or who wants their own mic calibrated:

Most of my suppliers are out of stock of ECM8000 and won't have a reliable supply until mid-March. I am able to get them one at a time and I am filling back orders but please be aware it may take a couple of weeks to fill new orders. As always, I do take pre-orders and I ship them on a first-come-first-served basis.
I normally turn around mic calibrations in 48 hours. Please be aware that I will be traveling on business from March 16 to March 23 so any units I receive for calibration after March 11 won't be returned until the week of the 23rd.

**Dennis H** · **03-07-09, 09:13 PM**

Quote:

Anechoic wrote:

I've heard it repeated by extremely trustworthy sources (including people who have written some of the books on my shelf and have written ANSI/IEC microphone standards) that it is indeed the case, but have never seen a detailed explanation as to how/why.

The condenser mic is a fundamentlally different transducer than a loudspeaker driver- the loudspeaker is a linear motor that can be modeled by 2-D array of point sources on a plane while a condenser mic is a capacitor of varying charge and their mathematical models exhibit very different behavior (for example the response of the condenser mic is independent of frequency below the diaphragm resonance). It is my understanding that dynamic mics and ribbon mics, which basically act as loudspeakers in reverse, are minimum phase but condensers are not.

Thanks, Herb. I guess I'll have to take your word for that. Just curious, how could you ever use a condenser mic to design a crossover if it's introducing excess-phase errors? Seems like all the measurement and crossover programs assume the cal file is minimum phase and experience shows they work pretty well.

**Anechoic** · **03-11-09, 04:59 PM**

Quote:

Dennis H wrote:

Thanks, Herb. I guess I'll have to take your word for that. Just curious, how could you ever use a condenser mic to design a crossover if it's introducing excess-phase errors?

About all you can do is trust in your cross-over design theory. That said most pro measurement mics are designed to have a relatively flat phase response up to 15-20 kHz, but that doesn't help you with an ECM8000 unfortunately.

Over the past week I made another attempt to get phase data for my measurement mics with no success. In the case of BSWA, in what I ca only guess is a language-barrier problem, no matter how many times I ask for "phase response" and "phase data" for the MP201, they keep sending me frequency response curves.

Quote:

Seems like all the measurement and crossover programs assume the cal file is minimum phase and experience shows they work pretty well.

I'll have to try and dig it up, but when I first started doing microphone measurements, I used to give phase results generated by the Hilbert transform. When I learned that condenser mics weren't minimum phase devices, I found a generic freq and phase response curve for a measurement mic. I scanned in the frequency response, ran the Hilbert alogorthim on the data and compared the resulting phase response to the measured curve - and they weren't the same. that little exercise scared me straight.

**Ricci** · **03-12-09, 01:33 PM**

Herb,

Do you know what the useable spl limit for the ECM8000 is? Seems to be about 120db or less?

**lovebohn** · **03-13-09, 02:44 PM**

Quote:

Ricci wrote:

Herb,

Do you know what the useable spl limit for the ECM8000 is? Seems to be about 120db or less?

Good question!

**Anechoic** · **03-13-09, 02:57 PM**

Quote:

Ricci wrote:

Herb,

Do you know what the useable spl limit for the ECM8000 is? Seems to be about 120db or less?

I've tested it up to 114 dB with no problems. When I get back from my trip I'll run some tests and see if I can find a definitive limit.

**Ricci** · **03-15-09, 01:45 PM**

Quote:

Anechoic wrote:

I've tested it up to 114 dB with no problems. When I get back from my trip I'll run some tests and see if I can find a definitive limit.

That'd be great. I seemed to have run into some output/ distortion limits with the mic while doing some close mic (2 or 3") low frequency tests. The apparent level that close up to the driver cone was somewhere between 120-125db.

**Anechoic** · **03-23-09, 10:47 PM**

Quote:

Ricci wrote:

That'd be great. I seemed to have run into some output/ distortion limits with the mic while doing some close mic (2 or 3") low frequency tests. The apparent level that close up to the driver cone was somewhere between 120-125db.

I just ran a quick test comparing the distortion of my one of my reference mics (ACO Pacific 7052, rated to 144 dB) to an ECM8000 at 1 kHz.

At 115 dB, my ref mic gets 0.13% distortion, the ECM8000 reads 0.54%

At 120 dB, my ref mic gets 0.22% distortion, the ECM8000 reads 0.81%

At 125 dB, my ref mic gets 0.46% distortion, the ECM8000 reads 1.37%

**Ricci** · **03-24-09, 05:16 PM**

Thanks Herb.

Overall that doesn't seem too bad, but it looks like you want to keep things under 120db if you can for the ECM8000. Do you think that there would be a difference if the frequency was lower at like 32hz or 40hz?

**Anechoic** · **03-25-09, 01:34 AM**

I'm falling behind trying to cal all those mics I sold last week, but when I get a chance, I'll run the test again at a lower frequency.

**Ricci** · **03-25-09, 02:28 PM**

Thanks again for being so forthcoming with data and help with odd requests and questions. I think this distortion info will be very interesting for a lot of people.

I'm going to go ahead and guess that there will be noticeably higher distortion on the LF test. Won't the lower frequencies cause greater deflection of the mics diaghram for a set SPL?

**Anechoic** · **03-25-09, 06:11 PM**

Quote:

Ricci wrote:

I'm going to go ahead and guess that there will be noticeably higher distortion on the LF test.?

Tests at 30 Hz:

At 115 dB, my ref mic gets 1.8% distortion, the ECM8000 reads 3.1%

At 120 dB, my ref mic gets 2.95% distortion, the ECM8000 reads 5.0%

At 125 dB, my ref mic gets 4.2% distortion, the ECM8000 reads 7.4%

FWIW, this is a different ECM8000 than the one used for the 1 kHz test.

**Ricci** · **03-25-09, 10:43 PM**

Well...That's eye opening. That could explain some of the things I was seeing. I can only assume that the distortion will get even higher the lower the frequencies get. Looks like you should keep things under 110db when using this mic to remain clean. Not that any one serious about high SPL measurements would be using an ECM8000 in the first place.

Do you think that the ECM will compress the signal somewhat above 120db?

**Anechoic** · **03-26-09, 01:41 AM**

Quote:

Ricci wrote:

Well...That's eye opening. That could explain some of the things I was seeing. I can only assume that the distortion will get even higher the lower the frequencies get. Looks like you should keep things under 110db when using this mic to remain clean.

That sounds like good advice.

Quote:

Do you think that the ECM will compress the signal somewhat above 120db?

I think that it depends on the mechanism that is causing the distortion, but my guess would be that you would see clipping rather than compression. I'll have to but the mic on an o-scope and check some waveforms at high amplitudes to see what's going on.

**Anechoic** · **03-26-09, 01:45 AM**

Quote:

Anechoic wrote:

Over the past week I made another attempt to get phase data for my measurement mics with no success. In the case of BSWA, in what I can only guess is a language-barrier problem, no matter how many times I ask for "phase response" and "phase data" for the MP201, they keep sending me frequency response curves.

BSWA finally got back to me with a phase curve (theoretical I suspect) for the MP201 microphone. I'm going to have to spend some time experimenting to see if it's useful, but if it is, I may be adding microphone phase measurements to my repertoire.

**Anechoic** · **04-01-09, 10:49 AM**

One other thing - when ordering the calibrated ECM8000s, I've gotten a lot of requests from people (most in this forum) for calibrations down to 10 Hz and up to 25 kHz. My normal default was to do 20Hz to 20kHz, but with so many people asking for the extended frequency response, and me having problems trying to keep track of those requests, I am now generating 10 Hz to 25 kHz FR by default for calibrated ECM8000s.. If you want 10Hz to 25kHz you no longer have to ask, it will come that way.

For microphones sent to me for calibration, those will continue to be 20 to 20 so you'll have to ask for the extended response.

I will also continue to perform 5 Hz calibrations and freq responses at up to additional 2 angles at no charge for the $85 and $100 mics, but again you'll have to ask for those. Also note that asking for those additional measurements may delay your order.

**Anechoic** · **05-04-09, 11:37 PM**

so I've been asked here and other places whether ECM8000 have a lot of unit-to-unit variation and I usually answer by pointing to a plot done by someone else that shows a bunch of units compared with each other.

I finally got around to putting together a plot of mics I've sold - this is about 40 mics and represents about 1/3 of the ECM8000's I've measured over the last year (with the bulk of them mics from the newest Behringer run with the serial numbers). Thought people might be interested in seeing it:

Another interesting observation: based on the last bunch of mics I received it seems that the high-frequency variation is based on the location of the mic capsule in the end of the wand - the greater the capsule is recessed compared with another unit, the larger the difference.

**Ricci** · **05-05-09, 11:26 AM**

Good stuff. Looks like the unit to unit variation has a range of around 6db or more on both ends of the spectrum.

. Calibrating your ECM or buying one with the calibration done already seems like a necessary evil.

**lcaillo** · **05-05-09, 10:33 PM**

This data needs to be made into a sticky to emphasize the importance of having some actual reference data rather than making assumptions regarding the response of a mic.

**Anechoic** · **05-05-09, 11:29 PM**

Quote:

lcaillo wrote:

This data needs to be made into a sticky to emphasize the importance of having some actual reference data rather than making assumptions regarding the response of a mic.

So long as you keep the title intact, feel free to use/distribute the plot for whatever.

**brucek** · **05-06-09, 08:18 AM**

Quote:

This data needs to be made into a sticky

It's been there for two days....

brucek

**Anechoic** · **05-08-09, 02:57 PM**

I've gotten a couple of inquiries about sound level meter measurements, so let me formalize the policy:

I will perform on-axis and random incidence (free field) measurements of any SLM for $60 (which includes return shipping). That will include measurements at A/C/flat weighting as desired (one, or all of the above). This will use the same free-field testing method as my mic meaurements and will be non-NIST traceable. You can see a sample report of a sound meter calibration here (C-weighting only in this example). The typical report will be given for 10Hz to 25kHz but I'll go down to 5Hz upon request.

If you have a sound level meter with a 1/2-inch microphone (like the Galaxy CM-140, Tenma/Extech/etc lines or any number of meters, but not the Radio Shack meters) I can also perform a NIST traceable calibration and adjustment of the meter at 125 Hz, 250 Hz, 500 Hz and 1000 Hz and 74/84/94/104/114 dB. If the pressure field-to-free field adjustment data is available, I can also give you data at 2000 Hz and 4000 Hz. The price for this service is $30, including return shipping. You get a much simplified report showing the 20 (or 30) data points plotted against the ANSI limits and a table with the relative levels.

The price for the combined free-field measurements and the calibration-level adjustment is $75 - that is, that's the price for mere mortals. If you mention Home Theater Shack when you contact me, I'll do everything for $65 (return shipping included). As with the mic cal, I should be able to turn around the unit in 48-hours after receiving it.

For those that don't want to part with their meter, you can buy an acoustic calibrator (that typically calibrate at one or two levels and one or two frequencies) for a couple of hundred bucks.

One thing to be aware of - the absolute calibration level of your meter will drift with changing temperature, humidity and altitude, so don't expect that any adjustment I can do will be spot on for ever. If you are really looking for your meter to be calibrated to a high degree of accuracy on a regular basis, it might be worthwhile to invest in an acoustic calibrator.

**Tchao** · **05-09-09, 06:18 PM**

Hello,
First, I have to say that my english is not fluent and I will try to do my best to improve it : I apologize for that

second, it's my first post, and I expect to be in the "tolerance 0" of this forum, because I did'nt understand the entire rules that I agreed ...
Third: thank you Herb for your explanations, even though I don't understand every thing.

My questions to Herb:
I have a microphone from my pioneer audio/video multi-channel receiver with mcacc auto-calibration system.
Q1:Is-it possible to send you this microphone (the only one I have) to have a file of its calibration (10-25KHz) to use it with REW software ?
Q2: when you wrote "65$ for the combined free-field measurements and the calibration-level adjustment ", is it the same price when the microphone comes from France ? (that's why my english is not good

)
Q3: is there somewhere an explanation about a "NIST traceable calibration" ? because in the example of calibration it's written that's it's not a NIST traceable calibration. So what a Nist Traceable calibration has special ? is it a certificate which proves that the calibration is under NIST standard and is as "true" as possible ?

Sorry for this questions so simple compared the others you debate here.

**Anechoic** · **06-16-09, 12:33 AM**

Sorry Tchao, I didn't see this post.

Quote:

Tchao wrote:

Hello,

My questions to Herb:
I have a microphone from my pioneer audio/video multi-channel receiver with mcacc auto-calibration system.
Q1:Is-it possible to send you this microphone (the only one I have) to have a file of its calibration (10-25KHz) to use it with REW software ?

I don't know - if the microphone has conventional power and signal connections, then yes, but if it uses some propriety Pioneer connection, then no.

Quote:

Q2: when you wrote "65$ for the combined free-field measurements and the calibration-level adjustment ", is it the same price when the microphone comes from France ? (that's why my english is not good

)

I would have to charge extra for the international shipping. And don't worry about your English, it's much better than my French

Quote:

Q3: is there somewhere an explanation about a "NIST traceable calibration" ? because in the example of calibration it's written that's it's not a NIST traceable calibration. So what a Nist Traceable calibration has special ? is it a certificate which proves that the calibration is under NIST standard and is as "true" as possible ?

In short, NIST-traceable basically means that the process and techniques used in the calibration can be traced back to techniques outlined in standards recognized by the U.S. National Institute of Standards and Technology (usually ANSI, IEC, ISO, ASTM, etc standards) and the instrumentation used in the process have been calibrated by NIST or a NIST-accredited laboratory. Essentially is means that the NIST-traceable calibration would hold up in a court of law.

Now the processes I use for the calibration are consistent with IEC 60268 methods and the equipment I use have been calibrated by NIST-traceable labs. If I wanted to go through the process of performing a statistical analysis of my methods using NVLAP methods I could probably assert NIST-traceability, but I don't want to deal with the potential liability issues so I just slap "not NIST traceable" on the reports to reduce the chance of lawsuits.

The point calibrations at 125, 250, 500 and 1000 Hz use an acoustic calibrator that meets ANSI S1.40 and has been calibrated by an accredited 3rd party lab so I have to problem asserting NIST traceability for those measurements.

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