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Cross-Spectrum Microphone Calibration Service - USA

141634 Views 367 Replies 78 Participants Last post by  cichlids

We are happy to announce and recommend Cross-Spectrum Labs for microphone calibration in the U.S. ... particularly the Behringer ECM8000 and Galaxy CM-140. Of course, they will also calibrate other microphones as well.

Cross·Spectrum Labs
P.O. Box 90842
Springfield, MA 01139

I have personally spoken with Herb Singleton, founder of Cross-Spectrum, and he as agreed to calibrate these units 10Hz to 20KHz provided you specify you would like calibration to 10Hz. The charge is $55.00 and includes the cost of return shipping via USPS for customers in the continental United States..

Cross-Spectrum will use the pressure method to calibrate the low frequency response and the quasi-anechoic free-field on-axis method to calibrate the upper frequency response. The pressure method is one of the most accurate methods of measuring the frequency response of a microphone, however baffle diffraction must be accounted for when using this method. Generally mic manufacturers will provide the variance to offset the baffle diffraction. Unfortunately, Behringer nor Galaxy offers these variance corrections. Therefore, the free-field method of measuring the upper frequencies will be more accurate for these units. These combined calibration methods will give us the best calibration file for the ECM8000 and CM-140 mics. This is one of the first labs we have found that will calibrate the CM-140 SPL Meter microphone.

Cross-Spectrum also offers calibrated Behringer ECM-8000 microphones for sale. The cost is $100 + shipping. Please see their Behringer page for more information.

Herb will be joining us here at the Shack and has agreed to answer any questions anyone may have. Please check out their site before asking questions... you may very well find your answer. If you do not find the answer you are looking for, you may post a question here in this thread and Herb will do his best to get you an answer. Remember he is busy in the lab and does not monitor this forum 24/7, so please be patient... or you are more than happy to contact him by telephone (see their Contact Us page).

Remember, if you are wanting your mic calibrated to 10Hz, you must include a note requesting same, otherwise the calibration may only be to 20Hz.

We appreciate Herb's willingness to work with us in providing this service to our members here at the Shack.
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Re: Cross-Spectrum Microphone Calibration Service

Hi Herb,

Good to see you posting here. Do you have any guess about the high and low end roll-offs of the Behringer? Say X dB/oct at Y Hz? With a 96K sound card and a 128K FFT, you could be generating measurement files covering .7-48K Hz. It would be nice to have some non-zero numbers in the cal file at those extremes, even if they were just a wild guess, so you didn't have your CAD software assuming there was a sudden big jump in the response. Seems like that could lead to phase errors in your crossover design.
Re: Cross-Spectrum Microphone Calibration Service

The ECM8000s have so much unit to unit variation that I'm not comfortable giving generalized numbers - well other than +/- 15 from 20Hz to 20kHz :).

As for the extremes, yes it is possible with a long FFT and wideband sound card to give a wide frequency response, but acoustically it's very difficult to generate reliable test data at those extremes. I could give numbers at those frequencies, but they would be meaningless.
Okay, forget the Behringer. Generally speaking, what is the slope of the roll-off of generic condenser mics on the high and low end? 2nd order? Steeper? I'm just trying to learn how these mics work.
Re: Cross-Spectrum Microphone Calibration Service

Thanks, Herb. To keep things simple, could we start with the response of a plain old electret capsule with no extra circuits? Most software takes 2-channel measurements these days so the response of the preamp gets subtracted out.
Re: Cross-Spectrum Microphone Calibration Service

I think the Behringer and the Nady are both OEM'd from Superlux in China -- probably the same mic with a different label. They seem to have lost their flat response when Panasonic discontinued the WM-60 and everyone switched to the WM-61. I thought it was a Superlux problem but I recently read a paper where someone built 8 mics using the WM-61, 4 with the stock wiring and 4 with the 'Linkwitz mod'. They all had the 10K hump although the ones with the mod didn't hump quite as high. The ones with the mod also had lower distortion at high SPL.

Herb, the Panasonic electret capsules don't really require a preamp like you describe for a traditional condenser mic. They work fine with just a battery and a resistor as long as they are feeding a fairly high impedance preamp or sound card. Superlux is including more of a preamp built in to make them compatible with a pro mic preamp with low impedance and 48V.

I was looking at some measured 5-48K curves for the Behringer and it looks like the tails are more like 24dB/oct. The tails don't really matter for someone measuring a room but they could make a difference for someone designing a crossover because of errors in the calculated phase using a Hilbert transform. As an experiment, I built a fake cal file extending down to 1Hz and ran a Hilbert transform to get the phase. Then I truncated the file to 20Hz and ran the HBT again. The phase error at 20Hz was about 70 degrees with the truncated file.
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Re: Cross-Spectrum Microphone Calibration Service

You have to be careful with using Hilbert Transforms on condenser mic freq response data since condenser mics aren't minimum phase.
Hmmm, interesting. I'm no expert but I can't imagine a mechanism where a mechanical transducer and analog electronics wouldn't be minimum phase. There have been a lot of discussions about that with speaker drivers, cone breakup modes, baffle diffraction, etc. and it always turns out (as far as I know) that it is behaving as a minimum phase system even though it doesn't appear that way at first. BUT.... the HBT is making assumptions and it's subject to computation errors if you don't do things like extending the tails beyond the measured values. You have to do that with speaker drivers and I'd imagine you have to do it with mics as well.
Re: Cross-Spectrum Microphone Calibration Service

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.
as of August, I'm going to offer 4 mic models: a) "Basic" with 0-deg freq response for $85, b) "Basic-Plus" with 0/45/90 deg for $90, c) "Extra" with 0-deg freq response, polar, noise floor, and sensitivity for $100 and d) "Extra-Plus" with the "Extra" package + 0/45/90 deg for $110. On the plus side, I'm going to do freq-response down to 5 Hz by default.
Do the Basic and Basic-Plus not include 1K sensitivity? Seems like everyone would need that.
Correct, the Basic and Basic-plus do not include 1 kHz sensitivity. I thought more people would want sensitivity but it turns out that very few people care about - in fact more people want the polar response than the sensitivity.
That's amazing. How are they measuring SPL? An RS meter that may or may not be within 3dB?

On another subject, can you calibrate above 24K for an extra charge? Many people are using 96K sound cards with a nominal 48K capability so it would be nice to have a general idea what the mic is doing up there, even if the calibration isn't perfect.
ARTA (along with some other programs, don't know about REW) has a setup routine where you calibrate your mic preamp and sound card with tones and a voltmeter so you can do your measurements in dB SPL if you can plug in the mic's sensitivity.
The crowd where I usually hang out is into designing speakers so having the levels calibrated is useful when you're mixing and matching drivers that may have been measured at different times and under different conditions. The most common way is to normalize everything to 2.83V/1m when you save the files.
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