Cross-Spectrum Microphone Calibration Service - USA - Page 21 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #201 of 368 Old 08-30-12, 11:46 AM
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Re: Cross-Spectrum Microphone Calibration Service - USA

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Anechoic wrote: View Post
There's also the used-equipment inventory at Modal Shop.
Neat, love used equipment sources--good call, thanks.

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Also remember that the noise floor of the mic will have to be at least 8 to 10 dB lower than the noise level to be measured, so to measure noise levels at 20 dBA, the mic will need a noise floor of 8 to 10 dBA.
OK so to verify RC30, the maximum acceptable noise level per the Nyal/Hedbeck paper, because the A curve is ~0dB at 4k, and the RC curve is at 20 there, to be able to verify that requirement you'd need a mic with 10-12 dBA noise floor? Am I incorrect because the RC measurement is in an octave band which will contain less energy and therefore correspond to a higher dBA noise floor requirement? I note that the chart in the link shows 40 dBA as the equivalent sound level of RC30--if true that would be a significantly easier requirement to meet (maybe I need to just read the ARTA manual finally, LOL).

With the LinearX mic you do have to pay extra for the clamshell case and mic clip. LOL. It also looks like it requires +9V so you have to build special cables rather than simply being able to use the phantom power on the common interfaces....
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post #202 of 368 Old 08-30-12, 01:19 PM
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Re: Cross-Spectrum Microphone Calibration Service - USA

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OK so to verify RC30, the maximum acceptable noise level per the Nyal/Hedbeck paper, because the A curve is ~0dB at 4k, and the RC curve is at 20 there, to be able to verify that requirement you'd need a mic with 10-12 dBA noise floor? Am I incorrect because the RC measurement is in an octave band which will contain less energy and therefore correspond to a higher dBA noise floor requirement? I note that the chart in the link shows 40 dBA as the equivalent sound level of RC30--if true that would be a significantly easier requirement to meet (maybe I need to just read the ARTA manual finally, LOL).
This is a hard question to answer since RC (as well as NC and NCB) are defined in the frequency domain, so the applicability of any particular mike will really depend on the specific characteristics of the mic as well as the spectrum of the background noise. The overall equivalent A-weighted level can very wildly for the same RC level. For example, if the background noise was a pure tone at 31.5 Hz at 60 dB, the A-weighted equivalent for that tone is 20 dBA, but it would still exceed RC 30.

All I can really say for sure is that microphone self-noise typically follows a 1/f pattern, so the self-noise may be more of a problem in low-frequency applications compared to mid- and high-frequency applications.
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post #203 of 368 Old 08-30-12, 02:15 PM
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Re: Cross-Spectrum Microphone Calibration Service - USA

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This is a hard question to answer since RC (as well as NC and NCB) are defined in the frequency domain
OK, I can see your point here.

Pros at this can't all just buy the ultimate B&K rig I'd imagine (although Nyal uses some type of B&K rig, don't know if it's "ultimate" -- but B&K's app note on KYDG says KYDG balked at the cost). And I wouldn't think they would all just buy a mic advertised for this use and hope. If there isn't a way to pre-calculate it, do pros do a test that verifies the self-noise of their chain?

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All I can really say for sure is that microphone self-noise typically follows a 1/f pattern, so the self-noise may be more of a problem in low-frequency applications compared to mid- and high-frequency applications.
That seems promising, given that's effectively the shape of the allowed noise curves under most systems--especially RC. Thanks for that, I was wondering what the typical spectral shape looks like since I don't think anyone's mentioned it yet.
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post #204 of 368 Old 08-30-12, 02:38 PM
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Re: Cross-Spectrum Microphone Calibration Service - USA

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OK, I can see your point here.

Pros at this can't all just buy the ultimate B&K rig I'd imagine (although Nyal uses some type of B&K rig, don't know if it's "ultimate" -- but B&K's app note on KYDG says KYDG balked at the cost). And I wouldn't think they would all just buy a mic advertised for this use and hope. If there isn't a way to pre-calculate it, do pros do a test that verifies the self-noise of their chain?
Yes. The simplest way is to get a dummy mic that matches the capacitance of the microphone you're going to use (something like this) and determine the equivalent noise floor. The more difficult method is to put the mic in a quiet environment and determine the noise floor directly. For my noise floor measurements, I have an old hearing-aid chamber that I bought off eBay a few years ago, that provides isolation of about 15-20 dB. Using it in my basement at night, I can get levels down to below 16 dBA (that's as low as my Larson-Davis, NTi and Sencore meters go with the mics I own). Building a heavier-duty enclosure with mounting it on a vibration-isolated platform, you might be able to approach 5 dBA or so. And of course there are anechoic chambers such as the Orfield facility with free-field background levels down to 0 dBA or below.

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That seems promising, given that's effectively the shape of the allowed noise curves under most systems--especially RC. Thanks for that, I was wondering what the typical spectral shape looks like since I don't think anyone's mentioned it yet.
If the noise source is a properly installed and balanced HVAC noise, the spectrum should resemble NC/NCB/RC curves. If the HVAC is improperly installed (unbalanced, or generating structure-borne noise) or it's a different source (outdoor traffic for example), the spectrum would change accordingly.
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post #205 of 368 Old 08-31-12, 07:20 AM
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Re: Cross-Spectrum Microphone Calibration Service - USA

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Yes. The simplest way is to get a dummy mic that matches the capacitance of the microphone you're going to use
OK gotcha. Now this is starting to make sense. One more question, that page says "The method is not usable for measurement of the thermodynamic noise of the microphone cartridge." I presume this is based on the presumption that is an inconsequential contributor to the total self-noise of the chain (so the preamp is virtually all of the self-noise), and that there are studies somewhere substantiating it; is this correct? Or is it just sort of standard practice so no one worries about it anymore?

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For my noise floor measurements, I have an old hearing-aid chamber
Ahh, makes sense again. I saw a page somewhere discussing mic testing in a chamber, and I'd envisioned it before that. That there's a commercial product only makes sense.

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If the noise source is a properly installed and balanced HVAC noise, the spectrum should resemble NC/NCB/RC curves.
LOL, I knew that--I was wondering about the spectrum of the self-noise, not the spectrum of the typical expected environmental noise/etc. I should have been more specific.

Thanks for your insights here.
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post #206 of 368 Old 08-31-12, 10:52 AM
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Re: Cross-Spectrum Microphone Calibration Service - USA

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OK gotcha. Now this is starting to make sense. One more question, that page says "The method is not usable for measurement of the thermodynamic noise of the microphone cartridge." I presume this is based on the presumption that is an inconsequential contributor to the total self-noise of the chain (so the preamp is virtually all of the self-noise), and that there are studies somewhere substantiating it; is this correct? Or is it just sort of standard practice so no one worries about it anymore?
Good question. Consulting my copy of Beranek's Acoustical Measurements, the self noise (in rms volts) caused by thermal fluctuations can be calculated using the equation

e = sqrt (4 * k * T * R)

where k= Boltzmann gas constant of 1.37e-23, T= absolute temperature in Kelvin, R is the resistive component of the mic impedance (usually in the range of a couple of hundred ohms)

For a temperature of 296 K (room temp), R set to 1000 ohms, that gives an rms voltage of 4e-9 volts. For a 10 mV/Pa mic, that corresponds to an RMS pressure of 4e-7 Pa, which is about -33 dB. So it looks like it is an inconsequential contributor.

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Ahh, makes sense again. I saw a page somewhere discussing mic testing in a chamber, and I'd envisioned it before that. That there's a commercial product only makes sense.
You could just as easily build one using heavy wood and foam, it was just easier for me to buy it on eBay, since I don't think I paid more than $50 for it.

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LOL, I knew that--I was wondering about the spectrum of the self-noise, not the spectrum of the typical expected environmental noise/etc. I should have been more specific.
Oops.

Here are some noise floor plots using an NTI XL2 and a dummy mic:





Here are some noise floor plots using an NTI XL2 and a PCB mic in my chamber (note the different y-axis scale):





The chamber measurements were made in the middle of the day, so they're probably a few dB high because of outdoor activity.
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post #207 of 368 Old 08-31-12, 11:35 AM
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Re: Cross-Spectrum Microphone Calibration Service - USA

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So it looks like it is an inconsequential contributor.
Does this only go for pro-style mics, which I presume don't have active components in the capsule? Because there are lots of pages about the Panasonic WM-61a (supposed that was/is used in the ECM8000) that supposedly has a FET in it, the (infamous) Linkwitz mod, etc.

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You could just as easily build one using heavy wood and foam, it was just easier for me to buy it on eBay, since I don't think I paid more than $50 for it.
Fair enough. There's one there now too but it's $150 start price for the auction. Not that I really need such a thing, LOL.

Thanks for the plots, I always like data.
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post #208 of 368 Old 08-31-12, 01:09 PM
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Re: Cross-Spectrum Microphone Calibration Service - USA

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aackthpt wrote: View Post
Does this only go for pro-style mics, which I presume don't have active components in the capsule? Because there are lots of pages about the Panasonic WM-61a (supposed that was/is used in the ECM8000) that supposedly has a FET in it, the (infamous) Linkwitz mod, etc.
That's a good point about the FET, pro measurement mic capsules (AFAICT) are relativey "pure" with just the diaphragm and a charged backplate. The pre-amps obviously have more electronics, but they are designed to be low noise (pre-amps go for about $300 - $500 new, $50-$150 used, plus you need a power supply).

The noise floor for WM-61a's that I've measured have been around 25 dBA.

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Fair enough. There's one there now too but it's $150 start price for the auction. Not that I really need such a thing, LOL.
I'm guessing that one actually works, unlike the one I bought.

Another option would be to check it a local cabinet maker, furniture maker or woodshop. A local cabinet maker made the enclosure of my reference speaker (12-inch cube) using scrap pieces of 1/2-inch mdf for $50.
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post #209 of 368 Old 09-02-12, 09:02 AM
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Let me just say thank you very much for all your responses. They were highly enlightening for me and I think I understand the basics of this area much better now.

I did think of one more pertinent question... for a hobbyist wanting to measure the background (or projector, pro amp, etc) noise in their HT, do you think (for absolute level) comparison to RS meter (eg I have the old analog model) is sufficient or would I need to consider getting a meter with calibration? Would it be better to just invest in an SPL calibrator? I read that meters are better than they used to be and that calibrators are a throwback (and double check) today.
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post #210 of 368 Old 09-02-12, 12:00 PM
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Re: Cross-Spectrum Microphone Calibration Service - USA

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aackthpt wrote: View Post
I did think of one more pertinent question... for a hobbyist wanting to measure the background (or projector, pro amp, etc) noise in their HT, do you think (for absolute level) comparison to RS meter (eg I have the old analog model) is sufficient or would I need to consider getting a meter with calibration? Would it be better to just invest in an SPL calibrator? I read that meters are better than they used to be and that calibrators are a throwback (and double check) today.
If you're just looking at overall dBA or dBC levels, the Radio Shack meter isn't actually too bad. However, if you're trying to get levels at particular frequencies or particular frequency bands, stay away from the RS meters as their frequency responses are all over the place, especially above 1 kHz.

Now with card to getting a meter with or without calibration - if you get a meter that is ANSI or IEC rated, generally the frequency response will fall within the rated tolerances even if you don't have a specific calibration curve for the meter. Just keep in mind the the Type 2/Class 2 tolerances are +/- 1.5 dB from 100 to 1250 Hz, +/- 2 dB from 40-80 Hz & 1600-2000 Hz, and become higher beyond those limits (up to +5/-infinity below 20 Hz and above 8000 Hz) so it's up to you to decide if those tolerances are acceptable.

As for getting an acoustical calibrator, again that depends on how precise you are trying to get. Out-of-the-box most ANSI/IEC rated meters (even the cheap ones) will be within 3-4 dB of the absolute level - most of the differences are do to unit-to-unit setting variations as well as factors that can affect ambient air pressures (such as elevation). If you want more precision, an acoustical calibrator is a good investment, you can get decent Type 2 calibrators from Amazon. A calibrator is also good for checking the meter if you drop it, which might shift the calibration set screw found on lower end meters.

Again remember that the noise floor for your meter needs to be at least 8-10 dB lower than the level you're trying to measure. The noise floor on Type 2 meters (including the Galaxy ones I sell) are around 30 dBA, so if you're trying to measure noise from quiet equipment, you might have difficulty with those meters.
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