Calibrating measurement mics.

Jay Barracato

Jay Barracato

Graduate Student
Jan 11, 2011
1,528
5
38
Solomons MD
As I have put together my new measurement system the last thing is a primary standard for my mics. Searching has really only led me to the Extech unit. I really don't need NIST traceable, really only reason is to convert the level meter in SMAART to the values I think in.



What do you use to calibrate your mics?



How often do you calibrate?



Any shortcuts I should know about?



If you are near Maryland and have a standard source, could I swing in and calibrate a coule of mics?



Thanks.

Jay
 
Re: Calibrating measurement mics.

I considered that. It is probably the route I will end up taking. I haven't needed exact levels or logging for any reproducible results.
 
Re: Calibrating measurement mics.

I have a microphone calibrator, although it's been so long since I've used it I'm sure the battery is dead (it's a WEIRD battery). Which also makes me question its own calibration, but I digress.



Every time I dig out my measurement kit, I at least make sure all my mics line up with each other, since FedEx and UPS are not exactly kind to my Pelican cases.



I'll see if I can dig it up and find a battery for it. If I can, you're welcome to borrow it if you'd like. Is your mic a 1/2'' or 1/4'' capsule?
 
Re: Calibrating measurement mics.

Thanks Brian,



It is officially a 6mm so 1/4 inch. If you want to send me a battery model/number I could hunt also.



Matching level with my PAA2 and olympic meters is probably enough for my use.
 
Re: Calibrating measurement mics.

Evan Kirkendall wrote on Tue said:
If you're talking SPL calibrations, I just play pink noise, and calibrate it with my rat shack meter. It works fairly well.







Evan
Click to expand...

Assuming your Rat shack is actually right
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I attended a sub get together (home theatre) a couple of years ago and lots of guys brought their SPL meters. All makes and models. My was recently calibrated-so I ''assume'' it was pretty close to a standard.



From the highest reading to the lowest reading the range was about 7-8dB
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SO just because it ''says'' so-does not make it so.



Consider what a ''real'' SPL meter costs, and then look at the cost of the ''cheapies'' and just think about why the other one cost more.



Remember there is a little screwdriver adjust on the side of the meter-so it is very possible to have it drift a bit.



A man with a meter is sure. A man with 2 meters is sure of nothing-unless it is traceable.
 
Re: Calibrating measurement mics.

But you also have to consider the cost. Most of the time I am more concerned with changes in db rather than the absolute measurement.



There is always a tradeoff in measurement, time and cost vs. the precision and accuracy needed.



Right now I really don't need more accuracy than my handheld meters, so they would be adequate as a standard. When I am hired to provide a sound level log that needs to be admittable as evidence, I will get a traceable standard, and charge as such.

 
Re: Calibrating measurement mics.

Jay,



A ''real'' calibrator will cost you about $500, I've looked into them before and never felt like dropping the money. In between ''I just want close'' and ''the right way'' there's not really anything. If you have a reasonably good measurement mic you may be able to use its stated sensitivity to set actual SPL within a dB or two, but you will need to know the gain of your measurement pre. Smaart should have this capability again eventually, and then Rational should give you mic profiles and if you have a mic on their list you just pick it and you're calibrated ''enough''.



I always just do the same thing Evan does, get close enough do the numbers mean something. Mostly I want to know the differences between my mics, but I want to know that broadband so even a simple calibrator doesn't do what I want.



p.s. If you do buy a calibrator, a pistonphone is the only way to go. The electronic ones are all way too unstable.
 
Re: Calibrating measurement mics.

Bennett,



That is good to know. I was looking at the Extech which is about $425 with NIST traceable certificate. I was definitely thinking that was more than I need. Calibrating against a non-traceable source (like the Extech) didn't seem any different to me than calibrating against another meter.



I figured if someone had one, it might be worth dropping in to do my pair of mics, otherwise Evans method was my first choice.



Actually, all my contacts in the chemistry department have left, but I wonder if I could contact NIST directly. They are only a little over an hour from me.
 
Re: Calibrating measurement mics.-Freq response

One of the problems with a calibrator is that all it does is give you a reference at 1 or 2 freq.



It does not show you the freq response of the mic or any way to check that.



I us multi mics in my alignment setup. I origionally used the origional Superlux 999 mics (before the quality dropped) and they held up real well over a couple of years.



I would always do my own ''calibration'' before each measurement session. I knew which one had the lowest output and would turn on a single loudspeaker and capture that trace. Then I would swap out mics in the same position and adjust the levels as needed so the matched the reference.



This also would give me a chance to keep tabs on the freq response.



Two of the mics started to ''drift'', so I got some real measurement mics. They are ''dead nuts on'' in response and level.



The claibrator assumes the response of the mic is totally flat. If it is not, then the measured SPL could be off.



I had a link (but have lost it) to a guy who measured something like50 of the Behringer mics. And overlaid them. There was quite a variance in level and response. If you don't know exactly where your mic is on the response curve-you could easily make the wrong decision.



Of course having a measured response curve of the particular mic (like you would get from rational) is a great start.
 
Re: Calibrating measurement mics.

Search ebay, because there's plenty of acoustic calibrators for sale there, some pretty good for the money! You can always send it in to get a current calibration. Just recently I got myself a Quest Type 1 calibrator for $60!!! I had it sent to Harry Brill, and he turned on his lathe several mic adapters for a very reasonable price to fit all my different test mics. I'm still waaaay ahead of buying a new calibrator for hundreds of dollars more!



I haven't sent my calibrator in to get it calibrated, primarily because it's only a dB or two off from a known standard that I compared to, which is good enough for my books and for most of my Smaart work. I also keep a Simpson and a Cirrus calibrator handy in case my Quest calibrator doesn't work, but really I don't forsee that happening anytime soon, it's a pretty solid device!



If I do find the need to produce legally binding measurement data, then I'll budget for renting the correct, calibrated gear and do it right.
 
Re: Calibrating measurement mics.

While we are on the subject...



I ran across as least two places on the net where measurement mics can be calibrated for a reasonable fee. Reports were very positive from several posts. Do the experts feel it is worth it to have a ECM8000 run for get a calibration file?



I will be getting SMAART 7 very soon so I wanted to be ready. I have used RTAs since 1980 but the SMAART world is a new adventure. I have the SD6 suite with FFT but have not used IR or transfer functions before. Looks to be very interesting.
 
Re: Calibrating measurement mics.

Hi Cummins,



Cross spectrum labs offers a service to measure your mics and come up with a calibration file. You can even buy new ECM8000's from them with the calibration data. This will prove very useful in an upcoming release of Smaart 7. I would budget the time to attend a 3 day Smaart class to get the full benefit of the software.



Arthur



http://www.cross-spectrum.com/
 
Re: Calibrating measurement mics.

Cummins Mebane wrote on Wed said:
While we are on the subject...



I ran across as least two places on the net where measurement mics can be calibrated for a reasonable fee. Reports were very positive from several posts. Do the experts feel it is worth it to have a ECM8000 run for get a calibration file?



I will be getting SMAART 7 very soon so I wanted to be ready. I have used RTAs since 1980 but the SMAART world is a new adventure. I have the SD6 suite with FFT but have not used IR or transfer functions before. Looks to be very interesting.
Click to expand...

Cummins,



One normally has a calibrator with one's self so one may calibrate one's mic that day with that preamp gain, etc. Knowing the sensitivity of a mic at one frequency, as Ivan said above, depends largely on how flat the mic is and whether it has drifted. Publishing the sensitivity for your mic may not help others with the same mic. There is plenty of variation between cheap mics. Even very good ($700+) mics, which may have uniform frequency response, may have sensitivities that vary by a decibel or more.



Long story short, calibrate your mic if you like but it will only hold true for a certain preamp gain. Also, you only need to go to the trouble if you need reasonably accurate SPL numbers. If you really need accurate SPL numbers then you have to use a type 1 mic ($1,500+) and a calibrator with trackable calibration itself ($500+).
 
Re: Calibrating measurement mics.

Thanks guys. Cross Spectrum is one of the places I was referring to.

I am mainly interested in the relative frequency response of the mic vs. the SPL.

There aren't any RA classes nearby but I learn very well from reading and hands on. Any book recommendations are appreciated.
 
Re: Calibrating measurement mics.

Arthur Skudra wrote on Wed said:
Definitely Bob McCarthy's book: Sound Systems Design and Optimization should be at the top of your list. There's a wealth of information at the Rational Acoustics forums.
Click to expand...



In addition to the forums, I found the white papers on the rational site to be useful. You just have to watch out for the fact that they were written for different versions of the software and things are not always labeled the same.
 
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