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Author Topic: Measurement Microphones  (Read 86716 times)

Mac Kerr

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Calibration
« Reply #60 on: January 01, 2009, 06:34:11 pm »

Christopher Wintz wrote on Thu, 01 January 2009 18:29

How would you compensate for this while using a mic calibrator, do you need to?

A mic calibrator is used to set a known reference level in your measurement system. It has nothing to do with frequency response, it creates a calibrated level at 1kHz. Using that known level you can adjust your meter or analyzer to match that level while the calibrator is on.

Mac
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Tom Young

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Re: Measurement Microphones
« Reply #61 on: January 01, 2009, 07:24:15 pm »

Yes, all microphones behave in this manner. Higher frequencies cannot bend (diffract) around the mic head and be picked up by the mic, as lower frequencies do. That being said, more expensive mic's (and especially in the measurement mic category) are designed to be flatter off axis (as well as on axis), than cheaper microphones can be. There is one exception (to this directional characteristic at high frequencies tendency) that I can think of: the diffusion sphere that Bruel and Kjaer makes for their omni-directional mic's to make them more omni-directional at very high frequencies. Look it up.

This has no bearing on calibration, which in this context is setting the microphone gain so that the SLM sensitivity results in accurate, *absolute* SPL measurements. Calibration is done most often at 1kHz (well below where the mic begins to become directional) and the driver in the calibrator faces directly into the mic diaphragm. FYI- the other "standard" calibration frequency is 250Hz. But 1kHz is significant/important because whether the SLM is in the Flat or A or C weighting position.... they are all flat (unweighted) at 1kHz.

Beyer Dynamic is basically referencing the difference between free field measurement and diffuse feild measurement.

If you google on those terms and read up on what you find, you will grasp more how this comes into play.

(edited for grammar, spelling and additional trivial calibration factoids)
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Tom Young
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HarryBrillJr.

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Re: Measurement Microphones
« Reply #62 on: January 01, 2009, 10:46:41 pm »

Christopher Wintz wrote on Thu, 01 January 2009 17:29

Not to beat a dead horse, but I have more questions about measurement mics.

I picked up a Beyerdynamic MM1 the other day.  Not the greatest mic on the market, but it was well within my price range and it looks like it will be a good performer.

In the spec booklet it mentions an energy build up that can accour in front of the mic at 10+KHz, if the make is facing directly towards the source, thus throwing off the frequancy response and creating some phase issues.  Which makes sense as the wavelangth would then be smaller then the diameter of the mic.

Two questions:
Are all mics susceptible to this? (And that all measurement mics should be perpendicular to the source during measurement...)
How would you compensate for this while using a mic calibrator, do you need to?


You should probably just go ahead and face the mic toward the source.  Good luck finding a calibrator adapter to fit that mic.  If I remember right it's not a standard size.  Perhaps Beyer makes an adapter.  I measured one of these against an M30 a few months ago and it measured quite well.  I have heard they are not all like that so you may be one of the lucky ones.
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Harry Brill Jr.
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Arthur Skudra

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Re: Measurement Microphones
« Reply #63 on: January 02, 2009, 12:48:39 pm »

Ray Rayburn scanned in a bunch of pages from an old Gen Rad manual on measurement microphones, the principles are the same regardless of the age of this material, but it would be a good read particularly about the grazing effects of microphones vs. their diameter (see "Microphones Pages 1-17").
http://www.soundfirst.com/GenRad.html

Pat Brown did an article recently (Vol 36 August 2008) comparing the response of several different measurement microphones at different angles, maybe this is a good time to become a member so you can download it from their server, and benefit from other stuff that will increase your knowledge of professional audio.  He found that the Earthworks M30 has the flattest response at 90 degrees!  Interesting stuff!
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Jens Brewer

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Re: Measurement Microphones
« Reply #64 on: January 03, 2009, 02:20:32 am »

Arthur Skudra wrote on Fri, 02 January 2009 12:48

Pat Brown did an article recently (Vol 36 August 2008) comparing the response of several different measurement microphones at different angles....He found that the Earthworks M30 has the flattest response at 90 degrees!


I asked him about his methodology for that test and his choice of the 4090 as the reference.  If you look at the extremely similar deviations of the other mics in fig. 2, it leads me to believe that 4090 was the one with the predominate anomolies.

I also mentioned that EarthWorks recommends that the capsule be aimed at the source, not 90 degrees.  In fact, out of the 8 mics he tested, he determined that only 3 should be used on axis.   Rolling Eyes   I think if Pat had chosen another mic as his reference for the angle incident test, the results would have been a whole lot different.  Ultimately, for what we're using these tools for, I'd say that having a good SOP is a lot more important than worrying about your angle of incidence.  I plan to keep aiming the M30 at the target even if my 15-20kHz area gets skewed a dB or two.

+1 on joining SAC though.....the library is an excellent resource, Pat is a great teacher, and the mailing list has some deep talent.
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Arthur Skudra

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Re: Measurement Microphones
« Reply #65 on: January 03, 2009, 12:06:09 pm »

Interesting you mention this Jens.  I wish Pat went into further depth with this particular article, because it seems to open up a lot of questions about methodology that are worth exploring.  Also the consistency of the data is important too, mic to mic within the same model!  How does he know the reference mic he used is correct?  And for the record, I find aiming the M30 to the source to be the optimal configuration for me, though honestly I need to spend more time digging into this further for myself.

Which begs a controversial question:

If we are willing to accept cheaper measurement microphones for live sound reinforcement optimization that may deviate in response above say 10 kHz as an acceptable alternative to the more expensive ones (that are ruler flat smooth up to 20, 30, or even 40 kHz), does measurement angle (of which deviates the most above 10 kHz) really matter that much?  How often do you find yourself tweaking a sound reinforcement system above 10 kHz?  To further clarify the context of what were doing, we're dealing with a live sound reinforcement system in a mid to large sized room, not a testing lab or recording studio.
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Irving Albert Hammond Jr.

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Re: Measurement Microphones
« Reply #66 on: January 08, 2009, 04:25:10 pm »

Has anybody compared the older tr-40 to the newer tr-40a.
I'm wondering why the change if the spec sheets are identical.

Just got a new tr-40a and its aesthetically different than the older tr40's.

This new one has a flat finish and the top grill part looks different and i bought this from a reputable company so i doubt its a fake

Its also missing the omnidirectional sign at the top.
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Charlie Jeal

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Re: Measurement Microphones
« Reply #67 on: January 08, 2009, 04:54:03 pm »

I was recently given a Peavey ERO 10 that certainly appears to be a measurement mic it's still in the original flighcase style box with the Peavey Architectural Acoustics logo on it.  Does anyone know anything about these?

Thanks

Charlie
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Tom Young

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Re: Measurement Microphones
« Reply #68 on: January 08, 2009, 07:03:23 pm »

I read a post somewhere (a few years ago), from someone who knew what they were talking about, that categorized this mic as "pretty dang good".

I suggest that you find an opportunity to do a comparison against a known quantity and see how it looks. You need to look at both frequency (amplitude) and phase response.
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Tom Young
Electroacoustic Design Services
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Email: dbspl@earthlink.net
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Charlie Jeal

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Re: Measurement Microphones
« Reply #69 on: January 08, 2009, 07:38:47 pm »

Thanks for that Tom,

So one for the "keep pile" then.

It does look pretty well made and I guess finding out more will give me something to do while I'm on "light duties" waiting to get this damn hernia fixed.

Charlie
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Re: Measurement Microphones
« Reply #69 on: January 08, 2009, 07:38:47 pm »


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