Topic: Some mic measurements, now with graphs

[Church-Audio]

Of course I do.  And this discussion shouldn't progress in the manner of "it's impossible".  It's not a matter of possibility, it's a matter of margin of error.  So first we have to define what the acceptable margin of error is, then ascertain whether or not we can achieve that margin with the test methodology.

The difficulties of attempting an on-axis, free-field measure in a non-reflection-free environment aren't that hard to understand.  It starts with the basic inverse square law of sound intensity.  So we know that the closer we get to a sound source relative to reflective surfaces, reflections become less prominent relative to the source signal.  If we can stay 10cm from our source, we probably don't have to worry much about the room's acoustics at all.  This can be easily demonstrated in any room with a speaker.

Two problems: first, it's not easy to find a perfect full-range driver.  That isn't really so terrible though, so long as we start with a calibrated microphone and a reasonably full-range driver, we can compare response via the subtraction method.

That works great for omnidirectional microphones, not too well for cardioid microphones.  This is of course due to proximity effect.  That's the second problem.  So if we want to get an idea of the far-field low-frequency response of the microphone, we need to get at least 1m away.

Now room acoustics become troubling, because the reflections are no longer sufficiently small compared with the source signal.  That can lead to some different inaccuracies.  So we probably can't get an accurate low-frequency measurement of a cardioid microphone indoors . . .

Or can we?  Again, if we start with a reference microphone at the same location, we will know the room's response once we adjust for the reference microphone's response.  And so we can derive the microphone under test's low-frequency response.

I am curious why you think the outdoors does not very closely approximate a reflection-free environment.  What is the margin of error there?  If the nearest reflection is say 20m away?  This can be calculated mathematically without too much trouble.  And verified experimentally, again with the reference mic at increasing distances from the speaker.  That result can be compared with the same test in the room, and the effect of the room can be derived.

Some of the other comments . . . the error in mic positioning should be a frequency-dependent effect.  I don't get the objection based on capsule size.  That is relevant if you are trying to measure an acoustic environment, because the microphone will impede upon that environment by its presence.  But if you are measuring the microphone itself, the acoustic effects of its capsule and housing are an integral part of the microphone's response.

I never actually said outdoors was not a good environment.. I simply said it was not as simple as placing a speaker outside and measuring the sound with it.. as a test source.. I am sure you can explain why.

Yes its pretty difficult to measure a microphone accurately, just ask AKG, Sennheiser, Shure.. How many $100's of thousands of dollars they have spent trying to do that "simple task" you speak of. Its not simple it never was and it takes real gear to get it right. Not just placing a mic in front of a speaker and calling a day. The single most important part of the equation has not even been talked about at least not be anyone but me. figure out what that is then tell me how easy it is Its not if it was there would be a lot of mic companies out there with "flat mics" selling them for lots of $$$.  Because the real truth is most mics made today are pretty flat. But flat in and of it self does not tell the whole story.



Chris

[Church-Audio]

You know what, I'm not really into mystical gnosticism, so you can either share what you know or not, I am not particularly interested in guessing what you feel like you have to hide.  As I recall you haven't published frequency response specifications for any of your microphones.

Strict laboratory techniques for microphone measurement are well-described in the relevant standards, what I am discussing is what margin of error can be expected for the home "lab" environment.  When you want to know if microphone X is relatively flat or has a 10dB peak at 6kHz, that degree of accuracy is not hard to achieve.

I would challenge the assertion that most microphones are flat.  Clearly most are not, especially directional microphones, which have the lion's share of the market.

  No secrets just basic facts its not easy to measure a microphone with any kind of meaningful accuracy with out matching the raw sensitivity of the mic and with out proper placement of a sound source even outside. I still stand behind that statement. 

[illconditioned]

Let me just step in for a bit.

I agree, instead of negative statements, please try to provide some suggestions as well.

One method I have found to be quite good is to put two *small* mics side by side.  Hold them in front of a speaker and play pink noise at approx 80dBA or so.  (I wear earmuffs when doing this, otherwise it sounds like I'm behind a waterfall.)

I record with a stereo rig, Edirol or whatever.  Then I play back and analyse the spectrum with say two to five seconds of averaging.

This is a "quick and dirty" sanity check, that the mics are both outputting a good signal, and that they have similar responses.  I've done this with: small omnis (Countryman B3, Sennheiser MKE2, etc) and with slightly larger mics (AT853, CA11, Sennheiser MKE40, etc).

Enhancements.  Get a calibrated mic.  I could also do one mic right after another, being careful to put both mics in exactly the same position.

I think the biggest problem is the room response, which can vary quite a bit with position.  The second is the speaker response, which again can vary quite a bit.

Anyway, this kind of "sanity check" is good enough for small (5mm) omni mics.  It would be a good way to match a dozen WM61a capsules, for example.

  Richard

[illconditioned]

Enhancements.  Get a calibrated mic.  I could also do one mic right after another, being careful to put both mics in exactly the same position.

I think the biggest problem is the room response, which can vary quite a bit with position.  The second is the speaker response, which again can vary quite a bit.

Anyway, this kind of "sanity check" is good enough for small (5mm) omni mics.  It would be a good way to match a dozen WM61a capsules, for example.

  Richard

That's an entirely valid way to compare small omni capsules.  The speaker and room response aren't too important, since they will be the same for all units under test, although it does help to be diligent about identical placement.

Treating the room and doing a few helpful things with the drivers can make the charts look prettier insofar as they will look flatter, but the spiky kind of graphs you get without controls are valid for subtraction.

Yeah, I'm just suggesting this as I discovered it by chance.  My *guess* is that a lot of people (like Coresound, etc) are using this technique.  I don't imagine all of these guys are as obsessive about testing as Chris Church .

  Richard

[notlance]

My listening room is about 7m x 4m x 2.2m and it is somewhat dead acoustically but certainly not anechoic.

Not to be harsh, but you have no idea what effect room acoustics are having on your tests.

The effects of room acoustics should be negated because the reference mic and the test mic are measured in the same room at the same positions with the same sound source.  Therefore, the room acoustics affect both mics equally and are canceled out when the reference mic curve is subtracted from the test mic curve.

[Church-Audio]

Enhancements.  Get a calibrated mic.  I could also do one mic right after another, being careful to put both mics in exactly the same position.

I think the biggest problem is the room response, which can vary quite a bit with position.  The second is the speaker response, which again can vary quite a bit.

Anyway, this kind of "sanity check" is good enough for small (5mm) omni mics.  It would be a good way to match a dozen WM61a capsules, for example.

  Richard

That's an entirely valid way to compare small omni capsules.  The speaker and room response aren't too important, since they will be the same for all units under test, although it does help to be diligent about identical placement.

Treating the room and doing a few helpful things with the drivers can make the charts look prettier insofar as they will look flatter, but the spiky kind of graphs you get without controls are valid for subtraction.

Yeah, I'm just suggesting this as I discovered it by chance.  My *guess* is that a lot of people (like Coresound, etc) are using this technique.  I don't imagine all of these guys are as obsessive about testing as Chris Church .

  Richard

Who is obsessive ? 

[Church-Audio]

My listening room is about 7m x 4m x 2.2m and it is somewhat dead acoustically but certainly not anechoic.

Not to be harsh, but you have no idea what effect room acoustics are having on your tests.

The effects of room acoustics should be negated because the reference mic and the test mic are measured in the same room at the same positions with the same sound source.  Therefore, the room acoustics affect both mics equally and are canceled out when the reference mic curve is subtracted from the test mic curve.

How can you guarantee the same position? when the capsule sizes are all different? And they are all different.

[Church-Audio]

No secrets just basic facts its not easy to measure a microphone with any kind of meaningful accuracy with out matching the raw sensitivity of the mic and with out proper placement of a sound source even outside. I still stand behind that statement.

Well I offered to lend OP a reference mic for which I know its sensitivity, on-axis response, polar response, and so forth.  The same lab reports you probably have.  And of course placement is important with respect to the sound source and the acoustic environment.  I am trying to help OP mitigate those factors by explaining some useful techniques.  You apparently just want him to give up.  I don't accept that.  You also haven't defined "meaningful accuracy".  How would you define that, in quantitative terms if you please?

I will try to demonstrate some of these techniques in the next few days . . .

I don't think you have a nist traceable mic.. You may think that it is some labs are saying that have nist traceable gear but unless you get a nist number with your calibration data its not nist. Also sensitivity is relative to the input the mic is connected to and the signal chain. You need a calibrator to "calibrate" the signal / sensitivity to any new source you connect your mic to and you then need to calibrate all the other mics under test to that same calibrator in order to make the tests anywhere near accurate. And eyeballing the curve is not enough. But again everyone has there own opinion.

[page]

My listening room is about 7m x 4m x 2.2m and it is somewhat dead acoustically but certainly not anechoic.

Not to be harsh, but you have no idea what effect room acoustics are having on your tests.

The effects of room acoustics should be negated because the reference mic and the test mic are measured in the same room at the same positions with the same sound source.  Therefore, the room acoustics affect both mics equally and are canceled out when the reference mic curve is subtracted from the test mic curve.

How can you guarantee the same position? when the capsule sizes are all different? And they are all different.

Ok, I've watched this quietly long enough; Ok, so you are exactly in the *same* space, you're off as much as 1cm in any direction (so your field of testing expands). Fair, it's not the exact same spot, but will it make that much of a difference? I think that depends on what you're going to use the results to infer.

I still don't think you have addressed Jon's bit of what meaningful accuracy and tolerances are, but maybe I just missed that.

edit: jon beat me to the post.

[Church-Audio]

My listening room is about 7m x 4m x 2.2m and it is somewhat dead acoustically but certainly not anechoic.

Not to be harsh, but you have no idea what effect room acoustics are having on your tests.

The effects of room acoustics should be negated because the reference mic and the test mic are measured in the same room at the same positions with the same sound source.  Therefore, the room acoustics affect both mics equally and are canceled out when the reference mic curve is subtracted from the test mic curve.

How can you guarantee the same position? when the capsule sizes are all different? And they are all different.

Ok, I've watched this quietly long enough; Ok, so you are exactly in the *same* space, you're off as much as 1cm in any direction (so your field of testing expands). Fair, it's not the exact same spot, but will it make that much of a difference? I think that depends on what you're going to use the results to infer.

I still don't think you have addressed Jon's bit of what meaningful accuracy and tolerances are, but maybe I just missed that.

edit: jon beat me to the post.

The bottom line is when you have different size capsules you have a huge problem with measuring frequency response because you don't really have a meaningful reference point. That's the really big part of the problem. How accurate are these tests its impossible to say since I did not do them but I can tell you that there will be huge problems because of the method being used. + - 40% Maybe more maybe less. The problem gets further complicated by the fact that this is not a true point source. And yes moving the mic 1 cm  can make a huge difference. When you are talking about measurement. I just wanted to make that point.. I did not intend for this to get all blown up like it has.

[runonce]

I didn't say I did, I said it was second gen.  That means it was a test done in comparison to a NIST (caps please, it's an acronym) calibrated mic.  Therefore, the margin of error increases the farther you get from that calibrated mic.  All of this I said up front.  Strictly speaking, I don't need a NIST-calibrated mic because I don't calibrate my own reference mic, I send it off to a lab.  Thus, I only really need a calibrated mic to validate my own test methods.

You still refuse to estimate the margin of error of my tests or notlance's.  Your position seems to be only you can measure frequency response because you say so.  Here is your problem:  if I can't measure frequency response, then neither can you.  If you can't, why do you have a calibrated microphone?  Instead, you need to send your mics to the test lab rather than doing your own test.  This I have done.

Why don't you call up Herb Singleton at Cross-Spectrum Labs (not a NIST lab, yes), and explain to him why his methods are complete bunk, because you know better?  Funny, but I think they were good enough for Crowley & Tripp.  Have you sold a mic patent to Shure?  I shure  haven't.

You also haven't raised a serious objection to the outdoor test.  That tells me you have probably never tried it.  It wasn't my idea, I got it (along with most of my other good ideas) from Harvey Gerst.  He's designed a microphone and speaker or two.  You could ask Mr. Satz for a character reference there, I believe they are acquainted.

Again, margin of error.  No, notlance won't hit +/-0.1dB.  Who cares?  What accuracy did he achieve?  What exact are his errors?  Specifics, please, not innuendo.

Your point about sensitivity is absurd.  The reference mic has a measured sensitivity . . . but even if it didn't, who cares?  So long as the chain does not change between the reference mic and the mic under test, you only need to compare sensitivities and normalize 1kHz.  The exception would be for mics with unreasonably high output impedance or complex output impedance.  Or mics that aren't phantom powered (or just professional-style XLR output mics).  You will have that issue because the sensitivity of plug-in power mics is a function of the supply resistance.  Even so, it doesn't really matter, just normalize response at 1kHz and state the test conditions.

I've been doing some simple tests tonight, results to follow shortly.

I was going to go through each point line by line but I give up! I don't have enough spare time to explain / educate you. I will say one thing you are taking this very personally. I was just making a statement that these tests were not 100% accurate and should not be taken as such. Talk to anyone that measures microphones for a living they will tell you the same thing there are way to many variables with the way these mics are being placed / tested. I don't consider Cross spectrum labs to be a good source of info.. Try talking to the guys as Nist or NRC like I have. And maybe you will get a better idea of whats involved.

It seems like you are trying to mischaracterize his tests on your own terms to discredit them, rather than accept them for what they are. I cant see why/how anyone would read his post and methodology - and think the goal was "100% accuracy"

Pretty sure this was not set up to be about "accuracy"...but rather a loose comparison.

My only observation is - for this loose comparison, one could use any mic for "the standard" and make all comparisons to that mic...and still garner some useful info. I think the OP just picked his best documented, flattest "known" mic...

His tests seem to emphasize the differences between the mics...not make reference-quality measurements. Those differences will always be the same, regardless of the comparison mic.

And despite your claims to the contrary, you sound more like the person, taking something personally, and making it a pissing contest...

Try talking to the guys as Nist or NRC like I have. And maybe you will get a better idea of whats involved...That's why I spend $150 a year getting my reference mics calibrated...You don't know so you don't have any point of real reference that's why I spend $1000+ for a proper nist traceable analyzer mic and spend $150 a year to get it calibrated not to mention the $500 calibrator that I use to calibrate the sensitivity to my preamp / program.

sounds like a pissing contest, Chris...

[page]

But 1cm is really pretty poor placement accuracy.  I think anyone could do better than that if they tried.

I agree it is. My intention was primarily to provoke the response of "how much difference does it make", not that it's difficult to achieve mic placement tollerances that are tighter.

Next, there is an even easier way to measure placement error:  measure a mic, remove it from the stand, put it back in the stand, and measure it again.  Without moving it, measure it a third time.  Now we know our placement error and also our overall resolution.

precisely (no pun intended). 

[Massive Dynamic]

This is all the data I have for now.  I do have a few more mics I plan on testing, in particular a Studio Projects LSD2 which I forgot about, but I need to attend to the rest of my life for a while.  It will get done when it gets done.

I'd be interested in seeing graphs of all the LSD2 possibilities when you find the time. Thanks.

[illconditioned]

Enhancements.  Get a calibrated mic.  I could also do one mic right after another, being careful to put both mics in exactly the same position.

I think the biggest problem is the room response, which can vary quite a bit with position.  The second is the speaker response, which again can vary quite a bit.

Anyway, this kind of "sanity check" is good enough for small (5mm) omni mics.  It would be a good way to match a dozen WM61a capsules, for example.

  Richard

That's an entirely valid way to compare small omni capsules.  The speaker and room response aren't too important, since they will be the same for all units under test, although it does help to be diligent about identical placement.

Treating the room and doing a few helpful things with the drivers can make the charts look prettier insofar as they will look flatter, but the spiky kind of graphs you get without controls are valid for subtraction.

Yeah, I'm just suggesting this as I discovered it by chance.  My *guess* is that a lot of people (like Coresound, etc) are using this technique.  I don't imagine all of these guys are as obsessive about testing as Chris Church .

  Richard

Who is obsessive ? 

OK, you got me there!!!

  Richard

[Church-Audio]

I didn't say I did, I said it was second gen.  That means it was a test done in comparison to a NIST (caps please, it's an acronym) calibrated mic.  Therefore, the margin of error increases the farther you get from that calibrated mic.  All of this I said up front.  Strictly speaking, I don't need a NIST-calibrated mic because I don't calibrate my own reference mic, I send it off to a lab.  Thus, I only really need a calibrated mic to validate my own test methods.

You still refuse to estimate the margin of error of my tests or notlance's.  Your position seems to be only you can measure frequency response because you say so.  Here is your problem:  if I can't measure frequency response, then neither can you.  If you can't, why do you have a calibrated microphone?  Instead, you need to send your mics to the test lab rather than doing your own test.  This I have done.

Why don't you call up Herb Singleton at Cross-Spectrum Labs (not a NIST lab, yes), and explain to him why his methods are complete bunk, because you know better?  Funny, but I think they were good enough for Crowley & Tripp.  Have you sold a mic patent to Shure?  I shure  haven't.

You also haven't raised a serious objection to the outdoor test.  That tells me you have probably never tried it.  It wasn't my idea, I got it (along with most of my other good ideas) from Harvey Gerst.  He's designed a microphone and speaker or two.  You could ask Mr. Satz for a character reference there, I believe they are acquainted.

Again, margin of error.  No, notlance won't hit +/-0.1dB.  Who cares?  What accuracy did he achieve?  What exact are his errors?  Specifics, please, not innuendo.

Your point about sensitivity is absurd.  The reference mic has a measured sensitivity . . . but even if it didn't, who cares?  So long as the chain does not change between the reference mic and the mic under test, you only need to compare sensitivities and normalize 1kHz.  The exception would be for mics with unreasonably high output impedance or complex output impedance.  Or mics that aren't phantom powered (or just professional-style XLR output mics).  You will have that issue because the sensitivity of plug-in power mics is a function of the supply resistance.  Even so, it doesn't really matter, just normalize response at 1kHz and state the test conditions.

I've been doing some simple tests tonight, results to follow shortly.

I was going to go through each point line by line but I give up! I don't have enough spare time to explain / educate you. I will say one thing you are taking this very personally. I was just making a statement that these tests were not 100% accurate and should not be taken as such. Talk to anyone that measures microphones for a living they will tell you the same thing there are way to many variables with the way these mics are being placed / tested. I don't consider Cross spectrum labs to be a good source of info.. Try talking to the guys as Nist or NRC like I have. And maybe you will get a better idea of whats involved.

It seems like you are trying to mischaracterize his tests on your own terms to discredit them, rather than accept them for what they are. I cant see why/how anyone would read his post and methodology - and think the goal was "100% accuracy"

Pretty sure this was not set up to be about "accuracy"...but rather a loose comparison.

My only observation is - for this loose comparison, one could use any mic for "the standard" and make all comparisons to that mic...and still garner some useful info. I think the OP just picked his best documented, flattest "known" mic...

His tests seem to emphasize the differences between the mics...not make reference-quality measurements. Those differences will always be the same, regardless of the comparison mic.

And despite your claims to the contrary, you sound more like the person, taking something personally, and making it a pissing contest...

Try talking to the guys as Nist or NRC like I have. And maybe you will get a better idea of whats involved...That's why I spend $150 a year getting my reference mics calibrated...You don't know so you don't have any point of real reference that's why I spend $1000+ for a proper nist traceable analyzer mic and spend $150 a year to get it calibrated not to mention the $500 calibrator that I use to calibrate the sensitivity to my preamp / program.

sounds like a pissing contest, Chris...

And that's why I removed the post.. I realized that it was a total waste of my time and a bit to harsh... And I did not want to really get into it with anyone but again I can see things got out of hand. I can see the post police are watching my every move Anyway I will leave you all to it.