Topic: Some mic measurements, now with graphs

[notlance]

These mics are all Milab LSR2000 cardioids, which are often used as vocal performance mics.

Graph 1: Milab LSR2000 4097 cardioid
Graph 2: Milab LSR2000 6054 cardioid
Graph 3: Milab LSR2000 6056 cardioid
Graph 4: Milab LSR2000 6137 cardioid

[notlance]

Up to now, every mic has been a condenser.  These next three mics are dynamics.  The first two are Beyerdynamic M201 mics, a fairly common hypercardioid often used on drums.  The third mics, the M101, is a little less common and it’s an omni version of the M201.

Graph 1: Beyerdynamic M201 35019
Graph 2: Beyerdynamic M201 043072
Graph 3: Beyerdynamic M101

[notlance]

The next few post are all ribbons.  This first group are Beyerdynamic ribbons, the M160 and the M130.

I was surprised at the lack of low end on the M160; is the ribbon damaged?  Since I do not have another M160 to compare it to, I can’t be sure.  The mic sound OK, and quite frankly the M160’s LF rolloff has not been an issue in use, but I tend to use it to mic instruments without much bass.

The M130s are quite variable, with better bass than my M160 sample.  The M160 and M130 are often used as an MS pair, and from my experience they work quite well in that configuration.  One thing I find interesting is how my M160 and the M130 SN 2652 are almost a perfect complement to each other in an MS configuration.  They seem to have built-in stereo “shuffler” equalization where the Mid LF are cut and the Side LF are boosted.  I’ve used my M160 and one of my M130s (don’t know which one) in MS to record a choir and it did sound quite good, IMHO.  But it’s hard to say exactly why.
OK I’ll shut up already and give you the data.

Graph 1: Beyerdynamic M160 hypercardioid
Graph 2: Beyerdynamic M130 2629 figure-8
Graph 3: Beyerdynamic M130 2652 figure-8
Graph 4: Beyerdynamic M130 3675 figure-8

[notlance]

Next is a Peluso R14 ribbon, figure-8.  Just bought this mic and have not had a chance to use it yet.

Graph 1: Peluso R14 00012 figure-8

[notlance]

This next mic is perhaps the surprise of the lot.  It’s a Busman Stereo Ribbon in a Blumlein configuration.  I was surprised how uniform it is (for a ribbon) and how well matched the channels are.  Now its high end above 10 kHz has some problems, and it’s Blumlein so you have to live with the limitation of that configuration, and it’s a ribbon with not much output, but I’ve made some nice recordings with it IMNSHO.  This is a 300 buck mic.

Graph 1: Busman Stereo Ribbon left channel
Graph 2: Busman Stereo Ribbon right channel
Graph 3: Busman Stereo Ribbon difference between left & right channels

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.

[Roger Gustavsson]

Thank you, for sharing this with us!

Roger

[page]

Thank you, for sharing this with us!

[Church-Audio]

Unfortunately you cant really use a cardioid mic as a measurement source.. And get reliable data from it when measuring omnidirectional mics. Also the mics capsule size will also have a huge effect on the disturbance of the sound field making it also less reliable for a measurement standard. You can get an "idea"
 but you can not base anything from the data you are collecting because of the issues mentioned above.. Measuring Microphone frequency response is one of the hardest things to do in acoustics. Its easy to measure a speaker.. But not a microphone. The substitution method simply does not work with microphones of different polar patterns and types. The other huge part of the puzzle that you dont have is a db calibration for the Sennheiser mic.. You cant have that with out using a calibrator on the mic then using the same calibrator on all the other mics so you have at least matched the sensitivity. I can see by your graphs that there are "similarities" in all the plots.. This indicates a flaw in your testing. You must be able to subtract the measurement errors from the graphs.. In other words you must be able to subtract the flaws in your sound source from your graphs by using the Mic calibration file and zeroing out your reference curve. But you cant do that because you dont have calibration data about your measurement mic. That's why I spend $150 a year getting my reference mics calibrated. So I have a reliable reference. I think its great that you did these tests... i think its also important to point out that they are in no way accurate. Very small shifts in placement between mics and the sound source will also give you bad results that are 100% inconsistent.

Chris

[kbergend]

Thanks for sharing all this, very informative!

[notlance]

Reply to Church-Audio:

I’m not sure my measurements are all that bad, and I’ll try to show why I believe they are valid.  Let me do this by answering your concerns in order.

“Unfortunately you cant really use a cardioid mic as a measurement source.. And get reliable data from it when measuring omnidirectional mics.”
I agree.  In fact, as I was setting up one of my omnidirectional mics for measurement I was thinking “This does not make sense since my standard mic is a cardioid, but let’s see what I get.”  Measuring figure-8 mics with a cardioid standard is probably suspect also.  That is why I listed the polar pattern of all the mics I tested, so the observer could view the graphs with whatever size grain of salt they deemed appropriate.

“Also the mics capsule size will also have a huge effect on the disturbance of the sound field making it also less reliable for a measurement standard.”
My reference mic has a capsule size that is just about the same size as most of the microphones I measured, so the disturbance to the sound field of the standard mic and the measured mic should be similar.

“You can get an "idea" but you can not base anything from the data you are collecting because of the issues mentioned above.”
OK, let’s toss out all the data taken on the omnidirectional mics, and consider the cardioids only.

I believe my data on those cardioid mics is valid because it is consistent with all the independently measured data I have available to me.  For example, I have 6 factory supplied frequency response graphs for the Sennheiser MKH800 and the Milab DC196 mics and my measurements are consistant with the factory measurements to a surprising degree, within about +-1 dB.  In some cases I do not have the frequency response graphs of my individual mics, but I can reference the microphone model’s frequency response spec.  My data for the Beyerdynamic MC742 and MC740 agree closely with the specs for these models, for example.  Did I just get lucky with my measurements, or are they at least reasonable?

In addition, what my ears tell me matches my measurements.  For example, the recordings I have made with the Busman SDC sound like they have a presence bump around about 5 kHz.  I can hear the high-frequency rolloff of the ribbons.  To my ears the Sennheiser MKH800 and the Milab DC-196 sound a lot alike, and they also measure a lot alike.  My ears are not calibrated in dB, but what I hear correlates to what I measure, so that is at least a reasonableness check.

“Measuring Microphone frequency response is one of the hardest things to do in acoustics. Its easy to measure a speaker.. But not a microphone.”
I agree it is difficult to measure microphone frequency response, but if the test is carefully set up and one considers all possible sources of error, it is not impossible. Apparently you are able to do it, so why can’t I do it also?  (And speakers are not that easy to measure either, BTW.)

“The substitution method simply does not work with microphones of different polar patterns and types.”
Yes, I agree and I have already acknowledged my error in comparing omni and cardioid mics.

“The other huge part of the puzzle that you dont have is a db calibration for the Sennheiser mic.. You cant have that with out using a calibrator on the mic then using the same calibrator on all the other mics so you have at least matched the sensitivity.”
I don’t care about sensitivity, and I don’t need to measure it for my frequency response measurements to be valid.  The only thing a sensitivity measurement would do is shift the frequency response curves either up or down relative to each other without changing the shape of the curves.

“ I can see by your graphs that there are "similarities" in all the plots.. This indicates a flaw in your testing.”
What are these similarities?  Many of the mics show a presence boost, but that is by design.  I guess I have not poured over these graphs long enough to see the common errors, although I have spent too much time on this little experiment already it seems to me.

“You must be able to subtract the measurement errors from the graphs.. In other words you must be able to subtract the flaws in your sound source from your graphs by using the Mic calibration file and zeroing out your reference curve. But you cant do that because you dont have calibration data about your measurement mic. That's why I spend $150 a year getting my reference mics calibrated. So I have a reliable reference.”
I understand I need to subtract the non-random measurement errors from the graphs.  I do not have a calibration file, but I do have some factory supplied frequency response graphs for the Sennheiser mics.  These graphs are shown in the attached files called “Sennheiser 10015” and “Sennheiser 10138”.  The mics are “flat” enough to use as a reference, certainly from about 100 Hz to 10 kHz.  I could create a calibration file from these graphs, but that would be tedious and applying that correction would not significantly alter the shape of the frequency response graphs I measured.  The Sennheiser mics have their worst deviation of about +1 to + 2 dB between 10 kHz and 20 kHz.  So would adding 1 to 2 dB from 10 to 20 kHz make a big difference in any of my measurements?  Would that change the plots from unusable to acceptable?

What if the Sennheiser plots are in error, and my reference mic are much worse than I am assuming?  Well, I can compare the Sennheiser mics to each other, and I find the comparisons are consistent with the plots supplied by Sennheiser.  For example, the Sennheiser plots show at 50 Hz the front capsule of SN 10138 is about 2 dB lower than the front capsule of SN 10015.  My measurements confirm this.  In fact, my measurements are in agreement with the Sennheiser plots across the spectrum.  I can look at the plots I get from the Milab DC-196 mics and see they agree with the factory supplied plots.  If the Sennheiser factory plots are wrong, then the Milab factory plots would have to error in a complementary way in order for my measurements to match the Milab factory data.  This is not very likely.

Nevertheless, I would love to have an appropriate calibrated reference mic, if for nothing else my own peace of mind.  Can you recommend an appropriate microphone?  And where should I get it calibrated?  And how do you know the calibration data you have is accurate?

This post continues below.

[notlance]

“I think its great that you did these tests... i think its also important to point out that they are in no way accurate.”
Actually, if I thought my tests were “in no way accurate” I would be doing a disservice to everyone here by publishing bogus data.  Since I thought my measurements were reasonable, I made them public.

“Very small shifts in placement between mics and the sound source will also give you bad results that are 100% inconsistent.”
So how much error is introduced by shifting mic placement?  And how small is a “very small” shift?  Is +- 5 mm a very small shift?  To find out, I performed the following test:  I placed a mic 1.16 m high and 1 m from a speaker.  For this test it does not matter what mic I used, but it happened to be a Sennheiser MKH800 Twin.  With a pink noise source, I plotted and saved the frequency response at that location.  Then I moved the mic 5 mm higher, and ran another frequency response plot.  I repeated the plots with the mic 5 mm lower, 5 mm closer, and 5 mm farther back from the center location.  The only variable in this test is the location of the mic.  So now if I subtract the response of the mic in the center location from the responses at the other locations I can plot the error introduced by a shift in placement.

These plots are shown in the attached file “Placement Error”.  This file contains four plots offset from each other in order to be readable.  From top to bottom they are 5 mm up, 5 mm down, 5 mm forward, and 5 mm back.  These plots should NOT be compared to each other, rather the deviation within a plot should be observed.  Note the entire scale of this plot is 10 dB.  Each plot is +-0.5 dB, and some are +-0.25 dB which indicates the magnitude of the error introduced by a shift in placement.  Now some of this error is random error introduced largely, I would suspect, by the pink noise source, particularly at low frequencies.  I ran some tests to confirm this random error, but I’m not going to go into it at this time because this is turning into a thesis as it is.

My conclusion is I can vary the mic placement by +-5 mm without significantly affecting the frequency response, at least using a pink noise source.  I chose +-5 mm because I know I can place a microphone within that error margin, and NOT have “bad results that are 100% inconsistent.”

I have host of information that tells me my data is valid.  My measurements are consistent with outside sources of information, it is constant with itself (i.e. one measurement does not contradict another), and it is consistent with what I hear.  I am sure you know what you doing and I know you have a good reputation on this forum.  But with all due respect the only thing telling me my data is bogus is you.  You have raised some valid concerns and I hope to decrease my measurement error based on your suggestions, but I’ll stand by this data.

[Church-Audio]

“I think its great that you did these tests... i think its also important to point out that they are in no way accurate.”
Actually, if I thought my tests were “in no way accurate” I would be doing a disservice to everyone here by publishing bogus data.  Since I thought my measurements were reasonable, I made them public.

“Very small shifts in placement between mics and the sound source will also give you bad results that are 100% inconsistent.”
So how much error is introduced by shifting mic placement?  And how small is a “very small” shift?  Is +- 5 mm a very small shift?  To find out, I performed the following test:  I placed a mic 1.16 m high and 1 m from a speaker.  For this test it does not matter what mic I used, but it happened to be a Sennheiser MKH800 Twin.  With a pink noise source, I plotted and saved the frequency response at that location.  Then I moved the mic 5 mm higher, and ran another frequency response plot.  I repeated the plots with the mic 5 mm lower, 5 mm closer, and 5 mm farther back from the center location.  The only variable in this test is the location of the mic.  So now if I subtract the response of the mic in the center location from the responses at the other locations I can plot the error introduced by a shift in placement.

These plots are shown in the attached file “Placement Error”.  This file contains four plots offset from each other in order to be readable.  From top to bottom they are 5 mm up, 5 mm down, 5 mm forward, and 5 mm back.  These plots should NOT be compared to each other, rather the deviation within a plot should be observed.  Note the entire scale of this plot is 10 dB.  Each plot is +-0.5 dB, and some are +-0.25 dB which indicates the magnitude of the error introduced by a shift in placement.  Now some of this error is random error introduced largely, I would suspect, by the pink noise source, particularly at low frequencies.  I ran some tests to confirm this random error, but I’m not going to go into it at this time because this is turning into a thesis as it is.

My conclusion is I can vary the mic placement by +-5 mm without significantly affecting the frequency response, at least using a pink noise source.  I chose +-5 mm because I know I can place a microphone within that error margin, and NOT have “bad results that are 100% inconsistent.”

I have host of information that tells me my data is valid.  My measurements are consistent with outside sources of information, it is constant with itself (i.e. one measurement does not contradict another), and it is consistent with what I hear.  I am sure you know what you doing and I know you have a good reputation on this forum.  But with all due respect the only thing telling me my data is bogus is you.  You have raised some valid concerns and I hope to decrease my measurement error based on your suggestions, but I’ll stand by this data.

You have your opinion I have mine. I just wanted to point out your tests are highly flawed. And they are highly flawed, I dont want people to look at graphs and make decisions on purchases based on false data that is not a service people need around here. I am sure you have convinced your self that you are doing the tests in a correct manor but I can assure you there is problems with your methods.. I have pointed them out. I did not want to get into a pissing contest. Its very easy to measure a speaker compared to a mic.. Also you are looking at Freq plots from Sennheiser mics how did they get the data they are printing out? How flat are the mics? did someone have there finger on the plotter? was smoothing used?? 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. If it was just a matter of looking at  a plot and saying hey that's flat I would have done that This is what I do for a living after all. But go ahead and do your tests this is how you will learn Its how I learned. But be sure to point out that the tests are NOT perfect so people don't go thinking they are.

Chris Church

[chris319]

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.

[Roger Gustavsson]

It is a pity that Sennheiser have no comments on how their microphones are measured. Neumann have their microphones measured in free-field conditions, the IEC 60268-4 standard.

I would have expected the omnis to have better responses (flatter bass) than shown here. Even low price omnis use to be very flat at lower frequencies.


Roger

[Church-Audio]

Nevertheless, I would love to have an appropriate calibrated reference mic, if for nothing else my own peace of mind.  Can you recommend an appropriate microphone?

Shoot me a PM and I'll lend you a reference microphone. It will be a second gen calibration, so that will limit your accuracy to around +/-0.3dB.  Accuracy above 10kHz will be limited due to placement error, but that's not terribly important for many mics as their response tends to drop above that point.  So +/-2dB at 20kHz is fine.

As for room acoustics, use the poor man's anechoic chamber:  go outside.  You can do one test outside to study the effects of your indoor tests.

John you have the free field calibration data for your mic? If not then there is not much point.. Also just going "outside" is not enough. I only wish it was that easy.

Chris