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?
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