Ed there are times I think you never read anything people post for information and learning. Mr. Fredriksen of B&K is a smart guy and knows his maths. There were three versions of his paper posted in the last few days showing how B&K does it. Then there was the earlier B&K fixture that used two actuators sort of like PMA did.
Mr. Fredriksen of B&K is the true expert on measurement microphone construction and measurement. I met him 45 years ago in Copenhagen, and he is the smartest guy that I have ever met on this subject. READ his apps and then we will come to understand, and not talk across each other.
Yes Bruel & Kjaer have their methods, but have you ever noticed their standards are 1", 1/2" and 1/4"? The threading is also in threads per inch. That is because these microphone designs and standards started with Western Electric and General Radio.
There have been many methods used. I think the oldest is the hot wire method.
Unfortunately the really old papers I don't think are on the internet.
The only B & K paper I have on distortion in microphones shows how the theoretical predictions match the measured response. I haven't found anything on non-reciprocal distortion measurement. Can you p0int to an actual paper that discusses how B & K measures distortion?
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Ed we’ll see. Tittle and author
>Edit. e.g. from 1920 https://royalsocietypublishing.org/doi/pdf/10.1098/rsta.1921.0011
George
So you actually don't, I'll have to remember that and not waste the time.
https://www.icacommission.org/Proceedings/ICA2004Kyoto/pdf/Mo4.E.2.pdf
Okay George,
There are really old papers on the internet! However the issue I have is folks proposing the information in some cites is not really what is there.
The hot wire microphone and a Wheatstone bridge were used in World War One as a method to detect artillery locations. Two separated by a distance hot wire microphones inside a tube with a slit were rotated until maximum meter deflection occurred during a gun fire sound wave. The the resulting angles allowed triangulation of the source. As this could be over a rise and out of site it did allow for suppression counter fire. BTY it wasn't really accurate but current units that work much better are these days being implemented in cities to aid police in their work along with the military versions.
Ed Thanks for the valuable look-back !
(for some reason, it is easier to find on the Internet a 1870 paper than a 1970 one)
George
(for some reason, it is easier to find on the Internet a 1870 paper than a 1970 one)
George
Unless I misread that copy of the paper and the original in B&K Review #1 2002 it was using a single frequency of 500 Hz. and testing for distortion vs. sound pressure level. What seemed to me to be of concern was distortion vs frequency at modest levels. In particular as I think most are aware that intermodulation distortion is not the same as harmonic or total harmonic distortion. In my experience IM distortion is often higher.
Tourny,
Their technique to get a clean sine wave is to feed a loudspeaker into a resonant pipe!
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If you read more of Mr. Fredriksen's papers you can see that the distortion modeled as that of a weakly non-linear system with no discontinuities matches reality fairly well. This means the THD and IMD are both predictable and directly related. Of course this alone would not meet NIST standards.
OTOH we ARE wrong because we have not taken into account the diaphragm displacement. The net displacement caused by the 13K and 14K is small compared to 1k so one of the tones must exercise the diaphragm at a low frequency for a valid test.
This works amazingly well, you can find a description of a DIY version on the ARTA site. I accidentally did the same thing years ago with a small speaker and short piece of mailing tube.
Please keep in mind B&K are focusing on level. Measuring distortion has not been a top priority. It's valid in terms of knowing the accuracy and uncertainty of the level measurements. There seems to be little interest below 1% in those circles because it won't degrade accuracy of level.
Maybe there is a way to create a predistorted signal to remove driver distortion for measurement purposes. Since the distortion is level related physical displacement can be used to separate source and receiver contributions?
Maybe there is a way to create a predistorted signal to remove driver distortion for measurement purposes. Since the distortion is level related physical displacement can be used to separate source and receiver contributions?
I think we can check it, however, please take into account that for the 13 and 14kHz test with two sources their intrinsic harmonic distortion appears >= 26kHz, so it does not fall into audio measurement range. If we use, e.g., 4kHz and 13kHz, we shall have intrinsic H2, H3, H4 and H5 of the source playing 4kHz. However, difference tone 13-4 would be 9kHz which is a non-harmonic frequency of the 4kHz, and its level should reflect microphone distortion. Maybe worth trying, as a next step.
If you are referring to foobar ABX, it can be cheated. No reason to hold it up as the only way to find truth.
Here is a real effort at an even more difficult variation: Digital Distortion Compensation for Measurement Setup
I always ask myself, with ABX test, if the subject of the test is to compare files or to evaluate the accuracy of my listening skills ;-)
So, I always request the presence of my lawyer and refer to the 5th amendment.
Both no? Way to many people on this site believe they can hear things that they cant. So first we must prove theres an audible difference, then we try to explain why, and then we talk about preference. Not the other way around. Sure ABx isnt perfect, but its better than sighted.
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