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In the Forum: Audio Discussions
In the Thread: Measuring reality: empirical mode decomposition
Post Subject: Difference Frequency Distortion measurementsPosted by Andy Simpson on: 2/8/2009
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 el`Ol wrote:
I don´t know whether the audio industry`s products are so sophisticatet that they need mathematical steam hammers to improve them. In a German forum there is a discussion about a diploma work from the TU Berlin where the student found out in psychoacoustic tests that the subtraction frequency of the intermodulation products is the non-linearity that interferes most with the perceived sound quality, and built a valve anplifier that is optimized in this respect.

Quote:

"Der "BLACK CAT 2" hat einen Dfferenztonfaktor von 0,002% und einen Klirrfaktor von 0,033 %, während der Telefunken HA990, als Beispiel eines handelsüblichen HiFi-Transistorverstärkers, einen Differenztonfaktor von 0,366 ( das 183-fache des BC2 ) und einen Klirrfaktor von 0,0048 hat."

What can be critizised is that he compares his amp with a quite old mass product, but he says it is representative for what average people have in their living rooms. However, finding out that the subtraction frequency factor of this mass product is two orders of magnitude above its THD, where in his amp it is one magnitude below, is quite shocking.


It is quite shocking and I would like to hear about the mechanism involved....

I realise that my reply is late here - but do you have any further info on this?

This type of test is more commonly called DFD (difference frequency distortion) and is sometimes used in microphone measurement to measure acoustic/mechanical non-linearity.

I have been up to my eyes in exactly this kind of test with my microphones in order to prove the improvement in linearity and am finding that the microphone capsule distortion in the average studio condenser mic is often hugely worse than any amplifier at musical SPLs.

For my work, this test involves a pair of loudspeakers cranking out each a sinewave at 110dB SPL. The two sinewaves differ in frequency by 200Hz and the microphone output is measured for the amplitude of a 'difference frequency' at 200Hz, which distortion product is independent of any distortion introduced in the speakers, so the test is reliable.

Shockingly, in the case of the condenser microphone used on high SPL source, played back at low SPL by the end-user, we might expect distortion from the microphone to exceed that of the speaker system.

I have intimated this to Romy on several occasions but it is clearer now than it has been before.

Andy

PS - Romy, in comparing those two A/B files, on the orchestral peaks (assuming peaks of 130dB @2kHz) a distortion figure of some 10% (-20dB) applies to microphone A and more like 0.1% (-60dB) to microphone B. Was that audible to you?

For anybody else interested, A/B files here: http://www.simpsonmicrophonesarchives.com/AB/

If anybody is interested in listening, I am interested in perception of differences - dynamics, clarity, TUNING (ie. audible presence of inharmonic products), etc.

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