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In the Forum: Audio Discussions
In the Thread: Measuring reality: empirical mode decomposition
Post Subject: Further on the subject of difference frequency measurements...Posted by Andy Simpson on: 2/15/2009
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 drdna wrote:
 Andy Simpson wrote:
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.
This is interesting. What physical methods are employed to minimize the distortion?


I'm not sure if you mean distortion of the measurements or of the microphone?


 Andy Simpson wrote:
If anybody is interested in listening, I am interested in perception of differences - dynamics, clarity, TUNING (ie. audible presence of inharmonic products), etc.
Well I listened to these files. There was an obvious difference. It did not seem to be blinded: the "A" files seem to have more distortion. I perceived the distortion as a loss of some of the correct sound which was then made into noise. The analogy: taking a fine wood carving, sanding it lightly and sprinkling the surface with the resulting sawdust. The noise floor and hence clarity and dynamics suffer, as well as correct timbre of sound.

In general, my impressions of distortion are:

harmonic distortion: usually too slight to be heard
intermodulation distortion: affects timbre, focus, presence
frequency response: affects "connection", emotional color

SO far...

Adrian


Thanks Adrian - your wood-shavings description is interesting, not least as where energy is 'shaved-off' the top and redistributed as harmonic & inharmonic products this is close enough to true.

While we could perhaps argue that perception of dynamics can suffer simply with added constant noise-floor, I would not say that it is directly noise-floor in this case, but actual compression.

We could measure a compressor unit the same way and would see similar products, but the perception of compression is usually attributed to the actual non-linear gain reduction, rather than the products directly.

I think I recall a conclusion from a paper which studied audibility of IMD in amplifiers, which hypothesized that the ear was not sensitive to the distortion products but to the expectation of linear projection of the sound - which would agree with the compressor perception.

Andy

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