In article , Paul B
wrote:
Thus spake Jim Lesurf: Snipped
I've now found the HFN letter by Baxadall [1] I had in mind when I
made some earlier comments. This used some microphones and a speaker
to add controlled amounts of distortion to the sound of a 'live'
piano. To sum up the results, he found that listeners could hear
distortion at levels around 2 percent with fair reliability, but by
the time it was down to 0.5 percent they showed no signs of being able
to hear it.

Yet if the same distortion was recorded and replayed without the piano
playing, it was easily audible.

At 0.5%? I'm sorry but I don't understand why Baxendall went the route
of adding distortion to a live recording.

He did not say. However it may be that this avoids the 'undistorted' sound
actually having to pass through any equipment at all. In particular, avoids
any 'masking' by the 'undistorted' sound being reproduced by an imperfect
loudspeaker.

Surely the temporal relationship between the live fundamental tones &
the added harmonics would be smeared, let alone the phase relationship
(& vary with listening position) unless it was just to refute a dodgy
theory.

He reported setting up the system for specific listening locations, so may
have dealt with the above to some extent. Also, any 'extra' imperfection in
the distortion superposition might as easily have made them more audible,
not less. However he didn't comment on this in detail IIRC.

His test was for the 'Bob Stuart Hypothesis' that added distortions
might have an effect when accompanied by the sound they altered even
if the distortion in isolation was inaudible. This was a
counter-hypothesis to PJW using nulling to argue that when you nulled
amp distortion and got no audible result, then the distortion must
have no audible effect.

I don't understand the postulates behind the Bob Stuart Hypothesis.

PJW Had conducted tests where the signals 'before' and 'after' a power amp
were cancelled out, thus producing a result which nominally only contains
the 'distortions' introduced by the amp. He showed that with some amps this
residual was too quiet to be audible. Then the hypothesis he made was that
given this, it could be expected to have no audible effect when the music
*was* playing. Hence that such distortions were irrelevant and claims that
the amp had a 'sound' due to them would be nonsense.

This assumes you can apply linear superposition as part of the argument.

BS argued that linear superposition may not apply in such cases due to the
nonlinear behaviour of human hearing. Thus it may be that distortion
components that were - in isolation - inaudible might have an audible
effect when combined with other, audible, sounds. e.g those that prompted
the distortions.

Thus Baxandall's test supported PJW and refuted Bob Stuart's
hypothesis.

That was the implication of the results.

As I would expect.

? Don't know why you say that. I would have 'expected' it from linear
superposition, but BS's counter hypothesis was a quite reasonable one, can
could be tested. It was tested...

The result was in line with some similar tests using 'grunge boxes'
which I have read about where even long-term use showed no signs of
the listeners being able to hear distortion at the same levels as
Baxandall found to show no sign of being audible.

I suspect we are fairly tolerant to harmonics.

But of course as soon as you play a chord, or play a note that has multiple
strings not in unison, then intermodulation may be created by nonlinearity.
Leading to non-harmonic components being added that were absent from the
orginal sounds...

FWIW There have been other tests reported which also indicate that with
music, people could not reliably hear distortions much below 1 percent.
However these generally used arrangements to distort a reproduced signal,
not arrangements as Baxandall used. Hence the choice of arrangement does
not seem to affect the results much - although there may well be a problem
with using speakers that themselves produce distortion as this may then
mean the listeners are comaring almost identical sounds regardless of if
the added distortion is employed or not! Hence, perhaps, Baxandall's
approach to avoid this.

Slainte,

Jim

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