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From: mmt@dciem.UUCP (Martin Taylor)
Newsgroups: net.audio
Subject: Re: Unmeasurable differences?
Message-ID: <631@dciem.UUCP>
Date: Tue, 17-Jan-84 13:57:27 EST
Article-I.D.: dciem.631
Posted: Tue Jan 17 13:57:27 1984
Date-Received: Tue, 17-Jan-84 14:58:20 EST
References: <2389@rabbit.UUCP>
Organization: D.C.I.E.M., Toronto, Canada
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It's a natural presumption that any subjectively detectable variation
in sound quality can be correlated with some variation in objective
measurements, but the question often is "what should we measure?"

The ear is a funny beast. Sometimes it doesn't hear what you might
think was the most awful mess, and other times its sensitivity is
exquisite. Training matters enormously, although the training may have
to be very specific to the listening task.  For example, one classic
old experiment used a specially invented signal whose total energy
was constant during the entire pulse of some tens of msec. But the
energy distribution across the ears was a function of time. It started
at 100% in one ear and wound up up at 100% in the other ear, with
a linear shift over the pulse duration. So it was heard as a left-to-righ
or right-to-left sweep. In one of the four quarters of the sweep, the
energy split reverted for about 10 msec (if I remember properly) to
100% in one ear. The observer had to tell which quarter had the
blip in it. People were trained for days and weeks, and still could
score only chance results, until suddenly they found out how to listen,
and then over the space of a day or two went up to essentially perfect
scores.

Another example: Take someone off the street and get them to listen
for lateralization of sound caused by an interaural time delay.
Typically, an inter-aural shift of some 100+ microseconds is required
for perfect lateralization (telling which side of centre the sound is).
After prolonged training, subjects may suddenly start being able to
do the task with only around 10 usec shifts.

It is quite likely that the standard measurement specs do not take
into account some signal parameters that are used by some listeners
and not by other *equally good* listeners.  I would trust subjective
discrimination tests to tell me that there was something different,
and would then look to objective measures to find out what those
differences might be.  After finding out, I would tend to look for
those differences in any later tests on new equipment.

There is still art in psychoacoustics.
-- 

Martin Taylor
{allegra,linus,ihnp4,uw-beaver,floyd,ubc-vision}!utzoo!dciem!mmt