[net.audio] Acoustic Feedback Problem, & Another turntable problem

gregr@tekig1.UUCP (Greg Rogers) (12/15/83)

Read on for a) A statement of an old problem
            b) A reply to the Acoustic Feedback Problem
            c) A different but related problem that almost all turntable
               users suffer from.  New to the net, but old to engineers.

As I have stated and explained previously in this newsgroup, playing a 
turntable with the cover DOWN has long been known and well documented,
as UNACCEPTABLE except under carefully controlled environmental conditions
that are unlikely to occur in a home situation.  The reason for this is
the buildup of static charge on the dust cover that will attract the
tone arm and reduce the tracking force.  Since the amount of charge will
vary with humidity, temperature, clothing of user, and many additional factors,
it is not a predictable level and hence cannot be compensated for in any
general sense.  By the way, the charge buildup is not usually caused by
foolishly rubbing off the dustcover (although that would do it), but rather
by transfer from a human body when the cover is lifted and then replaced.

It's possible that your problem may be caused by this reduction in tracking 
force making the tonearm-cartridge more susceptible to minute movements
of the turntable/tonearm/furniture system from the acoustically coupled
or mechanical coupled vibrations.  There are however any number of other
reasons that could be responsible for your problem.  In any case, if you
must persist in this unadvisable practice of closing the dustcover then
you should determine whether the feedback is a result of mechanical
or acoustical coupling.  Try placing the turntable temporarily on foam rubber
to isolate it from whatever furniture supports it.  You should also place
the speakers on foam rubber as well.  This is simple and will identify
if the problem is mechanical coupling.  It usually is NOT in cases of severe
feedback.  More likely (and harder to fix) the problem is acoustical
coupling.  In this case you will have to reduce the level of sound at
the turntable, at the frequency, which the feedback is induced.  You might
test this theory by moving the turntable away from the speaker (to reduce
the direct transmission) or away from the walls or cabinetry (to reduce
reflected sound) or simply putting a temporary barrier around the turntable.
Any of these actions may improve or make worse the problem.  This should
indicate you have found the cause.  The solution would then be to find
a way to reduce the acoustic level at the turntable permanantly.  

You may have concluded at this point that the proper solution is to leave
the dustcover open (you stated this solved your problem) which I have
of course recommended above.  HOWEVER, if you are very concerned about
getting the very best sound from your system (this is TRUE audiophile
stuff from here on {as opposed to subjective I can hear what you can't
audiophile stuff}), then you probably still have a problem.  Long before
the acoustical or mechanical coupling reaches levels that cause severe
feedback, it begins to alter the sound by superimposing itself on the
electrical signal produced be the cartridge itself.  This is a frequency
modified version of the original signal (frequency modified by the 
path by which it returns, and the coupling mechanism at the turntable) 
as well as one or more time delays caused by either an acoustical path
or a mechanical path or both.  Hence the electrical signal produced
by the cartridge and the subsequent sound of the system is modified
by earlier sounds etc.  This effect is simple to measure, hence easy to
document, and the only thing left is to argue about how audible it is.

It is likely that this effect is one of the prime factors which determine
why different turntables sound differently.  It is rather easy to measure
a turntables susceptability to the problem.  You might also begin to imagine
how a small degree of this effect might enrich the sound, add a bit of 
spaciousness to the sound, or under some conditions "improve" the sound,
but always at the expense of true accurate reproduction of the recorded
signal.  Of course you might also be able to understand that a little too
much of this will result in a noticeable reduction in sound quality in
many respects (clarity, imaging, ad infinitium).  I will leave it to your
imagination to conclude in which category "some audiophile"
turntables can be measured as belonging in, while other turntables of 
far less reputation but better engineered are measured as belonging in.  

The solution to this problem from a practical standpoint involves either
a better turntable/tonearm design and/or a reduction in the mechanical
and acoustical coupling discussed above to levels far below those
required to create the howling type feedback that started this discussion.
In this case the mechanical coupling problem is usually always significant.
I'm sure that all this discussion will be controversial (this newsgroups
been kind of boring lately anyway) but I find it much more interesting
discussing the sound of measureable effects, then trying to explain
why some people "claim" to hear effects that can't be measured.  The obvious
advantage of digital CD discs which suffer from neither of these problems
is important to ponder.  It helps explain several interesting "complaints?!"
against CD's (or are they virtues!?).  So flame on subjective "audiophiles"!

				Greg Rogers 
				Tektronix