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Path: utzoo!watmath!watnot!watdcsu!herbie
From: herbie@watdcsu.UUCP (Herb Chong [DCS])
Newsgroups: net.audio
Subject: Re: Turntable-platter resonances
Message-ID: <1468@watdcsu.UUCP>
Date: Wed, 12-Jun-85 13:43:08 EDT
Article-I.D.: watdcsu.1468
Posted: Wed Jun 12 13:43:08 1985
Date-Received: Thu, 13-Jun-85 00:27:29 EDT
References: <682@charm.UUCP>
Reply-To: herbie@watdcsu.UUCP (Herb Chong [DCS])
Organization: U of Waterloo
Lines: 80
Summary: 

In article <682@charm.UUCP> prk@charm.UUCP (Paul Kolodner) writes:
>Ahem.  I have a question.  Every now and then (as recently
>on this net), I see comments about turntable mats and 
>record clamps.  These seem like a fine idea, but I don't
>understand why people talk about resonances in the same 
>connection.  The air space underneath a record has a thickness
>of a millimeter or less, putting its resonance frequency
>way above the audio range.  Vibrations of the record might be
>resonant, but the speed of sound in vinyl is so much higher
>than in air that the resonance frequency ought to be real high
>there, too.  So, I'm a bit confused.  Any comments?

perhaps resonance is the wrong term.  anyway, vibrations in the air
column itself are neglible for all intents and purposes.  the reason
for clamps and exotic mats is to damp vibrations induced in the vinyl
of the record by the stylus and by acoustic feedback through the air to
the record.

having an airspace underneath the record allows a greater amplitude of
motion due to rapid air pressure fluctuations (sound).  removing that
airspace increases the force neccesary to induce record surface
motion.  a mat alone will not increase coupling.  a clamp is required
unless the turntable is designed to accomodate the added wear of a
weight.

vibrations in the vinyl are induced by the stylus assembly.  if you put
on a record and turn the volume all the way down, you will still be
able to hear the record playing.  the vibration of the stylus is enough
to excite air and vinyl movement so that is is audible.  the sonic
energy introduced into the vinyl of the record propogates until it
meets the edge of the record where most of it reflects back because of
the mechanical impedance mismatch of air and vinyl.  having an airspace
underneath the record also has the same impedance mismatch.  a clamp
and a properly designed mat provide closer mechanical impedance match
to the vinyl and more vibrational energy is transmitted to the mat so
that less is reflected.  the mat has a lot of internal friction so that
the vibrational energy is dissipated as heat.  the wave front that is
normally propogated to the record edge is damped before it is able to
reflect.

a properly designed clamp and mat must make the record contact as much
of the mat as possible, including the label and edge areas.  the
mechanical impedance of the mat must be matched to that of the vinyl so
that the transfer of vibrational energy to the mat is maximized.  the
mat must have high internal losses so that the vibrational energy is
dissipated as heat instead of being allowed to reflect off of the
platter and return to the record.  a very soft rubber surface provides
a good compromise between all the conflicting requirements.  a
conductive rubber surface helps by reducing static buildup if your
turntable platter or spindle is grounded.  although vinyl is not as good
a conductor of vibrations as steel or aluminum, it certainly is much
better than rubber.  you must remember here that the record stylus is
very sensitive to vibration of any kind.  vibrations on the order of
the wavelength of UV light can be detected by the stylus and easily
heard in reproduction.

the best mats i've seen are the Platter Matter and the Angstrom
Ultimat.  the Angstrom comes with a clamp that looks suspiciously like
the Nagaoka clamp.  hard mats provide inadequate contact surface and
sometimes have lower internal losses than the vinyl itself (there are
ceramic platter mats that do this) so that the vibrations are reflected
even better.  fibrous mats provide only limited damping (like the Linn
and Rega mats) and i personally recommend replacing them because they
pick up dust.  many people that i know who own Linns have replaced
their mats for the dust reason alone.  no platter mat which purports to
reduce record vibration should be used without a clamp or (if your
table can take it) a weight.

the benefits are better resolution of low level information, especially
in the midrange.  this usually is perceived as better ambience or
more detail.

Herb Chong...

I'm user-friendly -- I don't byte, I nybble....

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