NONLINEAR THERMAL
DISTORTION - A SOLUTION IS SOUGHT
This is only preliminary. Much
has been said and discussed re the mechanism to describe thermal distortion
in speakers. The fact is that speakers are rarely 1% or more efficient and
the conclusion is that 99% plus is usually expended as heat. Indeed, most of
your amplifier's Watts are not actually used, the process is extremely
lossy.
But the question is posed, since 99% of the power fed
to the Voice Coil is dissipated as heat, then your very guts tells you that
somehow it causes some form of non-linear distortion. It just has to!
Add to that, many claim to hear an improvement when
speakers are designed to deal with heat in an organised way. And I agree
with them.
Here we need to distinguish between Linear Thermal
Distortion (LTD) and Nonlinear Thermal Distortion (NTD). Evidence of LTD is
easy to prove, but NTD is an entirely different matter. LTD is easily to
measure even with basic instruments and also easy to model, all of which has
been done.
The Voice Coil (VC) is made of Copper (rarely anything
else). It has a Thermal Coefficiency of 0.4% per single unit of Centigrade.
So if a VC has a coil resistance, called Re in T-S parlance, then each C
degree of increased heat will cause 0.4% increase of Re. So if Re is 6 Ohm
at 20C, then at 21C Re has increased to 6.024 Ohm. If the VC is increased by
100C (not unusual) then Re would now be 8.4 Ohm (40% increase. The result
can be viewed two ways, that Qe increases proportionally to the increase in
Re and that the 'No' (nominal sensitivity of driver) will be reduced. But
even easier to understand is that the RMS current drawn from the 'voltage'
amplifier will be reduced and the same result, less power is drawn (current
amplifiers will not have this problem - but that is a different topic). The
driver will now be less loud by a known amount and this is LTD.
BUT...
It is not nonlinear. The reduction in output is
very gradual and due to the thermal mass of VC and to a certain extent the
motor/magnet assembly, the Re's rate of increase is too slow to show
up at audio frequencies, way too slow!
This means that the mechanism causing NTD is not
directly related to an effect that we know leads to LTD, but cannot explain
NTD. Does that mean that NTD does not exist. Most of us interested in this
subject still insist that it must.
Hence we have a mystery, one that has persisted for
decades.
A mechanism is now being discussed here in Sydney that
may well be the breakthrough we seek. If this mechanism is correct, then
both driver manufacturers and system designers (like ourselves) will be able
to deal with this factor in a much more focussed way!
It is still early days and there is still much to do.
But I believe that we have made a very good start in developing a conceptual
model (hypothesis) and then see if a mathematical model can be developed.
The Elsinores already benefits from the thinking
behind these developments and in time other designers will also be able to
apply to their designs.
Joe R.
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