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
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
Here we need to distinguish between Linear Thermal
Distortion (LTD) and Non-linear Thermal Distortion (NTD). Evidence of LTD is
easy to prove, but NTD is an entirely different matter. LTD is easy to
measure even with basic instruments and also easy to model, all of which has
LTD results in a loss of
sensitivity, here is how:
The Voice Coil (VC) is made of Copper (rare
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, a 40% increase.
The Qe will also be raised by the same 40%.
This lessens the sensitivity of the
driver but not necessarily efficiency as the nominal current is decreased.
But in terms of calculating sensitivity it is the effect on Qe that lowers
the BL factor - the force factor of the Voice Coil in the gap.
So 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
(unlike "current" amplifiers). The
driver will now be less loud by a known amount and this is LTD.
It is not
proved non-linear. 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!
Geddes has expressed this thought clearly and yet believes that
NTD may be for real and designs with that in mind.
The mechanism causing
LTD has not
yet been directly linked to
NTD, there is no explanation
where or how heat becomes or causes NTD. Does that mean that NTD does not exist. Most of us interested in this
subject still insist that it must. Somehow!
At least some of this heat finds
itself radiated into the motor assembly. I tried input a 500 Hertz 10V RMS
into an 8 Ohm driver with 1" Voice Coil - and after some time the motor
assembly went double the ambient temperature.
Interesting that not all parts of the motor assembly stabilises at the same
temperature. This was noted with some interest as the heat is clearly
radiated across the gap, the only origin. This leads to an interesting train
Hence we have a mystery, one that has persisted for
decades. But I will reveal one mechanism that may
help to or lead to a resolution. This 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!
an assumption that the Voice Coil gets heated linearly across its length,
yet the observation has been made that speakers failing due to thermal
overload tends to fail in higher proportion at a certain point - the front
of the Voice Coil. This tells us that sinking heat is less successful at
that point and varies across the length of VC, probably approximating as
U or more likely a J shape. Just as all parts of the motor
assembly did not heat up the same.
indicate that the VC temperature is not a "constant" versus its length and
hence a VC thermally loaded up at higher frequencies where there is little
amplitude (cone in low piston mode or flexing mode where cooling may well be
less efficient) and then when any subsequent lower frequencies (piston mode)
comes along and become non-linear as the Re fraction of the VC's Re that is
in the gap will be modulated at audio frequencies, If both low frequencies
and high frequencies are reproduced together, as we would find in music,
will they both become non-linear?
modulated at audio frequencies modulates Qe and hence BL also at audio
frequencies. If this is the case we now do have a non-linear
mechanism, we have NTD; and LTD is only limited to static signals. Music has
a broad spectrum that can excite in a way static measurements cannot
Any system that lessens thermal
load of the VC, such as increasing its size and mass, and also
reducing the need for pistonic cone movement will lessen NTD. The latter
means the area of the cone or "Sd" will need to be as large as possible or
at least increased, too lessen cone movement away from its static position. Small driver with small VCs and small cone area Sd will
be at a disadvantage as they attempt to move air. Indeed larger drivers with
larger Sd are also perceived as
sounding more effortless and lower sense of strain - exactly what you would
expect. The lowering LTD may well prove to also lower NTD and treating LTD
will also do the same for potential NTD.
It's interesting that at least one
designer, Troels Gravesen, has heard audible effects from the type of former
that the VC is wound on, preferring Aluminium over Kapton. Could it be that
the former has a significant effect over the VC's heat distribution? He also
notices the same effects on drivers he considers having an inadequate Sd,
the cone area. If the conceptual theory is correct, then larger Sd will
lessen the audible effects of non-linear VC heat distribution. Please read
his article: Vocals.
It is still early days and there is still much to do.
But I believe that we have made a good start in developing a conceptual
model (hypothesis) and then see if a mathematical model can be developed.
As in all things of similar nature, this needs testing.
The Elsinores already benefits from
this thinking as thermal load on the Voice Coils
are far less than average systems and in the order of 16 times below 500
Hertz, four times in the midrange and at least two times at high
frequencies. This performs well above most commercially available systems.
It is also a notably dynamic speaker system.
In time other designers will also be able to
apply this thinking to their designs. In many ways they are, at least
the good ones are, but mostly because of gut instinct. We may now finally
get some proof of that instinct and power of observation.