On 2006-03-08, John Phillips wrote:
On 2006-03-07, Serge Auckland wrote:

"Menno" wrote in message
...
When dealing with larger currents (speaker cables), the mechanical
characteristics start to play a role. Parallel conducters push/pull on
each
other due to magnetic (Lorentz) forces thereby creating a signal dependant
inductance and capacitance which modulates the signal going through the
cable.

The mechanical movement will be infinitesimal, when considering the current
flowing and the magnitude of the earth's magnetic field, consequently the
change in capacitance and inductance will be similarly tiny. Even if it
wasn't, the change in voltage across the 'speaker considering the source
impedance from the amplifier is very low would be again vanishingly small.

I was considering this. The standard formula for the attractive force
(F) between two parallel conductors of length l and separation d is:

F = k * I1 * I2 * l/d

The constant k is defined to be 2*10^-7 N/Amp^2 and I1 = -I2 for a
speaker cable (i.e. the same current returns in the opposite direction).

So, for a 1A current, conductors 2mm apart repel each other with a force
of 10^-4 Newtons per metre or about 0.01 grams force per metre.

I haven't looked up the elastic properties of dielectrics yet to determine
the mechanical movement (any references for PVC or similar?). However 10
milligrams per metre of length isn't a big force.

I have looked up the figures now. I estimate Young's Modulus to be 0.1
GPa for PVC and similar so the relative thickness modulation, delta-d/d,
is about 5 x 10^-10.

Even then the intermodulation produced by the varying capacitance will
be rather small as the amplitude and phase response of a loudspeaker
cable is primarily determined by the inductance not the capacitance.

So, turning the above thickness modulation into inductance variation,
delta-L/L is about 10^-10 give or take some small constants.

I haven't calculated the intermodulation from a 0.1 parts per billion
modulation of cable inductance but my engineering feel is that it's
about three-fifths of bugger all.

Lorenz forces may look good in theory to apply to loudspeaker cables
but the engineering says it's not significant (assuming I haven't made
a mess of the figures, that is).

--
John Phillips