Don Pearce wrote:
On Sun, 18 Jun 2006 13:36:03 +0100, Jim Lesurf
wrote:

In article , Glenn
Richards wrote:
Serge Auckland wrote:
No it isn't! Your experiment is being done with dc. Repeat the
experiment with 50Hz to one bulb and 10kHz to the second bulb, both
bulbs being fed through a single amplifier and fed through a simple
capacitor or inductor "crossover". You will now see that the bulbs
don't change their brightness.
Makes no difference, AC or DC.
Do the experiment Serge described and you will find that the results show
he is correct in what he says. Then as Don has suggested, investigate
diplexing.

Bear in mind that what he describes is the kind of technique routinely
employed and studied by electronic engineers. Indeed, I'd suspect that
more than one undergrad lab may have what Serge describes as an
experiment to show this to students. It is the basis of frequency
division multiplexing in transmission line systems. :-)


Series resistance is still the same, regardless of line frequency.

Actually, not necessarily so. It is quite possible for a cable to
have a resistance that varies with frequency. :-) However even if we
ignore this, what Serge says is correct.


That experiment was to illustrate a concept, not specifics. The
principle is exactly the same.
Only for the specific case you gave. The snag is that loudspeakers aren't
simply resistors.

Slainte,

Jim

I think that there is a far more fundamental problem at work here,
Jim. Glenn does not appear to understand that what we have is a simple
voltage divider, comprising the cable and the speaker impedance. A 1
ohm cable, combined with an 8 ohm speaker will result in a loss of
about a dB at all frequencies, and it doesn't matter what signals are
present.

If the effect he is describing were to be real, then picture a 1kHz
signal in combination with a 10Hz signal. The current due to the 10Hz
would be changing from a maximum to zero 20 times per second, so the
loss at 1kHz would be changing 20 times per second. The 1kHz would
actually be amplitude modulated by the 10Hz, rather than simply
superposed, which we know to be the case. His misunderstanding of the
physics really is happening at a rather fundamental level.

d

Absolutely! Glenn's theory would only be valid if cables were
non-linear, but we know that cables are extremely linear, pretty much to
the limits of measurement, so no modulation can take place.

S.