[sdiy] Speaker Impedance

[harrybissell]

Hi Bob

Ok let's try it another, another way. Essentially we agree except for
the 'why' of it.

Take the 8 ohms speaker. In this case I'll even say that the DC resistance and
the impedance are both eight ohms.

The resistor is a pure resistance, and is eight ohms.

The resistor and speaker are receiving the same voltage (current is equal)

The resistor (because it is essentially non-inductive in this case - think carbon
composition) dissipates all the power as heat.

The speaker, because of its design (as a coil) dissipates some power in making
a magnetic field and moving off-center, and holding that position against the
spring forces that would like to re-center it.  The rest is dissipated as heat.

The speaker is doing 'some' of its job... otoh its kind of stupid because the
function is to move air... and it is not doing that. And our hearing does not go to
DC either.

The "DC" resistance is not really that important to us, except that as you noted...
we wish it was low or non-existant.  Its not... too bad for us.

The "impedance" is the real object... because it expresses how much work we can
get out of the speaker at some real (hopefully perceptable) frequency.  We will also

be careful to band-limit our input so that we don't make waste heat for freqeuncies
we can't / don't want to hear.

The DC in the resistor produces only heat.  The DC in the coil produces heat
and displacement.  Since we don't hear it (the frequency is infinitely low...)
it produces no sound.

Maybe we can agree, one is useless... and the other one might as well be.

My comments about DC resistance are for a broad range of speakers, including
tiny little things you would not drive with a guitar amp.  The point is that you
can't
take an ohmmeter reading and tell what the 'impedance' is.   If the speakers have
different DC resistances, but the same impedance at a frequency, they will
draw the same current and produce the same 'power'... how much displacement
that causes is more mechanical efficiency that electrical.  At any rate, the
impedance
cannot be lower than the DC resistance.

And I might add I've been tempted to hook up speakers to one of my
inverters.  Do you have enough for 840KW ???   (loud, yes :^)
1200A at 700V...  1KHz.  Rock and Roll ???   :^P

H^) harry