The impedance between 4 and 8 ohm to an audio amp,
is almost insignificant.  Unlike if you where feeding this in
to a 1000 ohm load.  Wiring 2 speaker on the same amp
has the same effect as a difference between 4 and 8 ohm
impedance. So, don't be concerned unless your trying
something fancy.
                                                      Eric

David Beierl wrote:

> At 07:36 PM 1/28/2003, Ben McCafferty wrote:
> >Any of you stereo buffs:  I need to determine if my home speakers are 4 or 8
> >ohms, so I can set my receiver properly.  Unfortunately, there is no marking
> >on the case or actual speaker anywhere, except "75 watts, magnetically
> >shielded, design for t.v."
>
> Speaker manufacturers tend to make 4-ohm speakers because it's cheaper...
>
> If you really want to measure the impedance, you'll need to use a sine-wave
> test signal of 1000 Hz frequency, and put a non-inductive power resistor in
> series with the speaker; then measure the ratio of the voltages across the
> speaker and the resistor; the impedance will be in the same ratio as the
> voltages, i.e. Zspkr : Zresistor = Vspkr : Vresistor.  With a non-inductive
> resistor Z (impedance) should be the same as R at any reasonable frequency,
> so multiply that by the ratio.
>
> Since the speaker is an inductive* load, impedance will be
> frequency-dependent, but 1000 Hz should give you the rating number.
>
> *actually resistive + inductive and very likely capacitive as well, given
> the crossover circuits and all.
>
> david
>
> --
> David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
> '84 Westy "Dutiful Passage"
> '85 GL "Poor Relation"