[dflee]

I am an imbecile when it comes to electronics. Does a speaker retain it's rated Ohm rating with a jumper as it would without the jumper? I Bi=Amp and was told by a Gentleman that by removing the jumper the Ohm rating for the speaker went up. Is this a good or bad thing for the Ohm rating to change and does the sensitivity of the speaker change?

Later
Don

[wywires]

The "ohm rating" is a nominal number that comes close to the average impedance over a broad frequency range. In some cases the impedance curve for the mid/tweeter is different from that of the woofer. I've had speakers where the nominal impedance of the tweeter is 8 ohms and the nominal impedance of the woofer is 4 ohms. Inserting jumpers doesn't make this a 6 ohm system. Since the woofer draws more current than the tweeter, it's safe to call this a 4 ohm system.

[Letitroll98]

Alex is quite correct and to add a bit to his post, note that when he says "average impedance over a broad frequency range" that can easily vary from 4 ohms to over 20 ohms in a nominal 8 ohm speaker.  Here's an electrical impedance (solid) and phase (dashed) plot of a very benign speaker load (Polk RTi A1 in this case).



Note the wild swings of the solid line, which is plotted by the ohms scale on the left.  The dotted line is phase angle which is pretty close to zero at the lowest impedance swings, making this a very easy to drive speaker.

[dflee]

I always thought that as you add speakers to a load that the impedance changed and impedance was related to ohms. By disconnecting the woofer section from this load why would it not change? Does it have something to do with the internals of the speaker?

[wywires]

I always thought that as you add speakers to a load that the impedance changed and impedance was related to ohms. By disconnecting the woofer section from this load why would it not change? Does it have something to do with the internals of the speaker?

Impedance for sure would change if you disconnect the woofer.

[neobop]

Dflee,
If you take two 8 ohm drivers and hook them up in parallel, you wind up with a 4 ohm load. Disconnect one and it's 8 ohms. If this is a biwired crossover, it doesn't work that way. Assuming that both the high frequency section and the low frequency section are 8 ohms each, you can hook up either section or both sections and still have an 8 ohm speaker. Any difference is because of the impedance of the individual drivers.

If you're biamping with an electronic crossover it probably works more like the first way.  Two 8 ohm drivers in parallel = 4 ohms. Because you used the word jumper, I suspect it's a biwired crossover. If you're biamping with a biwired crossover, each or both sections is 8 ohms or whaterver the speaker is.
neo

[Æ]

If you have a two way loudspeaker, a woofer and tweeter with two pairs of binding posts, one pair of binding posts goes to the tweeter and one pair goes to the woofer. The impedance at the one set of binding posts will be whatever the impedance of the woofer is and the impedance at the other set of binding posts will be whatever the impedance of the tweeter is.
Impedance is a function of frequency, regardless of how many set of binding posts the loudspeaker has.

Don't confuse "Impedance" with "Resistance." Both impedance and resistance are measured in "Ohms" (not "Ohmage") but impedance can and will change as the frequency changes. Audio frequencies are typically in the range from 20 Hertz to 20,000 Hertz. (20Hz - 20KHz)

Pure simple resistance doesn't concern itself with frequency, because it is measured at no frequency (zero Hertz). A typical 8 ohm woofer might have an impedance that varies from slightly below 8 ohms at 1 Hertz to maybe 60 ohms or more at its resonant frequency (peak) and then back down to somewhere near 8 ohms again. After that a woofer's impedance continues to rise gradually as the frequency extends towards and beyond 20,000 Hertz.

If you use an 8 ohm woofer and an 8 ohm tweeter WITHOUT a crossover, your impedance would be 4 ohms! But who would use a woofer and a tweeter together without a crossover? A crossover is a frequency dividing network. As long as you are using a crossover with an 8 ohm woofer and an 8 ohm tweeter then your impedance will remain at or near 8 ohms straight across, not 4 ohms.

If you use a 4 ohm woofer and an 8ohm tweeter without a crossover, your impedance would be 2.66 ohms. But with a crossover, your impedance would be 4 ohms in the range of frequencies covered by the woofer and 8 ohms in the range of frequencies covered by the tweeter.
If you use a 4 ohm woofer with an 8 ohm midrange and a 6 ohm tweeter all with a crossover, then your impedance would be 4 ohms in the range of frequencies covered by the woofer and 8 ohms in the range of frequencies covered by the midrange and 6 ohms in the range of frequencies covered by the tweeter.

If you end up bi-amping a loudspeakers, one amp will "see" the impedance of the tweeter and the other amp will "see" the impedance of the woofer. If you use jumpers and one amp, then that one amp will "see" the total impedance as it relates to frequency, not the combined impedance.

[dflee]

AE
Great response! So it's the crossovers that eliminate the combining of the speakers.

Thanks
Don

[Æ]

AE
Great response! So it's the crossovers that eliminate the combining of the speakers.

Thanks
Don

Impedance is a function of frequency.
As long as you use either a crossover or separate amps to divide the frequencies, you'll be fine. Regardless of how you do it, you need to separate the frequencies by sending bass to the woofer and treble to the tweeter.