Total Impedance?

[mpotoka]

I was just wondering--if you have 3 8 ohm drivers in a 3-way design.... how do you calculate the total impedance? Can someone point me to some relevant reading?

I am also curious how doubling up a woofer or midrange effects the equation.

[Kal Rubinson]

You cannot calculate it without considering the crossover network.

Kal

[kevmurray]

Nominal vs Actual Impedance

The nominal impedance rating of the speaker will remain 8 ohms. The real world impedance is dynamic, that is it varies with frequency and is different for different drivers. One thing that is certain is it will never drop below the dc resistance of the driver which is active at a given frequency. For example an 8 ohm woofer with a voice coil dc resistance of 6 ohms will never allow the speaker's impedance to drop below 6 ohms in the bass region. Same goes for a similar midrange in the mid region and tweeter in the treble region. The crossover will factor in of course but it's primary job is "steering frequencies" to the appropriate driver. The nominal impedance in your example will remain 8 ohms. If a driver with a lower impedance, say a 4 ohm woofer, were thrown into the mix the nominal rating should be changed to 4-6 ohms to better determine if an amp is compatible. This is the short version on this subject, volumes could be written. Hope this helps.

[kevmurray]

oops, missed your second question. Doubling up drivers will only change the impedance in the frequency range in which they operate. Wiring them in parallel will half the impedance while wiring them in series doubles it. The crossover component values would have to be modified for such a change to keep the cut off point the same. Again this may change the nominal impedance rating of the speaker overall.

[mpotoka]

Thank you for the excellent explanation!

[servicetech]

Also note there can be a HUGE difference in impedance depending on frequency, especially for woofers. It's not uncommon for a woofer to have an impedance TWICE the nominal at the crossover frequency. This means and 8 ohm woofer may actually be 16 ohms at the crossover frequency and the crossover needs to be adjusted to compensate.