I suspect there are a large number of speaker co's out there which are rating their speakers at 8 ohms when in fact they are dropping to 6/4 or even less.

Hi Bob,

Here is a quick explaination. I work for a speaker company, pro audio, PA's etc. We build our own drivers, and in some cases we do make essentially the same driver with different impeadance values, 2,4,6,8 & 16 ohm. There are tradeoffs to be made. Higher impedance drivers have lower sensitivity, i.e. the amount of power you put in results in lower power out as your sensitivity goes down. On the plus side of the equation, lower impeadance drivers can handle more power. Axiom has choosen to design their product to provide good sensitivity and high power handling, with good frequency response. It seems to be agood mix, my M80's sound great.

This is due to the voice coil wire. Thicker voice coil wire allows more electricity to flow through it more easily than a thinner wire(lower impedance), and it can take more heat for a longer period of time, resulting in higher power handling. But it has more mass so it can reduce sensitivity. There are a lot of other factors involved (cone mass, surround & spider stiffness etc.). The general rules above aren't true in every case, but I think it illustrates the idea.

As previously mentioned impedance changes contiuously with frequency. It can vary a lot across teh spectrum. There is no industry standard for how the nominal impedance value is selected, I suspect some companies let their marketing department make the call. Some people use the lowest value, some take the average excluding the resonancy frequency peak(s).

Nice explanation! Thank You!

To be a pain: Axiom M80 uses 6 drivers per speaker. The more drivers, the lower the impedance, right? So are the Axiom drivers actually 8 ohm or more (16 ?) so that the final impedance on the amp is 4 ohms? Otherwise, it seems like the amp would have a 2 or 1 ohm load. Thus, does the sensitivity issue you explain apply here if the individual drivers are of higher impedance?

To be clear, I am not disagreeing with you, just trying to figure this out in my own uninformed way.

It doesn't matter what the individual driver impedences are, the amp only sees the entire load (impedance) as a whole.

Of course the load of the system (speaker) is determined by the collection of components, the impedance of each driver, the crossover, everything in the signal path, and how they are wired together (series or parallel). The wiring can be a combination of series and parallel, it doesn't have to be either/or.

6 speakers in the box can be configured in lots of ways. If all drivers are 16 ohm and are wired in parallel then the impedance of the system would be. 2.67 ohm.

1
_______________________________________
1/R1 + 1/R2 + 1/R3 +1/R4 + 1/R5 + 1/R6


If the same 6 drivers are wired in series then the impedence will be 96 ohm. You could have three in series (16*3=48) and 3 in parallel (5.34) and the resulting impedance of the system would be 53.34

Or any combination

I have gotten myself all confused.

Not to split hairs, but that would depend on how the "triplets" were connected... parallel or serial.

Bren R.

Yes, series in my example. I was trying to keep it simple.