There's a fundamental issue of a moving speaker system that you're overlooking. Flat impedance is NOT required for flat frequency response! In fact, if the driver had a flat impedance, the frequency response would be decided NON flat.

For example, every driver has a resonant frequency. Run the driver in free air, and you'll see the impedance peak at the resonant frequency. Yet the frequency response is flat! Why?

The impedance is really nothing more than the electrical mirror of the mechanical impedance of the system. When a voice coil moves in a magnetic field, it generates a voltage. This is the dreaded "back EMF" that so many have heard about (back because it's generated by the voice coil moving in the gap, and EMF because that's short for ElectroMotive Force, a fancy way of saying voltage).

Note that anyone can verify this effect - disconnect your driver from the amp, put an AC voltmeter across the speaker terminals, then push on the cone. You'll see the voltages jump all over. And in fact the amplitude of this voltage is directly proportional to the velocity of the voice coil - the faster it moves, the higher the voltage.

Anyway, what is impedance? Well, it's AC resistance, simply put. It's a ratio of voltage to current (V=IR, thus R=V/I). Applied voltage to supplied current. If you have 10 volts pushing 1 amp, that means you have 10 Ohms (10/1). Do this at different frequencies up and down the spectrum, and you get an impedance curve - V/I ratio at different frequencies.

So how does the back EMF come in? Well, let's say your amp is putting out 10V, and 1A. That is 10 Ohms, right? But what if the EMF the driver is generating is 10V, because of the motion of the cone? Well, now we have (10V from amp + 10V from driver) 20V and 1A in the system. Or 20 Ohms of impedance.

So what would cause the driver to move a lot with little input? Resonance. Resonance is a characteristic where very little input creates high levels of output. A little excitation to the system (a volt or two to the voice coil, causing forward motion) creates a LOT of output. Which means that you generate a LOT of EMF with very little applied voltage. Which means it's efficient (and is why you get high output with little actual current at the resonant point).

So in conclusion, the problem with a nonflat impedance is basically irrelevant; it isn't a problem at all. In fact, if it was NOT there, then things would REALLY be screwed! That is when we'd need lots of EQ. But thankfully the laws of physics dictate how things really work, and the nonflat impedance is just a confirmation that things are working right.

So, given that, XBL^2 doesn't really help the impedance issue, but that is a non-issue to start with (impedance in terms of nonflat response down around resonance).

Dan Wiggins
Adire Audio