Mac48025 3-6-2017
I’m told the impedence curve also influences how hard a speaker is to drive? The flatter the impedence curve the easier it is to drive?
Yes, generally speaking. Although if the impedance stays relatively high at most or all frequencies lack of flatness will be less significant than if it dips down to low values at some frequencies. For example the impedance curve of the Enzo XL that I linked to earlier varies a lot over the frequency range, but never goes much below 6 ohms, which minimizes the significance of that variation.
Also, the phase angle of the impedance can be a significant factor in many cases. That is shown as a dotted line in the impedance plots in Stereophile’s measurements, and corresponds to the markings on the axis at the right side of the graph. Very negative phase angles that may occur at various frequencies, especially if they occur at frequencies for which the magnitude of the impedance (the number of ohms) is low, will work in the direction of making the speaker a more challenging load for the amplifier. JA will usually comment on that. For example, in the Enzo XL review he commented that "the Tekton speaker’s impedance magnitude (fig.1, solid trace) remains above 6 ohms at all frequencies, and the electrical phase angle (dotted trace) reaches extreme values only when the magnitude is high. The Enzo XL is therefore well suited for use with low-powered tube amplifiers." The "extreme values" he refers to for the phase angle are in the vicinity of -45 degrees, but occur at frequencies at which the impedance magnitude is around 10 ohms or more.
Regards,
-- Al