Shardone - Some of this or that might be related to rainbows and puppydogs. If the driver is breaking up anywhere in its band I can't hear it. You couldn't either.
When the frequency response becomes very spikey above a certain frequency cone break up is often the explanation. The fall off in off axis response is probably beaming. IMHO the "whizzer" cone is there for a reason.
As Duke points out...our hearing is remarkably well able to deal with notches in the frequency response (spikes are actually much more acceptable than sustained drops)
So the break up may not be audible.
The uneven power response: Although the "whizzer" fixes the on axis to a large extent....I expect that the roll off in off axis response above 2 Khz will be clearly audible. The speaker will probably sound different as you walk from directly in front to 45 degrees off axis - this is not a bad thing - just the way it will behave and will make speaker positioning/toe-in quite important - thats all.
Duke,
I agree with you. Underdamped/high Q design is a much better explanation for the severe notch at 150 HZ. The cone will respond to cabinet resonance.
Here is another example
PMC GB1 of a speaker with a similar notch.
There is a superficial similarity in the tall narrow cabinet dimensions and overal response, however the GB1 is crossed over on the tweeter much lower and therefore has a more even off axis response at higher frequencies.
PMC = 6.1"W x 9.21D x 34.25" H
Zu Druid = 10 1/2"W x 6 3/8"D x 50"H
I agree that the notch at 150 Hz will likely be inaudible - many rooms may have sharp notches like this anyway from wall reflections/modes - so no big deal.
However an underdamped design will differ quite audibly from a critically damped design. As you suggest, longer oscillations in the low end may give more of a bass impression than the frequency plot alone may suggest.
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All goes to show that measurements are meaningful and do tell us something about the design and the performance.
Of course, I would NEVER recommend to select a speaker based on measurements alone. Nevertheless I would not recommend to stick ones head in the sand and totally ignore meaurements either.
Let me finish by saying Zu Druid's are undoubtedly good speakers. The high efficiency and dynamics from this design is likely to be extremely impressive even with very modest amplification power. My intent was never to bash these great speakers. Sorry if it sounded that way...but I was only trying to defend the value of measurements.