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Richard,
The inductance (Le) of the voice coil creates a 6db/oct rolloff starting around a certain frequency. I've seen drivers with different inductance specifications at different frequencies, so it's probably a nonlinear effect. Most woofers roll off steeply past a certain point, which is probably higher than the inductance's corner frequency because many drivers have a naturally rising response with increasing frequency, counteracting the inductance. (I think the big peaks before rolloff are breakup modes, but I don't know exactly what that means. It might be that the cone starts to flex and becomes ... un-pistonic.)
I forget the directivity rules for driver size, but I do think an 8" woofer starts to beam around 2KHz, for example.
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