Ported (vented) speakers
I suspect the answer to your conundrum is contained in your own words ....
Originally Posted by Diminish
The common thread here is that all three speakers in question are vented, that is, they have an opening of some shape and size. In a vented cabinet the woofer in motion at low frequencies will 'bounce' the air inside the box as an acoustic spring and sound will (in a certain relatively narrow frequency range) be phase inverted and emanate from the port.
The other speakers that I've owned have all been 2 way, bass reflex designs with similar configurations... [one] in particular, displayed quite a lot of excursion. Even with their 93 dB sensitivity, I don't feel that they did a better job of reproducing bass than the Harbeths.
As with any mechanical system, and very much like the design of a car suspension, the designer can balance many variable. He can design an F1 racing car with a taught suspension system in which the driver feels in intimate contact with the contours of the road. Such a design is only comfortable providing that the road surface is glass-like smooth and barely usable on conventional roads. Another compromise would be the Rolls Royce suspension: a big, wallowing, superbly soft suspension that soaks up every lump and bump and is slow to respond making high-speed cornering rather tricky. In the first case if the road-surface's undulations (at a particular travelling speed) exactly coincide with the suspension's natural resonant frequency then there will be violent, poorly controlled up and down motion of the car. In the RR example, it may not even be possible to identify the natural resonant frequency because it is so well damped.
So, what I'm suggesting is that the violent excursion you've witnessed is probably only evident over a relatively narrow low frequency range and power level. It reflects a particular design balance, possibly even a cultural preference for a different listening 'driving experience'. If you use a turntable as a source with its inevitable LF rumble and groove tracing noises this cone excursion effect will be magnified by the presence of lots of VLF to tickle the speaker's vent. That's really all there is to this! No new breakthrough! No reinvention of physics!
Alan A. Shaw
Harbeth Audio UK