Hi E.G.
Interesting post.
The problem manufacturers have with subwoofer measurement figures is the enormous variety of rooms in which they will be used.
A reservation I have heard from the manufacturers is that it would not be fair to their own products to quote figures. Because the next (competing) manufacturer would simply employ a more flattering measuring environment.
The wavelength of the frequencies they are dealing with would demand an enormous anechoic chamber to avoid room reinforcement. Probably much larger than a conventional floorstander speakers would need.
The small size of these subwoofer companies would suggest that building such a facility for their own use would be be financially crippling.
Until a suitable completely independant testing facility is available to all they would prefer to maintain a "level playing field" by not providing any figures at all.
My stand on this would be to suggest a test tower set above a conifer plantation. In still conditions it would be as close to anechoic as really matters. It has the advantage of cheapness.
Another problem is that should they provide any figures at all. It would only provide fodder to every cynical Tandy meter wielding enthusiast to "disprove" in his own (telephone kiosk sized) listening room.
There is also the question of the life of the drive unit in the box. Unless steep active filtering manages the subsonics. Then cone excursions on the increasingly fierce DVD recordings available would be pushed (and pulled) beyond sensible limits. Particularly with ported boxes which have very low resistance below roll-off.
Really the manufacturers have a nightmare scenario. They must provide a nicely finished box no larger than a house-brick to satisfy WAF requirements. Particularly in the ever smaller modern rooms. Yet modern DVDs provide tracks that would destroy their units if driven at anything like full power. All this has to be set against an acceptable price for the box. If the manufacturer provides close to perfect performance he is governed by the laws of physics and real life costs. That means (as you say yourself) BIG!
Regarding the bandpass speaker I would humbly suggest that such harmonics would be audible. As harmonics would have to be up in the audible range. The narrowness of the frequency response band is desirable (-3dB @ 20 & 120Hz) as they were deliberately matched to particular satellite speakers. (From David Purton's: "The Coupled Cavity Handbook")
Purton does indeed show a steep roll-off. My original measurement figures were -27dB @ 10Hz. However it seems to be freely accepted that the Tandy SPL meter reads -20dB at 10Hz. Hence my new and surprisingly flattering figure of -7dB.
NIMBY
