Thanks Alex. I wonder what that other board is then. I have a theory on the enclosure. Maybe Rudy will chime in here. Its true there is an ideal enclosure volume for each driver and rarely is there room for an enclosure big enough as there is here but I am wondering if this was also designed to resonate at the lower frequencies.
Normally when you design the enclosure you don't want the panels to vibrate and color the sound but maybe in this case thats what they wanted to do. Two large panels made of thin material are likey to vibrate.
Alex, if you haven't put that engine back together again maybe you could touch the sides as you blow the horn or start up the prime mover? If you grip it tight and the volume decreases you will know maybe that was the intent of that enclosure design.
I bet grabbing it would decrease mid to low end somewhat. I doubt they went that far in design as to count on "passive radiation". If there is a positive aspect ,it was likely a happy byproduct in the high-midrange and above due to the small size of the enclosure panels.
Theywould have trouble with passive low range frequency reproduction imo. You need big air movement measured in volume/amount for lows. The small single divers and tinny nature of our materials tell me the more solid that enclosure the better.
I think a 2 way speaker system will be the next big improvement with the shrinking amp boards hopefully giving the designers enough room for a tweeter to remove some demands from the bass drivers.
There are easier ways of designing bass enclosures (passages & port sizing can do TONS moving air correctly, Bose showed you that .E.g., You don't even see much if any passively radiated sound parts in hi-fi speaker enclosures made for improved phone/device sounds. Enclosure size, then passages, & porting works wonders on mids and lows. Passive radiation is usually a low cost fine tune method and then still needs deep movement of the powered speakers to be effective.
Getting into larger car and home systems, actual passive radiators are a bit more common, but still rare for the most part.
They usually resemble a flat speaker vs a cone and are made of a dense material not subject to rattle by high frequency, but still reactive to mass air movement of lows from another driver ... like a flat sand filled pouch suspended by foam... I.e. you make a bigger speaker via surface area of two speakers without a second magnet driving #2, and#2 is dampened from making highs. Any more explanation dives pretty deep but the end result can be a less "buzzy" wider range speaker with one heck of a kick when somebody uses a hard stick/strike on a bass drum, especially a pedal bass drum with a solid hammer on it. Deep lows, usually 100% electronically created vs analog electrically boosted.. and deep kettle drums suffer slightly but low mids& mid lows separate..pop.
They are also usually used with sealed enclosures, though I've set up two of my own systems with ports that improved things over two drivers in phase or opposed, sealed or ported, in mix and match system improvements (flatter & about 3db better on an over 80db kick, 6db over 90db, better mids and high end lows via the driven speaker choice. (better bass guitar delivery and sharp kicks).
(I designed some vehicle audio for fun and friends, and more for use as outdoor open air systems... not waking neighbors or disturbing other folks at intersections 😇 at least not too often😨. Usually just to drown out the idiot next to me who wasn't being as thoughtful 😈 Like running a sleeper cross ram wedge motor but seldom using it's full potential until weekends at the track 😁 (98db constant with bass peaks at 108db; not quite top end quality or competition range but at half their costs. The world record was 116db back then. Good enough for a park full of frisbee players. I hated a bunch of cars playing different songs and you couldn't hear any well 😉)