It's an example of what I described above with the driver (active radiator) rolling off and the passive radiators, which are tuned to resonate at the lower frequencies, picking up the vibration from the driver and amplifing it. Call it what you want but the effect is casued by the low frequencies at higher SPL (Sound pressure level). A sealed sub can do this too. I have plans on the drawing board for a sub with a 15" driver in a 5 cubic foot sealed box with either a 500 or 1000 watt amplifier. If I ever get around to building it, it will make a nice end table. It'll do 16Hz no problem but there's no way you'd ever call it a "small cube"