Are we to believe speakers ring in the sub-hertz*** regions and this noise actually transcends into audible noise that the human ear easily detects?
Great question.
More to the point though in the application of loudspeakers, is that fact they reproduce the entire audible frequency range, and with that energise the enclosures.
If one where to consider the speaker enclosure as a very rigid balloon, that when the bass/mid-bass driver moves in and out will shake, pressurise and depressurise, especially when driven hard.
The bracing effectively creates smaller surface areas with distinct sizes of wall material, depending on the engineering qualities of the enclosure. This creates smaller nodes where frequencies can excite the enclosure walls.
An easy way to test this out on your own speakers is to play a frequency sweep of white noise, at levels you listen at in your own system. Assuming the track has the same amplitude throughout, you will hear the accumulated sound of the cabinet and the signal the driver is reproducing.
I heard a single Focal BE 1038 subjected to such a sweep and heard multiple large frequency breakouts from the cabinets, clearly colouring the voicing of the speakers. This largely evidenced what we heard when listening to a pair that had been traded in.
https://www.youtube.com/watch?v=oSB-nmlbkdA Nikola Tesla vs Mythbusters - this has some good information regarding this phenomenon.
So the bracing inside a speakers enclosure will create different frequency nodes that can be excited depending on the engineering controls of energies that are introduced to by the drivers, throughout the audible frequency range.
Engineering controls of vibrations can vary in effectiveness considerably, and will have an impact on the overall voicing the loudspeaker** delivers.
** distinctions are important when explaining anything, I often use the term drivers instead of speakers, because often people use the term speaker/s for that which I mostly use loudspeaker.**
*** Sub-hertz, are you referring to a frequency cycle under once cycle per second?