Townshend Springs under Speakers

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?

a time and phase correct speaker with inert enclosure and pistonic drivers is the very definition of outlier.

If the floor is wobbly, springs can work, if the speaker is on solid ground, 3 spikes is preferred.

If one would consider a solid reinforced concrete floor to be "not wobbly" it has been my personal experience that sprung isolation works significantly better than on a so called "wobbly floor".

The main reason I concluded was that instead of the floor being excited by the energies created in a loudspeaker enclosure deforming, that the higher percentage of energies are deforming the springs. The floor no matter what it is, has a resonant frequency, however the more it can withstand deformation the higher the percentage the spring will be required to deform in order to effectively hold the loudspeaker as motionless as possible.

The concept that energy is stored in the springs used to uphold a loudspeaker is not entirely accurate, it deforms and returns to status quo with the load, and will move above and below the status quo holding the mass placed upon it. The ideal is to have an engineered spring loading for the mass it’s to isolate a broad range of frequencies, all within the audible range.

Such an engineering precedence is used in correctly assembling cars with different rated springs, depending on the result desired. One spring does not suit all applications, and so, it is the same with different loudspeakers.
I will suggest that the more rigid the car structure, the body, the more effective the suspension system can be engineered to support it. The results are much more predictable.

Last time I checked, I was not driving my speakers down a bumpy road

you have it upside down, your loudspeakers ARE the bumpy road!

As is one with 11 band EQ below 120 hz with performance measured….wait for it…in room. 

Thank you @prof