Bohanna has the most correct outlook.
In normal Home construction, any Sub system that has the capabilities of the one mactoast
possesses can produce enough SPL to displace the existing volume of air pressure in a room of 300 sq. ft.2x over. This pressure increase will impact a substance like Drywall, and the drywall will amplify the effect much the same as a Drum Head does upon being struck by a Drum Stick, only in this case the surface area affected will be far more broad.
And...should the Ceiling be constructed in a 24" O.C. method instead of 16" O.C., that conclusion is almost doubled. The Drywall becomes a Diaphragm, a "Eardrum" just waiting to collect resonant air pressure and turn it into some sort of reactive energy.
Few really have a correct understanding of just how conductive such air pressure changes can be into a resonant solid (...wall or Ceiling...). Even the much slighter changes in air pressure that vocals produce are primarily responsible for such vocal sound being conducted through walls. One does not hear "air Pressure"...but Air Pressure delivers resonant frequencies to a location, and "holds" them there long enough to transfer the energy of those resonating frequency wave into a susceptible medium. Be it Drywall, Wood, even Steel, (...and of course a Membrane like a Eardrum...) the efficiency of transfer is directly related to the amount / level of energy that can be absorbed.
In the Sound masking industry, Resonant frequency producing Audio Transducers are used to inject / create a inaudible resonant barrier within a structural solid that will either eliminate or greatly reduce the ability of such a material to absorb any exterior influence that is not able to displace and overcome the amount of energy within the solid.
A 18" Sub that is not tuned specifically to a room, and that is within a room whose surfaces are not protected with enough absorptive material, makes for a tremendous opportunity for non- harmonic resonant distortion within the solid to occur. This is because when even accurately produced resonant frequencies hit a non-dampened solid such as Drywall, (...which is by nature not really wanting to be agreeable with the insertion of such energy...) , and it does not have the potential to accurately reproduce the frequencies at the level it is being encountered, those deep Bass frequencies instead become inserted "vibrations" which are neither sympathetic nor accurate. No...they are just adverse vibrations that can indeed damage Projector Lamps, shake loose mechanical parts (Lens Shift,,,Zoom and Focus settings...Solder points) and cause many such vibration related issues.
Basically, what you have going on is akin to someone striking a ceiling lightly but repeatedly with a Cloth-Wrapped Fist that is larger than a Garbage Can lid.. While the point of strike might not even be close to the PJ, the continual concussive transfer of vibration is amplified and spread throughout the solid.
Yep...that's a very "layman-esque" explanation, but it applies to the question quite well. The more excessive the quest for oppressive Bass, the more consideration needs to be given toward dampening the room's surface.
Oh....BTW, mounting a PJ on a "Shock Absorber" doesn't work. That is not the answer, in fact that can often make visible shaking worse. The solution is to dampen the energy being transferred into the solid the PJ is mounted upon.
(...it took me so long to compose the above, Bohanna
had time to make another post.