i didn't know about the mattress taking up all the floor space .. but now that you said something about it, i feel a whole lot more confident about what to say.
i realize the plywood might seem thin.. but sometimes the metal has different amounts of pores (think a sponge) and if i were you.. i would seriously throw in whatever radio (or anything that makes loud noise) and stand outside to hear just how loud it is for yourself.
because knowing what is safe and what isnt from outside is really good rest when you are in the middle of a movie.
i'm here laughing a bit about the queen size mattress, because where i would want some big speakers.. you use that area for storage it seems (or at least a place to toss your shoes and whatever odds & ends).
since you were specific about how close the speakers are going to be.. i don't think it is irrational one single bit to shop and browse those little 2 inch speakers.
they've been adorable for quite some time (i'd say since about the year 2000 or 2002) and from what i've seen stumble onto my lap, the sound from those tiny speakers just keeps getting better and better.
(and i've been tracking them since the 1990's when bose first introduced that size to the world)
now with the iphone and mp3 players, there's been a large increase in quality audio coming from those speakers .. and if you can check the frequency response of the speaker before buying it.. you could seriously build something that already has a flat frequency response.
i setup my microphone inside my aunt & uncles house .. they've got a home theater in a box setup with the smaller 3 inch speakers.
i dont know exactly how big the room is.. i'm thinking if you put 5 recliner chairs side by side, the room is that wide, and about half as long.
the speakers didn't really need much tweaking, i had to adjust the treble one or two taps on the receiver .. and then rotate the gain knob on the subwoofer to get an averaged bass response on down the frequency chart.
so what i am saying to you is, if you go with those small speakers - looking at the frequency response, you could save yourself the time and effort of using a microphone to dial in the frequency response and still get a flat response.
since you are going to use the bass shakers, you get to be a basshead if you want to be because you can crank up the vibration much louder than the rest of the system.
but be aware.. if the vibrations from those shakers make it to the outside paneling, it might be possible for your neighbor to hear it.
i'm not saying it will be loud enough to be annoying, but as they say.. the cricket is making noise or it isnt.
and really.. i havent heard bass shakers in a long time, but i know i've felt some that were quite strong - and i know some that are beyond quite strong.
have you put any thought into how you are going to mount the bass shakers?
i can imagine something about the bass shaker providing a better response (the feeling is more detailed) when the shaker is mounted solid, compared to allowing the shaker to float in a cushion.
one person might assume the bass shaker is silent.. but in reality, it can be louder than a portable boombox .. comparable to the newer boomboxes with a subwoofer - under some teenager's volume control might i add.
if it were me, i would need to know what is under the cushion of that bench.. no matter if the pad is there or not.
perhaps regardless, you could try some of the sound dampening rolls or sheets ... and i say this because it is like a gelatin (well aim for the gelatin stuff, not the hard stuff).
the gelatin could help lessen the vibration if you lay it down on the wood, then put another piece of wood on top of it .. and then just mount the bass shaker to the top wood.
the only problem, and i think it works out rather nice, is not enough gelatin to stop the vibration.. but if you dont screw down the wood with the bass shakers, you could simply lift the wood up and lay down another layer.
the one thing of fear about using the gelatin is it can actually increase the sound of the vibrations because it is functioning like a bigger speaker cone, except that the vibrations are simply moving slower.
...and in that case, it would be smarter to try and stiffen the wood to prevent it from vibrating at all.. and the sound dampening rolls (or sheets) might not be as effective as something else that dries good & hard (i'm thinking maybe truck bed liner).
whatever you use, hopefully you've got the time to air out the camper before getting in there.
and if i had to make only one choice, i would probably ... well it isnt really easy because either one could be the better choice, it all depends on how much wiggle & vibration the floor will allow.
because if you go hard, it might turn a 6 inch bass shaker into an 18 inch cone.
if you go soft, it could turn the 6 inch bass shaker into an 18 inch cone (because of the floor vibrating) .. but sometimes the vibrating can spread more, and even work it's way up the wall turning the vibration into a 32 inch cone.
see.. you remember the episode of mythbusters where they tried to destroy a bridge using a transducer piston & harmonic frequencies?
they were looking for the specific phase of the bridge, to lock onto that and use it to shake the entire bridge violently until it fell apart.
if you went with soft, and it just happened to be near that special frequency.. you might be making half the trailer vibrate, and when that happens.. usually the middle of the floor will turn to a second octave ... the vibration goes up and down, but it spreads wide.. and because of the wide, the up and down increases.. and because the up and down increases, there is enough time for the shaker to vibrate that area a second time (and that is what really makes the loud noise).
most of the time the metal will be worked to do one of two things:
1. dont let the vibration spread
2. if the vibration does spread, aim for the 2nd harmonic to literally 'ring' opposite .. because we know opposite soundwaves cancels eachother out.
so yeah.. i retracted one warning and actually suggested you look into it with high expectations, but then i gave you a new warning about how it is possible for the bass shakers to make an embarassing amount of noise that comes as a suprise.
dont make the mistake of using a speaker cone for bass, not without those impulse response files to help bring up the decibel level .. and it doesnt matter about being familiar with the make and model of the trailer, because sometimes the silent ones grow louder with age.
and that means, potentially, even the impulse response files wont help (not with their current level of potency).
if the carpet in there can be squeezed, maybe you can just lay a board on top of it and use the carpet as the gelatin.
otherwise.. for some real fun, i would lay one spring mattress down (or air) and then lay a board on top of it (the board needs to be about the full size of the bed, not some small square) and then put another mattress on top of it.
that will get you lots of solid vibration (with the potential to be rather accurate & detailed too) and because of the mattress on the bottom, you might be able to crank up those shakers without a single noise being heard from outside.
coil, foam, or air mattress.. dont make the mistake of using a piece of wood that is too small.
since the mattress is already in there taking up the space, it should seem logical to lay another one down on top of it to get the best chance of silence.
(obviously two coil mattresses would be more solid than the air mattress, and actually the foam would probably be the most solid with the best vibration isolation, yet transfering every last detail from the shaker)
it is the details from the shaker that really helps transform the thing into a silent speaker.. because it goes from stupid toy rumbles to music.
i hope that helps you seize the day .