There are two way sound escapes a room. One is by air, simply projecting itself through walls and floors. The other is by conduction, sound actually travels better through wood or even water than through air. So, if the Sub is causing the walls or floors to vibrate that sound is transmitted extremely well.
Though it might be a bit much for you budget, it still illustrated the principle. The Auralex GRAMMA or SUBDUDE are two devices that can prevent the driving of sound down into the floor -
auralex gramma - Google Search
auralex subdude - Google Search
These are wood (plywood, MD) platforms that sit a top of some high density foam. High density foam meaning it holds it shape very well, as opposed to being soft and squishy like upholstery foam.
I think in some cases they simply use section of the Acoustical Foam that they also make -
Auralex Acoustics - SonoFlat Panels
Even though the color samples show Pyramid Foam, these are flat panel foam.
Set a piece of plywood or MDF on one of these flat foam panels, and you sub on top of that.
You need high density foam to keep the platform from flexing or vibrating, which would result in loss of movement to the speaker itself.
Now, you could make something like this yourself. But a 2 foot by 2 foot or 18 inch by 2 foot panel ready-cut at the building supply store, and put perhaps 3 layers of craft foam under it. Craft Foam can be purchases at any Fabric or Hobby/Craft store.
If you could find some carpet or hardwood floor underlay, several layers of that might work. Though it depends on the thickness of the Underlay.
I was in a building supply store and saw sheets of rubber that were intended for roofing, but would work just a well glued in a couple of layers under a Sub Platform.
But it gets down to what is available to you. If you are desperate, some simple 1" thick upholstery foam placed under a wooden platform would help.
Again, the degree of results is related to the degree of ingenuity of the builder.
This is a form of Isolation. You are isolating the bass from the floor.
But at least you understand the concept.
One additional concept would be Absorption. If you can absorb bass while it is in the room, you can reduce the amount that escapes -
http://www.avforums.com/forums/hi-f...72-primer-acoustics-absorption-diffusion.html
Bass Traps are just thick soft pads that absorb bass. Typically the deeper they are, the deeper the signal they absorb.
At the link above, you will find several videos for making sound absorbing panels. Again, the deeper they are, the deeper the bass they will absorb. Though if you are absorbing bass in roughly the 100hz range you are probably getting the bulk of it.
This can be accomplished in a variety of ways, depending on how much money you have to spend.
You can get Rockwool insulation. You can get ridged fiberglass. Ridged fiberglass is not that ridged, it just hold its shape better than the soft fluffy fiberglass house insulation, though you can also use the fluffy fiberglass house insulation. You can also use thick upholstery foam. Though upholstery foam tends to be expensive.
However, you can go to just about any general merchandising store (Tescos, whatever) and get a convoluted (egg carton cut) foam mattress pad. You simply can't get more foam for your money than a simple foam mattress pad. One large pad cut down to create several equal sized layers, then stuffed in a cloth envelope would get the job done. You can use single layer or double layers of foam mattress pads to absorb echos in a room or to stop midrange reflections.
If you want to build a simply wood frame for the sound absorbing material, as you can see from the videos in the link above, that is relatively easy and cheap to do.
Again ... ingenuity will take you a long way.
Steve/bluewizard