Moira:
Actually, the way sound moves through walls is basically the same as it moves through floors and ceilings, and understanding how that happens also explains why putting any sort of noise absorbant material BETWEEN the floor joists is not likely to do much good at all.
In a nutshell, the way a sound wave travels through a wall is astonishingly simple:
The sound wave hits the wall and makes it move. The movement of the wall reproduces another sound wave on the other side of it. Ditto for floors and cielings.
And, that extremely simple mechanism by which sound propogates through walls, floors and ceilings of a building is what gives rise to something called "The Mass Law" of noise transmission, which says:
A) That for every doubling of the mass of the wall (per square foot), the amplitude of that reproduced sound wave on the other side of the wall is reduced by 6 decibels, or 1/4 of it's previous value (cuz decibels are a logarithmic measurement).
B) That for every doubling of the frequency of the sound wave hitting the wall, the amplitude of that reproduced sound wave on the other side of the wall is reduced by 6 decibels, or 1/4 of it's previous value.
And, it's simple to understand why in terms of mass.
The heavier a wall is, the more inertia it has and the less it moves in response to an applied force. So, when a sound wave hits a heavier wall, that wall moves less and the amplitude of the sound wave the wall reproduces is less, so the noise on the other side of the wall is quieter.
Similarily, the heavier a wall is, the more inertia it has and the less quickly it can change it's direction of motion. So as the frequency of the sound wave hitting the wall increases, the more the inertia of the wall prevents it from moving in response to that sound wave.
And, now you know the reason why when someone in an apartment block is having a party at 3:00 in the morning, all the other tenants can hear a BOOM-BOOM-BOOM noise coming from that suite, but can't tell what song is playing. The reason is that the inertia of the walls, floors and ceilings effectively prevent the mid-range and high frequency sound waves from propogating through the building, and all you're left with are the lowest frequencies getting through.
And, that is ALSO the reason why newer apartment buildings built with 3/8 inch plasterboard walls on metal studs have more noise complaints between tenants than old buildings with 3/4 inch thick plaster on 2X4 fir studs. It's the inertia of those plaster walls and ceilings that kept homes and apartments quiet in the past.
So, while there are underlayments you can buy to cushion foot falls on the floor, the real solution to stopping noise propogation from one floor to another is to make the floor/ceiling construction between them heavier. So, if you live in an apartment building with concrete floors, you will have little noise coming from the floor above or the floor below.
You can also install soft, sound absorbant materials in the lively room. Soft materials like carpets, draperies, upholstered furniture absorb sound waves so that the sound wave that reflects off a soft surface will be less intense than the one that hit it.
Also, because of the way sound propogates through walls, floors and ceilings, if you can somehow prevent the movement of one side of the wall in response to the impact of the sound wave from causing the opposite side of the wall to move, then you've got the problem solved too.
And, in fact, vibration isolation is very effective in reducing noise propogation through walls. Basically, the idea here is that you build two separate parallel walls so that the movement of one side of the wall doesn't necessarily result in the movement of the other side. Consequently, a sound wave hitting one of those two parallel walls won't result in the other wall reproducing that same sound wave. However, while this works well with walls, it's not a practical solution for floors and ceilings which require solid support against the force of gravity. One way of doing that, however, is through the use of two separate joist systems; one set of joists holding up the floor above, and a completely separate set of joists holding up the ceiling below. And, in fact, my building is built that way (with two completely separate sets of joists between each floor), but I still get complaints when one tenant plays his stereo too loud. But, the complaining tenant is not complaining about the sound level. He's complaining that the low frequency BOOM-BOOM-BOOM vibrations he hears are annoying.
Hang on, lemme get you some web sites that'll explain more about noise transmission through buildings...