SamRadford said:
Not much to add to what's already been written but some years ago I discussed this with Bestobell Acoustics (don't know if they are still in business) and with Dunlop. The answer to bass frequencies is as follows:
It requires a very heavy, flexible "curtain'. They suggested I achieve this by glueing a very very low density foam to the wall (mostly air) and glueing a sheet of lead-powder filled PVC sheet onto the foam. However, the lead-filled PVC was incredibly expensive and I ended up using the thickest, heaviest solid rubber I could lay my hands on, which turned out to be a few dozen rubber foot mats. Tricky to glue in place I can tell you! To further cut down the high and mid-range frequencies (and for cosmetic purposes) you can build a plasterboard wall backed with 13mm coarse fibre underlay, leaving a small air gap between that and the rubber "curtain". This loses you around 40mm of room width per wall, which ain't too bad.
Hi SamRadford,
i hope your isolation works out for you, and please don't take my comments the wrong way, and i don't mean any disrespect to the expert you consulted...
But that is simply false. The logic is ... intuitive. it's easy to imagine a big, limp curtain absorbing all the low frequency sound and making much quiet on the other side. Similarly, sometimes you see someone toutint very stiff walls as the best way to stop low frequencies. And again, the logic is very intuitive - it's easy to imagine those cross-braced walls being so stiff that they resist being moved by low frequency noise.
but the former (limp mass) is false in every way, and the latter (stiff) is false on structures as large as walls. If you had a simple panel - like a 1m x 1m plasterboard panel - this panel would exhibit a strong resonance, and then below that resonance it's stiffness would in fact work for you and resist lower frqeuency noise. But on something as large as a wall, the panel would have to be about a meter thick before this stiffness could work in your advantage. a meter of solid plasterboard (a meter of layers of 16mm plasterboard wouldn't be nearly as stiff).
finally, stiffness helps us out only below that strong fundamental resonance in the panel, and so it's not that graet of an idea to ever try to use stiffness as a sound isolation tool because if you suceed in getting the wall stiff enough to resist sound at a relevant frequency, your wall will be very bad at higher frqeuencies around that resonance...
as for limp mass.... a simple mental experiment can help clarify that situation. Imagine that we got all the scientists at NASA together to develop the most amazingly limp material ever seen. so limp that it moved perfectly with the sound and absorbed it all...
well that would stop all of the sound, right? no, it would TRANSMIT all of the sound. sound = vibrating air, and sound makes its way through walls by vibrating the walls, which then vibrate air on the other side, making new sound. If you have this magical super-limp material that vibrates perfectly ... it offers no resistance to sound transmission at all. Indeed, the very nature of sound transmission is RESISTING vibration
but the point is moot. because a stiff mass (like plasterboard) and a limp mass (like the mass loaded vinyl) offer exactly the same performance for a given size over most of the frequency range because the resistance to motion is provided by mass, not by stiffness or by limpness. mass
and in real walls, resistance to vibration is a function of bascially two things
1) mass. the heavier the walls, the more they can resist vibration from airborne sound
2) resonance. the more the walls resonate, the easier it is for airborne sound to vibrate them.
Other things help sound isolation as well, like mechanical decoupling (resilient channel or double stud walls, etc.). Those function by helping prevent vibration from moving from one side of the wall to the other - by breaking the mechanical path.
Insulation in walls helps because (if the wall is decoupled and no mechanical path is available), then the sound has to try to make it to the other side via the air, and the insulation can absorb something.
But at low frequencies, most decoupling schemes are working against you, not working for you, and it is reasonable to view the low frequency behavior of most common walls as a function of mass and resonance.
good luck,
Brian