Sound insulation is a whole science on its own, but in essence is a way of trying to reduce sound transmission of two main types of sound: Impact and Ambient
Impacts are the lower frequency sound waves that are caused by things like footsteps whereas Ambient tends to be higher frequencies associated with chatting, TV, radio etc.
The basic treatment for sound is using heavy weight mass. Basically this mass takes more energy to move and so sound is absorbed into it and hence deadened. However getting mass between floors is quite difficult. People have used ingenious and often daft ways of dealing with it. Pouring sand inbetween the joists for instance is an excellent way of reducing sound, but not so clever when sorting out any ceiling problems!
So given that mass can be an issue, how else to tackle sound? Well we have to be clever and look at using a range of products that absorb the different frequencies of sound. Mostly in homes we are looking at ambient sounds and these can be treated by using a mix of higher density materials coupled with lighter fluffy materials that will muffle higher and mid range frequencies.
The key ways of reducing sound between floors are:
1. Reduce incidence of impact sound – by having carpets rather than floor boards reduces sound creation, the impact noise made by two hard surfaces (floor and shoe) is greater than that made by a soft and hard surface.
2. Introduce a rubber strip / cover between the floor and the joists. This can be applied where you are replacing the floor boards (and maybe want to keep them exposed). There are two main options here. A. to just apply between the joist and floor (strips of rubber) or B. to use a whole acoustic sheet across the whole floor (either under or over the floor boards)
3. Install insulation between the joists. This can be any insulation, but the more dense the better, so look for the kg per cubic metre figure. Some natural wood fibre insulations can be as high as 80kg/m3, but most are between 25 and 40 kg/m3. Conventional insulations are also available and most have an ‘acoustic’ version. Remember here that you need to leave an air gap and not fill up the whole space as by leaving an airgap it provides a void that doesn’t transmit resonances between the different materials.
4. On the ceiling you can increase mass by using woodwool boards (these come up to 50mm thick!), but most people will install a double layer of plasterboard. A double layer of plaster board does provide mass, but it can also amplify some frequencies as it is an even board with its own resonance, whereas the woodwool boards are not as even and hence their resonance is more complex and hence it absorbs sound better)
5. Ensure that there are no gaps in the treatment. Sounds have a great knack of finding holes and so you need to ensure that the insulation is even, the boards are taped, the rubber strips / sheets are taped / constant etc. So care needs to be taken around the edges of the room.
By having a range of materials in your floor you will help to keep the sound down as each material has its own role to play. They all absorb different frequencies of sound and so by having some ‘fluffy’ stuff and some heavy stuff and a range of interfaces between them you will radically reduce the sound transmissions between floors.