Stud wall sound insulation question

[jfb]

I have a 70mm thick stud wall to insulate for sound.

I also have a lot of 75mm rockwool sound insulation.

 

Is it better from a sound point of view to just fit the whole 75mm in or I could easily cut in half for 37.5mm thick with an airgap?

 

I seem to remember it is best to have an air gap and if I slice in half it will go twice as far. Presumably 50mm with a 20mm gap would be best but trying to work best with what I have.

[SuperJohnG]

You have to have air gap otherwise it transfers the sound right through. 

 

 

No idea how it would fair cutting it in half though, can imagine that will be a pain. 

[Conor]

Split it in half. Just tried it with a bit left over and comes away pretty easily. Just grab it in the corner and start pulling. Might be easier to cut to length first.

[saveasteading]

I don't agree. Sound insulation isn't dense enough to transfer sound by impact/reverberation through to the next room. In any case the stud is probably doing that. For higher frequencies and especially airborne sound, keep the full thickness.

Especially make sure the full area us filled to the edges as sound leaks through gaps.

[nod]

You will get virtually the same results by splitting 

Most of the sound insulation we put in on party walls is 25-50

Staple to the back the first side 

[mike2016]

I put in full rockwool inside the stud between 2 bedrooms, acoustic putty behind the wall power socket and after fitting 12.5mm Fermacell on the side I tore open, acoustic sealant at the bottom. You could double up the plasterboard or use a resilient channel to further deflect/attenuate sound instead of Fermacell. Rockwool is cheap, do it once! 

[jfb]

2 hours ago, mike2016 said:

acoustic putty behind the wall power socket

interesting - hadn't come across that.

presumably you mean something like this:

https://www.insulationexpress.co.uk/speedline-acoustic-putty-pads

 

though I can't seem to find it in smaller quantities.

[mike2016]

Amazon did it back in the day but it's listed as unavailable now. Was £3.65 for 4 x double sockets worth!! Try this:

https://www.noisestopsystems.co.uk/shop/wall-soundproofing/acoustic-putty-pads/

Or google "fire putty 2G"?

[saveasteading]

I had never heard of this. Great idea for closing the weak link in a wall. Any permanently flexible putty would do for a one-off. Plumbers mait?

[jfb]

I’ve ended up using some left over air tightness tape around the sockets. Has to be better than nothing. 
presumably it is the airborne transmission of sound this is dealing with mostly.

[mike2016]

All the noise from that box bedroom I did this with comes around the two doors, not through the wall. Was very effective from a typical stud partition with just a sheet of plasterboard on each side!