I’m building a stud wall downstairs to make an open plan space into a dining kitchen and a front room.
Saturday nights I like to cook with Craig Charles on the radio and Mrs BigJohn sometimes likes to watch people dancing on the telly in the other room.

I’ve got soundblock board to cut down the noise crossover and I was just about to finish it off when I thought about sticking something inside the cavity to dampen the noise further. Trouble is, all the conventional stuff is just for thermal insulation (not needed) and costs a packet. And logic tells me it’s all about the weight of the insulation.

Should I just
a) leave the voids
b) stuff it with old plasterboard offcuts, newspaper, cardboard etc.
c) do it properly.

It’s not as if either of us has the music on really loud, and it hasn’t caused any conflict yet, it’s just that if something cheap will make a noticeable difference I might as well do it. Except I don’t really do c) above.

Saturday nights I like to cook with Craig Charles on the radio

You and Craig want to buy yourself a cooker.
It takes ages to fry an egg on a radio. The heat output is abysmal.

Oh.. c) Do it properly.

I’d agree with

c) Do it properly.

but what’s stopping you sticking all the board up for a test?

edit: don’t do b) as I think anything solid can reduce your sound insulation badly.

Cheap answer? Double 12.5mm plasterboard on the noisy side, joints offset, stuff with insulation, and I just used old carpet underlay neatly between studs as well as rockwool. Be fussy about gaps round each side – expandy foam reverberates sound, so use proper filler/plaster.

Use the to hold the plasterboard.

http://www.noisestopsystems.co.uk/soundproofing-accessories/soundbreaker-bars

it’s an additional barrier to sound transmission.

+1 for double boarding. DO it on both sides, and you can still buy door linings deep enough to reach outer faces.

Stuff 100mm rockwool into the 75mm voids of your studwork.

Works fine, and not too expensive.

Two layers of board are better than one and the denser the insulation the better. Also, the deeper the void the better. However, there is little point going mad and spending a fortune because most of the noise will travel under/through/around the door any way. Sounds to me like a good excuse to buy some bluetooth noise cancelling headphones!

Should I just
a) leave the voids

No, a wall with a cavity containing some mineral insulation in will perform better than one without mineral wool in the cavity

b) stuff it with old plasterboard offcuts, newspaper, cardboard etc.

None of this will have any positive effect. Also it might be detrimental if it bridges the cavity, allowing sound to transfer

c) do it properly.

Mineral wool in the cavity will definitely help, but don’t pack it in as this may bridge it.

And logic tells me it’s all about the weight of the insulation.

This isn’t true. The layers of plasterboard are doing most of the work. The mineral wool serves to damp any resonances within the cavity. These can reduce the overall sound insulation of the wall ‘system’ as a whole.

Also, to make your wall effective you need to think about where it meets the existing structure. You could end up with sound ‘flanking’ over the top and around the sides.

You should cut back into the plaster and extend/beef up the stud work to block up any cavities that allow sound to bypass the new wall. See the bottom row of the image below for examples

I used a rockwool sound slab – “Rockwool Sound Insulation 1200 x 600 x 50mm (8.64m2 per pack) pk 12” according to my invoice.
List was £45/pack + V.A.T. but I understand cost is under half, so see what your local builders merchant has in stock and haggle?

(Also available in 70mm & 100mm thickness, but not recommended getting larger and trying to compress – it’s already dense.)

Thanks for all that. Looks like I’m going to have to do it properly then.

Two layers of board are better than one and the denser the insulation the better. Also, the deeper the void the better

I agree with your first and last points but mineral wool with a density of over 10kg/m3 is a dense as you need to go, especially if you double board.

Build a decoupled wall? The below image gives the general idea, with double boarding etc pottential for use in addition.

I’m now itching like it’s summer in Glentress and the midges have been out.

Just stuffed a load of acoustic glass mat into the voids, which seems to have done the trick. It’s not silent but it’s good enough.