My wife's started working in the room above my office, and we can hear every word the other says. The makeup between us is plasterboard - solid timber joists - 22mm chipboard.

 

I've sealed any cracks so there's no direct route of transmission for the sound. Is the next logical step to add rockwool or similar between the joists? If so does the density of it matter or like in a stud wall that there's just "something" there?

Edited January 20, 20242 yr by Sparrowhawk
Corrected chipboard thickness

It’s as much about different materials making up the structure so if there’s no acoustic insulation then adding some will help but it will only help not eliminate it. My build up has 100mm acoustic insulation, 15mm plasterboard everywhere with some double boarding, plastered rather than Ames taping, all joints/spaces sealed. It’s hard to tell yet what the acoustic transmission is like as we don’t have any doors fitted yet. 
 

There’s a lot of marketing about dense acoustic wool but there’s evidence that as long as it’s at least 10kg/m³ there’s not much improvement by going ever more dense. Thick plasterboard, double boarding, resilient bars, sealing gaps,  etc is what you need to do. Here’s a good thread on the subject. 
 

 

Edited January 20, 20242 yr by Kelvin