I would tend to avoid ordinary plasterboard for most jobs. It is too easily damaged, doesn't retain fixings & feels flimsy when rapped with the knuckles. Acoustic plasterboard seems a lot tougher & easier to work with.

Acoustic plasterboard shouldn't be that much more expensive than the ordinary stuff. Are you comparing equal weights? It generally costs about 35-50% more for a 30-35% increase in weight per square meter. Its worth paying if you need to meet a given specification in a limited thickness, but you will be able to meet the same specification without it if space isn't an issue.

You need isolation as well as weight. So you should budget for some resilient bars or additional framing. How about fixing resilient bars to one face of the existing wall & 2 layers of 15mm acoustic plasterboard to that? If space is not a problem, build an independent metal stud wall just inside, but not fixed to the existing wall & fix 2 layers of 15mm ordinary plasterboard to that.

Any higher performance than that is likely to lead to acoustic bypass via floor/ceiling voids.

David

Been dealing with such things recently. I would suggest, in order of priority

1) Staggered double stud, gap filled with lots of dense but cheap heavy-ish flexible insulation quilt such as sheepwool or rockwool acoustic.

2) additional layer of plasterboard
or
2) denser board such as acoustic plasterboard or Mg Silicate board

3) additional layer of board

Biggest benefit will come from 1). Whether rest are worth doing also will depend how otherwise quiet the surroundings are. If you can't do the staggered double stud then it is still worth doing two layers on single stud.

Watch out for (and do not be taken in by) ridiculous pseudo-science (I hate that) on many of the acoustic products web sites. Do not fall for miracle products that claim to fix all.

There is this old reference but the pictures don't seem to work any more

http://www.sae.edu/reference_material/pages/STC%20Chart.htm

Someone on the old Yahoo self-build forum used Fermacell either side filled with blown Warmce in a B&B and reported excellent soundproofing and impression of solidity. No figures I'm afraid.

And building out of solid heavy weight masonry

I used Knauf soundboard when I reboarded the party wall in our back bedroom. Originally it had hardboard nailed to battens on the 9" concrete block wall.

To be honest I haven't noticed any difference, but I could hardly hear the neighbours before, though as I had to reboard anyway to redecorate I thought I might as well use sound board.

It is *much* heavier than ordinary plasterboard so quite hard to handle in large sheets - definitely a two person job!