What size air cavity should I use?

The larger your air cavity, the better the RW you will achieve. As a rule of thumb, a 2dB increase in RW can be seen for every 50mm of cavity size. This example shows how varying the size of an air cavity affects the RW of a 200mm wide brick wall:

ST12No air gap. RW: 54dB

ST1350mm air gap, 13mm plasterboard panel. RW: 56dB

ST14100mm air gap, 13mm plasterboard panel. RW: 58dB

ST15150mm air gap, 13mm plasterboard panel. RW: 60dB