On 2 Oct, 18:58, Señor Chris wrote:
I'm installing an independent ceiling just below the existing one for
sound insulation purposes, so the only load it will carry is an acoustic
quilt above and plasterboard below. Assuming maximum span of 4m and
plasterboard thickness of 25mm, what size joists do I use and at what
spacing ? Is there a generic formula I can use if I vary the span and
plasterboard thickness ?

The formula is that for a timber beam. It requires some engineering
knowledge to do from first principles but there are bound to be
tables available on the internet or in a library. Basically if it is
lightly loaded as it is - just quilt and plasterboard and its own self
weight - you will not need a huge depth for a span of 4 m. I have used
225 (9") for a similar span that is a floor load that will be
reasonably highly loaded so 150 mm sounds about right to me at 400 cc
odd. Any less and your deflection would be too great and dont forget
duration of load effects if you go too small
Chris