def ampdb(x) :
       return pow(10.0,x/20.0)

so
ampdb(0) =>  1
ampdb(-6) => 0.5
ampdb(-12) => 0.25

Victor (these email formatting errors are doing my
head in, I can't see what I am doing wrong...)

Victor Lazzarini
Music Technology Laboratory
Music Department
National University of Ireland, Maynooth