A material thickness question

I remember seeing somewhere a formula for thickness of (I believe) fibreglass insulation for absorption. It was something like material thickness= Speed of sound (ft/sec)/4 x target frequency(Hz) Does this ring any bells with anyone? Is the formula true? Thanks Josh

Josh, > I remember seeing somewhere a formula for thickness < Theoretically (only!) the ideal thickness is at least one quarter the wavelength of the frequency of interest. However, even one fourth of that is enough to make a big improvement. If you space the material off the wall so the air space equals the thickness, that's as good as using material twice as thick. HERE is a link to a program I wrote (PC only) that calculates all the quarter wavelength distances for a given frequency, and vice versa. --Ethan

Thanks Ethan. I knew it was a theoretical calculation. I just remembered the formula and was trying to find it on the internet to no avail. I was starting to wonder if I made it up or something. Josh