A material thickness question

[lomky]

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

[Ethan Winer]

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

[lomky]

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