Airborne sound insulation of thick walls

The airborne sound insulation with respect to a diffuse sound field is discussed for the case of thick walls of typical building materials. It is shown, with the help of an analytical expression valid for arbitrarily thick plates, that the classic thin-plate expression for the transmission loss can be used approximately two octaves above the limit usually imposed on thin-plate solutions. Thus, the new frequency limit can be calculated from the condition that the wavelength of the free bending wave equals three times the plate thickness or, alternatively, that the corresponding Helmholtz number equals 2. From this new limit and a couple of octaves higher, the transmission-loss curve forms a plateau with pronounced dips due to the thickness resonances of the P wave. The thickness resonances of the S wave do not, on the other hand, influence the transmission loss.