Empirically Predicting the Sound Transmission Loss of Double-Wall Sound Barrier Assemblies

A study of exhaust noise in a motor vehicle using statistical energy analysis

“…Also, in this case the SBA is complicated, being structure/foam/ plastic where the structure is a steel sandwich construction with a bituminous core. This is very dicult to model, but recent work has been carried out by Wentzel and Saha [6] who measured the TL of a number of motor vehicle SBA and produced an empirical model for predicting the TL. They model the SBA as a double wall with a foam inserted between the metal structure and a hard plastic membrane.…”

“…where 0 =1.18 kg/m 3 is the density of air, c 0 is the speed of sound in air, d is the foam thickness (cavity depth) and ss and sp are the surface density of the steel structure and plastic membrane respectively. Firstly, below the mass spring mass frequency for the SBA the TL is given by [6] TL 20 log s f À 47:2; f < f dw 2…”

“…where s ss sp : Above f dw the total TL for the assembly is given by [6] TL TL s TL p 20 log fd À 28:7;…”

“…where TL s and TL p are the TL for the panel on each side of the foam which can be calculated using equation (2) with the appropriate value of s , f sw c 0 =12d and the TL for the SBA above that frequency is given by [6] TL TL 1 TL 2 IL À 5;…”

“…In modern motor vehicles the`sound package' [5] can make the prediction of airborne noise transmission dif-®cult. Wentzel and Saha [6] recently published empirical methods for predicting the transmission loss (TL) through sound barrier assemblies (SBA) which are positioned between the engine compartment and the saloon. These methods are applied in the present work to estimate the sound power transmitted from the engine compartment through the SBA into the saloon.…”