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The four-microphone standing wave tube system has proven useful for measuring the absorption and transmission loss of various fibrous and non-fibrous absorbers. The system is fast, repeatable, accurate and compact. This paper discusses the advantages of the four-microphone system for measuring the transmission loss in lateral-flow absorber systems. The original four-microphone round impedance tube system and the migration to a four-microphone square tube system are discussed. The four-microphone square tube system allows effective study of filled and partially filled cavities.

The damping effect that is observed when a fibrous acoustical treatment is applied to a thin metal panel typical of automotive structures has been modeled by using three independent techniques. In the first two methods the fibrous treatment was modeled by using the limp frame formulation proposed by Bolton et al., while the third method makes use of a general poro-elastic model based on the Biot theory. All three methods have been found to provide consistent predictions that are in excellent agreement with one another. An examination of the numerical results shows that the structural damping effect results primarily from the suppression of the nearfield acoustical motion within the fibrous treatment, that motion being closely coupled with the vibration of the base panel. The observed damping effect is similar in magnitude to that provided by constrained layer dampers having the same mass per unit area as the fibrous layer.

In recent years several variants of lightweight multi-layered acoustic treatments have been used successfully in vehicles to replace conventional barrier-decoupler interior dash mats. The principle involved is to utilize increased acoustic absorption to offset the decrease in insertion loss from the reduced mass such that equivalent vehicle level performance can be achieved. Typical dual density fibrous constructions consist of a relatively dense cap layer on top of a lofted layer. The density and flow resistivity of these layers are tuned to optimize a balance of insertion loss and absorption performance. Generally these have been found to be very effective with the exception of dash mats with very high insertion loss requirements. This paper describes an alternative treatment which consists of a micro-perforated film top layer and fibrous decoupler layer.

Flat, constant thickness composites that consisted of a microperforated top layer plus a fibrous decoupler layer were tested for random absorption and transmission loss (TL) performance. The top, microperforated layer consisted of a relatively thick film that contained small, precise micro-perforations. For reference, top layers that consisted of a resistive scrim and an impervious film were also included in this study. Two fibrous materials of constant thickness were used for the decoupler layer between a steel panel and the top microperforated film. The composites' absorption and TL performance were also modeled using the well-known transfer matrix method. This method has been implemented in a commercially available statistical energy analysis (SEA) software package. A comparison of testing and modeling results showed reasonable agreement for absorption results and even better agreement for transmission loss and insertion loss results.

In recent years, interest in microperforated panels (MPPs) has been growing in the automotive industry and elsewhere. Acoustic performance prediction is an important step toward understanding and designing MPPs. This paper outlines a start-to-finish procedure to predict the transfer impedance of a particular MPP based on its hole geometry and to further use this information in a simple plane wave application. A computational fluid dynamics (CFD) approach was used to calculate the impedance of the MPP and the results compared to impedance tube and flow resistance measurements. The transfer impedance results were then used to create a computationally efficient acoustic finite element (FE) model. The results of the acoustic FE model were also compared to impedance tube measurements.