In muffler design, concentric-tube resonators are often used to provide high-frequency attentuation. These resonators are constructed by shaping a rigid shell around a length of perforated tube, forming an unpartitioned cavity. Often the entire length of tube is perforated. Because of the length of the assembly, it is not possible to use the simple Helmholtz resonator theory to predict the attentuation. Beginning with a one-dimensional control volume, a mathematical model was derived which accounted for mean flow in the tube and for the wave motion in the cavity and the coupling between cavity and tube via the impedance boundary of the perforate. For impedance in the linear regime, a closed-form solution for the resonator transmission loss was obtained. With this restriction and for the case of zero mean flow, excellent agreement was obtained with experimental results. Limited parametric studies suggest that the performance of this type resonator is quite sensitive to the porosity of the perforate, and that unusually large bandwidths of attenuation may be obtained for select geometries at zero or near-zero mean flow conditions. As mean flow is increased, however, the model suggest that such large bandwidths may not be attainable in practice.

A simple method is presented for modeling perforated muffler components such as concentric resonators with perforated flow tube, and expansion chambers and reverse flow chambers with perforated inlet and outlet tubes. The theory includes mean flow, but is confined to those configurations having one acoustically long dimension. It is based on a segmentation procedure in which each segment is described by a transmission matrix. The four-pole parameters of a component are then found from the product of the transmission matrices. The four-pole parameters for configurations having through flow, cross flow, and reverse flow are presented. Because the product matrices are dimensionally small and because no inversion is needed, computational time is much lower than other methods such as finite element or finite difference. This allows rapid and economical modeling to be performed where iterative solutions are required because of dominating finite amplitude effects, for example.

The study was designed to investigate 2-month-old infant preferential attention to a feature found to be characteristic of mothers' speech to their infants. A modified infant-control auditory preference paradigm was employed to assess infants' differential attention to synthetically generated and naturally produced rising and falling intonation contours. Analysis of these data revealed that the infants attended more to the rising naturally produced intonation contour. A reverse pattern of greater attention to the falling contour was found with the synthetically generated stimuli. In addition, inspection of the results permitted the conclusion that the infant-control preference paradigm was a viable method for assessing the 2-month-old infant's preferential attention to auditory stimuli. The results are discussed in terms of their relevance to the study of the infant's developing language reception abilities.[*] Thanks are extended to the members of the Kansas University Infant Laboratory -

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This paper is the second of two papers describing a method for modeling muffler components having perforated flow tubes, as for example those found frequently in automotive applications. In the first paper the theoretical model was developed, whereas here the theory is applied to demostrate specifically the utility and potential of the model. The applications are idealized in the sense that effects which occur simultaneously in perforated-tube mufflers are treated separately. These effects are (1) the nonlinear impedance of the perforation due to finite amplitude sound pressure, and (2) the change in impedance of the perforation with mean flow. The first effect is demonstrated with a straight through resonator, and the second with a cross-flow chamber. Predicted results for transmission loss in both cases compared quite well with measurements. Crucial to the success of the modeling was a proper description of the perforate impedance. The confidence instilled by the results prompted an inquiry into the nature of the two muffler components. It was determined by modeling that, contrary to popular opinion, these devices can be very dissipative, even though they contain no recognizable dissipative materials. The controlling mechanism is the high resistivity of the perforation induced by the acoustic/flow environment.