Two different methods of analysis are presented for the transmission loss prediction of a multilayer panel with internal Helmholtz resonators. One of the methods, devised at the Lockheed Aeronautical Systems Company, is based on the pressure ratio methodology of Beranek and Work [J. Acoust. Soc. Am. 21, 419–428 (1949)], where the acoustic resonators are included as a side branch. The other method uses a transfer matrix approach where the resonators are embedded in and move with the trim panel. Comparisons are made between numerical results obtained from the two types of analyses. Except for a minor shift in the frequency of maximum transmission loss, the agreement is very close. The shift is a function of the ratio of the nozzle throat area to the resonator surface area. When this ratio goes to zero the results from the two approaches become identical. Experimental results are shown for a double-wall panel with resonators tuned to 240 Hz and are compared against predictions with good agreement. The results show that the use of tuned acoustic resonators in a double-wall structure can provide large increases in transmission loss at selected frequencies. In addition, both theory and experiment show that for singly tuned resonator arrays the large transmission loss increase at the resonance frequency is accompanied by a large decrease in the transmission loss over a limited frequency range above the resonance frequency.

The diffraction of sound around barriers partially defeats efforts to control noise by interruption of line of sight. Recent legislation, which limits allowable highway noise in adjacent communities, has stimulated interest in this problem. Currently, solid walls are used to provide partial acoustical shadows. This approach is expensive, partly because of nonacoustical engineering requirements. A review of Fresnel diffraction theory suggests that either transparency gradients or phase velocity gradients or both may be used to provide deeper shadows. Elemental Fresnel theory is slightly modified to introduce transparency or phase gradients near the diffracting edge and parametric studies are reported which define configurations that deepen the shadows. Scale model tests are described which display the trends predicted by the theory. This agreement leads to the fascinating conclusion that the best way to improve the performance of a solid wall is to cut a substantial portion of it away. Structures are suggested for the practical implementation of the method into noise barriers. A literary analogy suggests that such structures be called Thnadners.

Lined ducts containing flow and intense sound have been extensively studied in recent years. Parametric studies have revealed so many variables that design procedures are obscured. Analysis by solution of the convected wave equation has been complicated by uncertainties about the actual modal distribution and by uncertainties about the actual boundary impedance. As a result, when discrepancies occur between experimental and analytical results it is difficult to interpret whether the wave-equation solution, the modal assumption, the impedance value, or the test procedure is deficient. The simultaneous refinement of all four factors has resulted in a very satisfactory correlation between theory and duct test results and provides a basis for lucid duct design procedures and the evolution of new liner materials. A duct test facility has been built which appears to provide equipartition of energy. The impedance of nonlinear materials has been redefined and measurement methods modified. Contours of equal duct attenuation in the impedance plane have been generated and prove to be simple in shape. From these, a design methodology is deduced which may be applied to any liner material and which eliminates many redundant parameters. The new methodology has led directly to the creation of new liner materials.

Lined ducts containing flow and intense sound have been extensively studied in recent years. Parametric studies have revealed so many variables that design procedures are obscured. Analysis by solution of the convected wave equation has been complicated by uncertainties about the actual modal distribution and by uncertainties about the actual boundary impedance. As a result, when discrepancies occur between experimental and analytical results, it is difficult to interpret whether the wave equation solution, the modal assumption, the impedance value, or the test procedure is deficient. The simultaneous refinement of all four factors has resulted in a very satisfactory correlation between theory and duct test results and provides a basis for lucid duct design procedures and the evolution of new linear materials. A duet test facility has been built which appears to provide equipartition of energy. The impedance of nonlinear materials has been redefined and measurement methods modified. Contours of equal duct attenuation in the impedance plane have been generated and prove to be simple in shape. From these a design methodology is deduced which may be applied to any linear material and which eliminates many redundant parameters. The new methodology had led directly to the creation of new linear materials.

No abstract