Transmission loss of a multilayer panel with internal tuned Helmholtz resonators

Abstract

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.