Transmission loss of honeycomb sandwich structures with attached gas layers

Layering gasses of differing acoustic impedances on a panel substantially reduced the amount of sound energy transmitted through the panel with respect to the panel alone or an equivalent-thickness single species gas layer. The additional transmission loss derives from successive impedance mismatches at the interfaces between gas layers and the resulting inefficient energy transfer. Attachment of additional gas layers increased the transmission loss by as much as 17 dB at certain frequencies. The location and ordering of the gasses with respect to the panel were important factors in determining the magnitude of the total transmission loss. Theoretical analysis using a transfer matrix method was used to calculate the frequency dependence of sound transmission for the different configurations tested. The method accurately predicted the relative increases in transmission loss observed with the addition of different gas layer configurations.