1. Impedance and absorption coefficient spectra acquired with Configurations 1 and 2 (see Fig. 4) are shown in Figures 7 and 8, respectively. A number of conclusions can be derived from these results. Measured absorption coefficient spectra for these two samples clearly demonstrate the ability to achieve broadband sound absorption with just two distinct chambers. Good absorption is also achieved down to around 0.5 kHz. However, these samples have thicknesses of 3.0” and 3.5”, respectively, which is much larger than is typically available (nominally 2”) for conventional liners mounted in the walls of aircraft engine nacelles.
2. Figures 9 and 10 present results for the corresponding samples fabricated without slots. Initial attempts to predict the impedance spectra for these two configurations assumed the sound propagation was confined to two straight paths. The first was from the surface of the liner to the PEEK film (1.2” and 1.5” for Configurations 1 and 2, respectively) and the second was from the surface to the backplate (3.0” and 3.5” for Configurations 1 and 2, respectively). The air gap below the PEEK film was assumed to be dead space. Predictions made using this assumption (not shown) proved unsuccessful in aligning the predicted resonance and antiresonance frequencies with their measured counterparts.
3. Frequency,kHz (a) Normalized Resistance.20 -10
4. Frequency,kHz (a) Normalized Resistance.20 -10