Micro-perforated (MPP) panels have several advantages over tradition fiber and foam absorbers. They are non-combustible, acoustically tunable, and lightweight. In most cases, they are spaced from a wall, and that spacing determines the frequency range where the absorber performs well. The absorption is maximized when the particle velocity in the perforations is high which is approximately at 1/4, 3/4, 5/4 ... acoustic wavelengths from the wall. Larger cavity depths improve the low frequency absorption. However, the absorption is minimal at multiples of a half wavelength and at low frequencies due to the limited cavity depth behind the MPP. By designing customized backings, the cavity depth can be strategically increased and varied. This will improve the absorption at low frequencies and can provide absorption over a wide frequency range. In this work, backing substrates based on work by Wirt have been developed to enhance the absorption performance. Different MPP and backing substrate combinations are analyzed using Maa's theory and plane wave theory and the broadband absorptive behavior is validated via measurement in a square impedance tube.
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