Acoustic properties of micro-perforated panel absorbers backed by Helmholtz resonators for the improvement of low-frequency sound absorption

Micro-perforated panel absorbers backed by Helmholtz resonators are introduced to improve sound absorption in the low-frequency region where conventional micro-perforated panel absorbers cannot provide sufficient absorption. The neck of the backing resonator is designed to be extended into its cavity to reduce the total thickness of the proposed absorber. The acoustic impedance of the proposed sound absorber is investigated by using the transfer matrix method. It is shown that the absorber has two peak frequencies a few percent lower and higher than the uncoupled resonant frequencies of the resonator and the micro-perforated panel absorber. One anti-resonant frequency in the absorption coefficient curve is related to the coupled frequency of the two separate absorbers. The nonlinear acoustic impedance is also addressed in order to apply the proposed absorber to the mitigation of acoustic loads of launcher fairings. The proposed theory is verified by comparing the estimated normal incidence absorption coefficients with the measured ones both in low and high sound pressure environments. The applicability of the proposed absorber is demonstrated by numerical examples of noise reduction in a shallow rectangular cavity and a launcher fairing.