Prediction of the sound absorption of micro-perforated panels whose holes are extended in length by tubes

Maa's theory, which models the specific acoustic impedance of a narrow air-filled hole in front of an air cavity using a lumped parameter model, works well for short holes at low frequencies. However, as the length of the hole or the frequency increases, the acoustic velocity of the air in the hole varies along the length of the hole as well as radially. It is necessary to use the transfer matrix method (TMM) rather than Maa's lumped parameter model to calculate the impedance of the hole and the cavity behind it. The peak sound absorption of a micro-perforated panel (MPP) in front of an air cavity can be moved to lower frequencies by extending the length the holes in the MPP with rigid or flexible tube bundles in the air cavity behind the MPP without increasing the total depth of the sound absorbing system. Previously Maa's lumped parameter model had been used to predict the sound absorption coefficients of these tube bundle sound absorbers. These predictions are only correct up to a certain frequency which depends on the length of the tube extended holes. This paper predicts the sound absorption to higher frequencies using the TMM.