Multi-chamber micro-perforated panel absorbers optimised for high amplitude broadband absorption using a two-point impedance method

Abstract

An optimised, multi-chamber, micro-perforated panel absorber (MC-MPPA) with micro-perforated adjoining panels is presented in this paper. An MPPA is a clean and efficient noise absorber with a simple structure making it an increasingly popular choice over porous material-based solutions. For the basic configuration, the bandwidth is typically limited by the perforation size and the frequency range by the backing cavity depth. In order to improve these parameters, innovative variations of the MPPA have been proposed in the literature, but the development of a solution which delivers both a broadband frequency response at low frequencies, with a shallow cavity has yet to appear. This paper proposes such a technological breakthrough by employing a graph-theory-based method to the MPPA problem. By using the two-point impedance method (TpIM) based on graph theory, a model for multi-chamber MPPAs which are coupled with each other in two dimensions through perforated side panels can be developed, an onerous task for standard equivalent circuit analysis due to the generation of non-planar circuits. The resulting model lends itself to optimisation and thus allows the differing geometric parameters of a tessellated network of chambers to be combined to maximise the sound absorption in a frequency range for a particular depth. An overall absorption coefficient of 0.83 can be achieved experimentally in the frequency range of 660 Hz to 2 kHz with an MC-MPPA of only 22 mm thick. In addition, when the air cavity depth is slightly increased to 50 mm, an overall absorption coefficient of greater than 0.82 can be achieved in the 281 Hz to 1 kHz frequency range. Experimentally, an absorption coefficient of approximately 0.82 is achieved at 340 Hz at an air cavity depth of 50 mm, which is a depth-to-wavelength ratio of 20, making the MC-MPPA a deeply subwavelength absorber. Results have been verified theoretically, numerically and experimentally.