Studyon Improving Soundabsorptivityof Micro-perforated Panelby Combining Mechanical Impedance Plate With Micro-perforations

Micro-perforated panel(MPP) absorber is mainly composed of micro-perforated panel and the air-cavity behind it. The MPP absorber has the characteristics of high acoustic resistance and small acoustic mass so that the sound absorbing materials usually filled in the air-cavity can be spared, making the structure simple and environment friendly. The MPP absorber can work at high temperature, high pressure and high humidity. The sound absorption mechanism of a MPP absorber is cavity resonance and the acoustic absorptivity can be tuned by adjusting the parameters of the MPP. But the thickness of the air-cavity is always confined in application, making the acoustic absorptivity in low frequency range not good enough, which limits the wide applications of the MPP absorber.This paper is mainly focused on enhancing low frequency absorption. The sound absorption property of mechanical impedance plate(MIP) absorber is summarized. A method that boring a few micro perforations in the MIP is proposed which leads to a new sound absorber: mechanical impedance plate with micro-perforations(MIPMP).The sound absorptivity of the MIPMP absorber is studied both theoretically and experimentally. To enhance low frequency absorption,the MIPMP is mounted in the air-cavity of the MPP which makes up a composite absorber, the theoretical calculation and the experimental measurement of the sound absorption property is performed.MIP absorber is composed of rigid thin plate and viscoelastic material which is attached around the edge of the plate. The MIP absorber is a damped single-freedom mass-spring system in essence. Incident sound waves reflect on the plate surface then resonates the MIP, so that the sound energy is lost as heat because of the damping. The sound absorption mechanism of a MIP absorber is mechanical resonance. The resonance frequency of the MIP absorber can be tuned in low frequency and the sound absorbing coefficient can be very high by using proper viscoelastic material. Placing the MIP into the air-cavity of the MPP can improve the sound absorption of the MPP in low frequency range. The absorption broadband of MIP absorber is not wide.A new sound absorber is proposed that boring a few perforations on the MIP, in which the perforation ratio is less than 1%, which achieves both cavity resonance and mechanical resonance at the same time. A sound absorptivity calculation model of the MIPMP absorber is derived and experimental measurements by standing wave tubetesting system is performed. The results show that the MIPMP absorber creates two absorption peaks which are caused by mechanical impedance and micro-perforations.The absorption peak mainly caused by mechanical impedance rises at first then drops meanwhile moves towards high frequency as the perforation ratio increases, and it rises with increasing air-cavity thickness meanwhile the resonance frequency is almost invariant. The absorption peak mainly caused by micro-perforations goes down, moves to higher frequency and gets wider as the perforation ratio increases, and it moves towards lower frequency and drops slightly as the air-cavity gets wider. There is an optimal perforation ratio at which the sound absorption property is best. The perforation ratio of the MIPMP should be set near the optimum perforation ratio to get best sound absorbing performance.To take advantage of the good sound absorbing performance of MIPMP in low frequency range, the MIPMP is mounted in the air-cavity of the MPP to form a new composite sound absorbing structure: MPP+MIPMP absorber. The calculation of sound absorption coefficient is derived through impedance transfer method and measurements by standing wave tube testing system are performed. The results show that the composite sound absorbing structure creates three absorption peaks from 200 Hz to1600 Hz, two of which are in low frequency range. Compared with MPP+MIP absorber,the first absorption peak of the MPP+MIPMP absorber lowers by 60 Hz, the bandwidth broadens to 110 Hz and the minimum absorption coefficient is not less than 0.42 which is more than double of the MPP+MIP absorber’s. The method of MPP+MIPMP absorber is conducive to improving the sound absorption performance in low frequency range.