Broaden noise reduction range in low frequency by a HR + MPP structure based on impedance matching method

Abstract

Broadening the noise reduction range in low frequency has always been an important problem in the research of acoustic metamaterials. Traditional muffler like Helmholtz resonance cavities (HR) only have high absorption coefficient in its resonant frequency. By designing multi-layer or gradient air cavities, the range of frequency broadened is still limited. As is well known, microperforated plate (MPP) has a wide sound absorption frequency in the middle and low frequency. Taking advantage of the physical property of HR and MPP structure, we designed a composite structure by the method of surface impedance matching. Based on the noise reduction coefficient higher than 0.9, this structure realized a 967 Hz width noise reduction in the frequency below 1000 Hz. More than twice as the multi cavity HR structure we researched before. It is demonstrated that, by the impedance matching method, there is almost no reflection in the contact surface between HR and MPP. Therefore, the low resonant frequency and relatively medium frequency can be well absorbed in HR and MPP structures respectively.