Practical single MPP absorber

Abstract

The construction and properties of microperforated panel (MPP) absorbers are discussed. MPP is a plate perforated with numerous sub-millimeter orifices forming a resonant sound absorber with a cavity behind. It has been shown that low values of the perforate constant k and the orifice diameter d are essential for the MPP to have high absorption over a wide frequency band. To find the exact absorption limits, one can assume for that k = 1 as a start, because both specific resistance and wide frequency bands of absorption require k to be one or less. The orifice diameter d is chosen as 0.1 mm, so that the peak absorption coefficient (resonance absorption) is at 1000 Hz, and the higher sound frequencies may be included in the absorption region. Is it possible for a single MPP to cover the whole absorption region required in practice? The half-absorption limit and 0.5 absorption coefficient limit were used as criteria for comparison with different load resistances. MPP absorbers designed for use typically absorb sound over approximately two octaves, and the new absorbers with low values of k and d are found to be better for r = 1 (relative acoustical resistance equal to the characteristic impedance ρc in air), over about 2.5 octaves. This increases to around 3 octaves for r = 2 or 3. In addition, the MPP in a reverberant sound field will absorb over a wider frequency ranges due to increased high frequency absorption. If the increase in range is enough, the absorption region will join forces with the secondary absorption regions, due to the multi-resonance property of the resonance structure, to form a continuous absorption region. The design of a single MPP absorber is given and its realisation is discussed.