Practical absorption limits of MPP absorbers

The construction and properties of the absorption curve of microperforated panel (MPP) absorber are discussed. The absorption limit of the absorber had been shown that low values of the perforate constant k = d √f/10 and the orifice diameter d (in mm) are essential for MPP to have high absorption in wide frequency band. To find the exact limits, take 1 for k as a start, so that the specific resistance and absorption coefficient are bath close to unity. And the orifice diameter d is chosen as 0.1 mm, so that the peak absorption coefficient (resonance absorption) is at 1000 Hz, and high sound frequency may be in the absorption region. Is it possible for a single layer of such an MPP to cover the whole absorption band required in practice? The half-absorption limit is not a good criterion, because low absorption comes in also in some cases.. The 0.5 absorption coefficient is suggested for the lower limit of absorption region, to be the standard for comparison. Absorption curves were drawn for different load resistances, of absorption coefficients versus frequency. Ordinary MPP absorber absorbs in slightly larger than two octaves, the curve for r = 1 (load resistance equal to the specific impedance in air) is slightly better than this, 2,5 octaves. The new absorbers with r > 1 have frequency intervals much better than these, some satisfy high absorption in broad band of frequency. To realize these will be important progress of the MPP absorber.