微穿孔板的实际极限

Practical absorption limits of MPP absorbers

对微穿孔板吸声器的构造和吸声特性进行了分析。曾认为低穿孔常数和孔径小可导致宽频带内的高吸声特性。为求得极限,从穿孔系数为1,孔径0．1 mm开始,因为负载声阻和吸声系数可接近1,高吸声系数的共振频率可达1000 Hz,高频率可在吸声范围之内。单片微穿孔板可否用于平常噪声控制,在宽频带内吸声较大?为了比较,半吸声带宽不适用,因为可能包括低吸声系数。建议用吸声系数0．5作为实际吸声底线,为比较微穿孔板的统一标准。更以吸声系数的频率曲线以进行详细比较。负载声阻为1(声阻等于空气特性阻抗)时,微穿孔板的吸声范围约为2．5倍频程,比常用的微穿孔板稍好。负载声阻大于1时,吸声的频率范围显著增加,有些适合宽频带高吸收的要求。将其实现可能是微穿孔板吸声器的重大发展。

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.