On the use of micro-perforates for machinery and vehicle noise control

Abstract

A micro-perforated plate (MPP) is a perforated plate with holes typically in the sub-millimeter range and perforation ratio around 1%. The values are typical for applications in air at standard temperature and pressure (STP). The underlying acoustic principle is simple: To create a surface with a built in damping that effectively dissipates sound waves. To achieve this, the specific acoustic impedance of a MPP is normally tuned to be of the order of the characteristic wave impedance in the medium (400 Pa*s/m in air at STP). The traditional application for MPP's has been building acoustics, normally in the form of a so called panel absorber to create an absorption peak at a selected frequency. However, MPP's made of metal are also well suited for machinery and vehicle noise control. For instance MPP's have the potential to be used instead of porous materials in dissipative mufflers, which not only can save weight but also offer a non-fibrous alternative. Furthermore, since MPP's have a large steady flow re...