Micro-perforated structures as sound absorbers - a review and outlook

Sound absorbers employing micro-perforated panels or foils have long been used as attractive measures in room acoustics. So far, however, all applications have tried to activate the holes in such structures as fluid mass elements in a Helmholtz-resonator arrangement in front of a rigid backing with an air cushion in between acting as a spring element. With a slightly modified realization of the geometry of the micro-holes it now becomes possible to use micro-perforated surface structures as stand-alone, bilaterally active acoustic elements with some obvious practical benefits for their users: the new absorbers function with a minimum of space, material and costs required. Maa's ingenious idea of a sound absorber, which can do without any damping material incorporated in it, has thus found a very useful extension "with immense possibilities ahead" as he put it when reading the manuscript.