Measuring characteristic values of a new metallic absorber to predict the transmission loss of ducts with a porous absorbing lining

The propagation of sound inside porous absorbing material can be described completely by use of the characteristic values, the complex wave number k(ind a) and the complex impedance Z(ind a) . These characteristic values can be calculated using different acoustical models, e.g. the model of the homogeneous medium or the model of DELANY/BAZELY. But in case that for some special absorber no model exists, or the model is not valid due to clogging or deformation of the porous material, the characteristic values cannot be calculated correctly. This paper describes the direct measurement of the characteristic values, dependent from clogging or deformation. With the help of these characteristic values it is possible to estimate the parameters of the absorber or to predict the transmission loss of a silencer that is lined with this particular porous absorber. This measuring method is usefull especially for the new hollow metal sphere absorbers, for which no exact and universally valid acoustical models exists. These new porous absorbers can replace common porous absorbers with equal acoustical properties in places, where conventional absorbers cannot be used due to their disadvantageous technical properties. Entnommen aus TEMA