Analysis of Sound Propagation in Lined Ducts and Parallel Baffle Silencers

A test campaign was carried out, by means of an experimental apparatus compliant with EN ISO 11691 and EN ISO 7235, with the aim to determine the acoustical attenuation produced by several lined ducts and parallel baffle silencers, varying both the geometry and the physical properties of the sound-absorbing material. A second experimental campaign was carried on, by means of a standing wave tube, to study the effects of confinement on the acoustical properties of porous sound-absorbing materials. The data obtained showed consistency with the data found in the literature, i.e. that over a specific percentage open area of the perforated facing, equal to about 20%, the facing itself is "transparent" to the sound and has the only behaviour of a mere support of the sound absorbing material. This information has been useful in order to avoid the modelling of the perforated facings in the comparative study developed to analyze the behaviour of a finite element model versus two of the most widely used theoretical methods to estimate the noise attenuation in lined ducts, from Sabine and Kurze. A commercial silencer (FCR series SQ-A model 110-600) of known performance was tested in the above mentioned test facility and later modelled by means of FEM and Sabine’s and Kurze’s theories for a frequency range from 125 Hz to 8000 Hz: the numerical approach has been found to be the best choice to tackle the problem of the study of sound propagation in rectangular ducts, compared to the high cost of field surveys and experiments or to the poor and nowadays inadequate accuracy of existing analytical methods.