On the Acoustic Impedance of a Fibreless Sound Absorptive Element 2015-24-2462
The acoustic impedance exhibited by a new type of element for noise control, the Micro-Grooved Elements (MGEs), has been widely investigated in this paper.
The MGEs are typically composed of two overlying layers presenting macroscopic slots and a number of micro-grooves on one of the contact surfaces. The micro-grooves result in micro-channels as the layers are assembled to form the element. Similarly to Micro-Perforated Elements (MPEs), the MGEs have been proved to provide effective dissipation of acoustic energy by the means of viscous losses taking place in the micro-channels. However, in contrast to the MPEs, the MGEs use the grooves, instead of the holes, in which the air is forced to pass through. It results in more cost effective elements, which have been found to represent an adequate alternative for fibrous materials, typically present in silencer units.
The design of the MGEs has been largely improved since the year 2012 and a number of configurations, provided with different internal geometries, have been produced during the last three years. For this reason new and more general models are needed, allowing to describe the acoustic behavior of those elements. In this study, the impedance model provided in the paper  has been extended and generalized in order to be applicable to a wider variety of MGEs configurations.
The methodology proposed here comprises a systematic investigation on the impedance contribution of all the constituting parts, e.g., inlet/outlet layers and internal micro channels, treated as series impedance. Experimental results have been carried out by extracting the transfer matrix from the 2-port data and by assuming constant particle velocity throughout the elements.
The acoustic impedance of the micro-channels has been studied in detail, since it gives the most important contribution to the global performance of the MGEs. Therefore, the impedance end corrections of the micro-channels have been determined by interpolating the impedance values of micro-channels with different lengths. Finally, the non-linear impedance contribution and the mutual interactions between the constituting parts have been experimentally evaluated and modeled by using ansatz equations.