Sonic Crystal Acoustic Attenuation Applied to Exhaust Air Systems

Abstract

In a society where most people spend their time inside buildings or vehicles, the need to filter the air is essential to ensure a certain level of comfort. While incorporating an exhaust fan into these enclosed spaces improves air quality, noise is generated and often creates discomfort. Better fan design helps reduce noise, but even with good design, uncomfortable noise pollution remains. To block the noise generated by the fan, without however blocking the airflow, an acoustic metacage, made of sonic crystals, can be used, taking advantage of their stop-bandproperties. This work presents a discretized modeling by transfer matrices of a network of sonic crystals that can form such a metacage. Like the finite element method, the approach discretizes the crystal lattice by periodic elements. Each periodic element is assembled to the others, either in series or in parallel, according to the serial or parallel transfer matrix methods. Thus, complex shapes of crystals can be modeled to better adapt to a medium with flow (e.g. : NACA profile). The modeling is applied to cylindrical crystals of different diameters under normal acoustic incidence. The results of the modeling are compared to sound transmission loss measurements made in an impedance tube without flow. The comparisons are good, but a correction must be made to certain elements to consider the inertia added by the constrictions between the crystals. Such a correction is proposed in this work based on a geometrical tortuosity.