We present a thin subwavelength material that can be flush mounted to a duct and which gives a large wide band attenuation at remarkably low frequencies in air flow channels. To decrease the material thickness, the sound is slowed in the material using folded side branch tubes. The impedance of the material is compared to the optimal value, which differs greatly from the characteristic impedance. In particular, the viscous and thermal effects have to be very small to have high transmission losses.Grazing flow on this material increases the losses at the interface between the flow and the material.
The acoustic effects of in-parallel resonators is compared to the behavior of a classical single degree of freedom resonator for which the resistance and the mass are in series. In-parallel resonators serve to enhance the acoustic absorption for mid-frequencies thereby extending the active frequency range of perfect acoustic absorbers. Two implementations of these in-parallel absorbers are presented and investigated experimentally as well as numerically. In the first demonstration, the resistance is a perforated plate with a wiremesh and the oscillating mass is the air that fills a tube passing through the plate. The second implementation consists of a thin flexible beam that oscillates and where the resistance is due to the micro-slit resulting from the cutting of the beam.
In recent years, the control of low frequency noise has received a lot of attention for several applications. Traditional passive noise control techniques using Helmholtz resonators have size limitations in the low frequency range because of the long wavelength. Promising noise reductions, with flush mounted aluminum patches with no size problems can be obtained using local resonance phenomenon implemented in acoustic metamaterial techniques. The objective of this work is to introduce locally resonant thin aluminum patches flush mounted to a duct walls aiming at creating frequency stop bands in a specific frequency range. Green's function is used within the framework of interface response theory to predict the amount of attenuation of the local resonant patches. The two-port theory and finite elements are also used to predict the acoustic performance of these patches. No flow measurements were conducted and show good agreement with the models. The effect of varying the damping and the masses of the patches are used to expand the stop bandwidth and the effect of both Bragg scattering and the locally resonant mechanisms was demonstrated using mathematical models. The effect of the arrays of patches on the effective dynamic density and bulk modulus has also been investigated.
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