Sound transmission through panels and shells filled with porous material in the presence of external flow

Abstract

With increasingly tighter regulations on noise exposure during flight, aircraft designers have been compelled to innovate structures that minimise noise transmission into the cabin space. Porous material is widely used as a passive noise control medium because of their light weight, low cost, and broad band sound abatement effectiveness. The present work, inspired by the need to be able to predict noise transmission characteristics for commonly used constructions, incorporates the effect of flow into the calculations. Three types of sandwich configurations {bonded-bonded, bonded-unbonded and unbonded-unbonded{ are considered. Biot's theory is used to simulate the poroelastic material. The sound transmission though a double-walled panel lined with porous material in the presence of external mean flow is considered, first. The transmission loss is found to increase with increasing Mach number of the external mean flow. This is then explained on the basis that external mean flow increases the impedance of the panel. Mismatch in the characteristic acoustic impedances of the exterior and the interior results in the change of transmission loss. Transmission loss increases gradually when the pressure difference between air gap and that in the exterior decreases. A bi-objective optimization study is carried out to simultaneously minimize the sound transmission and the structural weight. The effect of laminated composite face plate in the structure is also brought out. Sound transmission through a system of double shells, lined with poroelastic material in the presence of external mean flow, is studied next. The transmission characteristics of the sandwich construction are presented for different incidence angles and Mach numbers over a wide frequency range. It is noted that the transmission loss exhibits three dips on the frequency axis as opposed to flat panels where there are only two such frequencies. Results are discussed in the light of these observations. Flow is shown to decrease the transmission loss below the ring frequency, but to increase this above the ring frequency due to the reduction of stiffness and the damping effect added by the flow. Finally, sound transmission through double-walled cylindrical shell lined with poroelastic material in the core excited by the exterior pressure fluctuation due to the turbulent boundary layer is investigated. The peaks of power spectral density of the inner shell kinetic energy due to shell resonance, hydrodynamic coincidence and acoustic coincidence are discussed. The results show that if the high frequency is interested, an air gap, even if very thin, between the two face shells provide superior sound insulation.

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