Abstract
The growing noise exposure of residents, due to a rising number of flights, causes significant impacts on physical health. Therefore it is necessary to reduce the noise emission of aircrafts. During take-off, the noise generated by the jet engines is dominating. One way to lower the noise emission of jet engines is to build an absorption silencer by using porous liners. Because of the high thermic and corrosive attacks as well as high fatigue loads, conventional absorbers cannot be used. A promising material is sintered metal fibre felts. This study investigates the suitability of metal fibre felts for the use as absorption material in silencers. The influences of pore morphology, absorption coefficient, determined with perpendicular sound incidence, as well as geometric parameters of the silencer to the damping are identified. To characterise the material, the parameters fibre diameter, porosity and thickness are determined using three-dimensional computer tomography images. The damping potential of absorption silencers is measured using an impedance tube, which was modified for transmission measurements. The essential parameter to describe the acoustic characteristics of porous materials is the flow resistivity. It depends on the size, shape and number of open pores in the material. Finally a connection between pore morphology, flow resistivity of the metal fibre felts and damping potential of the absorption silencer is given.
Highlights
The exposure to a high noise level, such as aircraft noise, causes insomnia, makes it difficult to concentrate and incurs further health damage [1,2]
Sintered metal fibre felts of the alloy 1.4404 have a high porosity, which is a basic requirement for dissipative sound absorption, and are available with different pore sizes
The damping that can be achieved with a silencer depends on the absorption coefficient α of the porous material, and on the pore morphology, and the dimensions of the silencer
Summary
The exposure to a high noise level, such as aircraft noise, causes insomnia, makes it difficult to concentrate and incurs further health damage [1,2]. Besides the aeroacoustic noise generated by flow around the aircraft, the engine noise is the most significant source of noise [3]. To reduce this noise, the future objective is to line the exhaust side of the engine’s hot gas path with porous absorbers. The future objective is to line the exhaust side of the engine’s hot gas path with porous absorbers This would create an absorption silencer for the core engine. It is possible to optimize the flow resistivity to the frequency needed to be damped These characteristics make sintered metal fibre felts well suited for the studies of silencers. A previous study, determining the absorption coefficient for a wide range of porous metals, supports this approach [4]
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