The effect of cylindrical waveguide outlet features on the directional pattern

Regular paper

Karolina Kolber

AGH University of Science and Technology

Wednesday 3 june, 2015, 09:20 - 09:40

0.3 Copenhagen (49)

Outlets of ventilation and air condition ducts as well as exhaust of combustion engines are typical sources of arduous noise and therefore any attempt to reduce the sound level emitted by outlet of such systems is considered as deserving attention. One method of influencing noise emission properties of acoustic duct outlets consists in undertaking an intervention in their directivity patterns and shaping them in a way minimising the noise emission towards the areas occupied by people redirecting at the same time the sound energy flux into regions where it will not pose any significant problem. If the problem of the semi-infinite cylindrical unbaffled waveguide is concerned, its analytical solution is known and allows to determine directivity patterns for the related outlet configuration, but must be accepted that for more complex geometrical forms of cylindrical duct outlets it will be necessary to use advanced numerical methods. The paper presents a number of directivity patterns obtained on the grounds of both laboratory measurements and numerical simulations for circular duct outlets characterised with various geometrical features for which analytical solutions, due to their complexity, are not known. In the experiments, the examined systems were excited with various arrays of point sources (both axisymmetrical and non-axisymmetrical) located in the vicinity of the duct model’s anechoic termination. Directivity patterns registered for duct outlets characterised with different geometrical features were compared to results of simulations carried out with the use of the finite elements method. Both measurements and simulations were carried out for a model of the semi- infinite cylindrical duct with rigid walls and radius of 77 mm. The measurements were taken in the 340 m3-anechoic chamber operated by the AGH’s Dept of Mechanics & Vibroacoustics in the spherical system of co-ordinates as a result of which a number of high-resolution 3D directivity patterns were obtained.

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