The effect of cylindrical waveguide outlet features on the directional pattern
Regular paper
AGH University of Science and Technology
Wednesday 3 june, 2015, 09:20 - 09:40
0.3 Copenhagen (49)
Abstract:
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.