The 2.5D MST for sound propagation through arrays of cylinders parallel to the ground
Invited paper
Division of Applied Acoustics. Chalmers University of Technology
Wednesday 3 june, 2015, 11:20 - 11:40
0.9 Athens (118)
Abstract:
In this work sound propagation through arrays of cylinders oriented parallel to the ground is of
interest. The structures are placed in a three-dimensional domain and are insonified by a monopole or
incoherent line source. Assuming a cross-sectionally invariant structure one can efficiently obtain the
3D pressure field for such arrangements by post-processing a series of 2D solutions - a technique
usually referred to as a 2.5D transform. Since the initiation of the 2.5D transform for outdoor sound
propagation it has been successfully applied together with frequency domain methods such as the
Boundary Element Method and the Equivalent Sources Method. However, to predict for sound
propagation through sonic crystal noise barriers the 2D Multiple Scattering Theory (2D MST) is often
used, and has proven to be very efficient. We therefore introduce the 2.5D MST to solve for 3D
scattering by clusters of acoustically rigid cylinders. It will be shown that only a few simple
substitutions applied to the 2D MST kernel allows us to solve for imaginary wave numbers, which are
needed in the 2.5D transform. The proposed method is numerically validated for two basic cases: (i) a
point source above rigid ground, and (ii) off-axis scattering by a cylinder in free-field. Both are shown
to be in excellent agreement with the respective reference calculations. We further demonstrate some
calculation results for sound propagation through graded index sonic crystals, and find that off-axis
insonification of these structures shifts the characteristic frequency response upwards, as could be
expected. Finally, we also present calculation results for infinite and finite incoherent line sources and
display the existence of a spectral smearing effect for both source types.