  Investigating the sound field in and around ducts is an important topic in
acoustics, e.g. when simulating musical instruments or the human vocal tract.
In this paper a method that is based on the boundary element method in 3D
combined with a formulation for infinitely thin elements is presented. The
boundary integral equations for these elements are presented, and numerical
experiments are used to illustrate the behavior of the thin elements. Using the
example of a closed benchmark duct, boundary element solutions for thin
elements and surface elements are compared with the analytic solution, and the
accuracy of the boundary element method as function of element size is
investigated. As already shown for surface elements in the literature, an
accumulation of the error along the duct can also be found for thin elements,
but in contrast to surface elements this effect is not as big and a damping of
the amplitude cannot be seen. In a second experiment, the impedance at the open
end of a half open duct is compared with formulas for the radiation impedance
of an unflanged tube, and a good agreement is shown. Finally, resonance
frequencies of a tube open at both ends are calculated and compared with
measured spectra. For sufficiently small element sizes frequencies for lower
harmonics agree very well, for higher frequencies a difference of a few Hertz
can be observed, which may be explained by the fact that the method does not
consider dampening effects near the duct walls. The numerical experiments also
suggest, that for duct simulations the usual six to eight elements per
wavelength rule is not enough for accurate results.
