Die Wirkung von zylindrischen Aufsatzen an Schallschirmen

摘要

This mainly theoretical work deals with the reduction of the sound field in the shadow region behind barriers by means of an attached cylindrical body at the edge of the screen. The reflecting attributes of the barrier's "hat" are described by a locally reacting impedance. The considerations start with a repetition of the diffraction by ideal soft and hard cylinders to demonstrate the governing principle of tangential powertransport parallel to their surfaces: the impedance must be chosen in such a way that the tangential intensity on the edge of the screen is as low as possible. The incoming power is thus turned to "harmless" directions. Subsequently, the differences due to impedances with mass, spring and damping behaviour are explained from simple physical concepts. The computations give a considerable improvement in the level reduction in the shadow compared to the "naked" screen for diffraction angles that are not too small. It is shown that this is essentially not due to absorption; in fact the influence of absorbing bodies on the diffracted field is poor. The necessary impedances allow for a certain range, which is - for spring-like types -limited by some optimal impedance. The paper closes with computed examples for a headpiece with a resonance - like frequency behaviour and for a body made from absorbing material.