Acoustic feedbacks of loudspeaker-room-microphone environments with directivity

Diploma thesis (pdf 13.700 KB)

Everybody is familiar with the unwanted howling sound of acoustic feedbacks in public adress systems or at live-concerts. Especially in monitoring situations, where a large amount of the radiated sound is fed back into the microphone, howling, also known as the Larsen effect, occurs quite often. Ironically, this unwanted sound has been used in many movie soundtracks to express the presence of a sound reinforcement system. Literature on acoustic feedback has focused on diffuse sound fields, phase and amplitude conditions under which feedbacks occur. Directivity was usually only taken into account statistically, considering the directivity index.

The aim of this thesis is to develop a feedback simulation which includes measured complex directivity patterns of both the loudspeaker and the microphone. Therefore a simple acoustic mirror source room model is used to simulate the direct sound path as well as the wall reflections including the directivity in magnitude and phase. Hereby, the impact of several factors can be investigated: Orientation and location of source and microphone, the directivity factor and room and absorption parameters.

The simulation results are evaluated for several situations regarding their plausibility and the usability for feedback estimation in different applications. Finally, to investigate the general behavior of feedback-prone systems and to find adequate simulation methods, the influence of several parameters of the simulation shall be isolated or simplified: For instance, the impact of phase data in the directivity patterns and the influence of the reflected sound in comparison to the direct path.