Bound states in the continuum in open acoustic resonators

TL;DR

This paper investigates bound states in the continuum within open acoustic resonators, demonstrating their occurrence through destructive interference and predicting their properties analytically using a coupled mode theory approach.

Contribution

It introduces a numerical and analytical framework to identify and predict BSCs in acoustic duct-cavity structures, highlighting the Friedrich-Wintgen interference mechanism.

Findings

Multiple BSCs occur due to destructive interference.

BSCs are detected at Fano resonance collapse points.

Two-mode approximation accurately predicts BSC properties.

Abstract

We consider bound states in the continuum (BSC) or embedded trapped modes in two- and three-dimensional acoustic axisymmetric duct-cavity structures. We demonstrate numerically that under variation of the length of the cavity multiple BSCs occur due to the Friedrich-Wintgen two-mode full destructive interference mechanism. The BSCs are detected by tracing the resonant widths to the points of the collapse of Fano resonances where one of the two resonant modes acquires infinite life-time. It is shown that the approach of the acoustic coupled mode theory cast in the truncated form of a two-mode approximation allows us to analytically predict the BSC frequencies and shape functions to a good accuracy in both two and three dimensions.