Acoustic scattering singularities via quasi-Bound states in the continuum
Non-Hermitian systems enable advanced control of wave propagation by exploiting engineered losses. This introduces an additional degree of freedom that permits the emergence of exceptional points (EPs). In this letter, we theoretically and experimentally demonstrate the control of scattering singularities in a non-Hermitian acoustic system using quasibound states in the continuum (qBICs). Through Friedrich Wintgen interference, the losses of a two port cavity are tuned until achieving critical coupling, yielding narrowband coherent perfect absorption (CPA) with a quality factor of 140. Additionally, by coupling two distinct resonators, we observe the emergence of an EP, where both eigenvalues simultaneously coalesce and vanish, resulting in narrowband unidirectional absorption. Our results establish a connection between qBICs and scattering singularities, and offer a route toward acoustic devices featuring narrowband resonances and tunable radiative losses.