Group velocity control in PT-symmetric acoustic systems

Achieving a group velocity that exceeds the speed of sound in a waveguide presents a significant challenge in acoustic wave engineering. This task becomes even more demanding when aiming to enhance velocity without altering the waveguide’s geometry, such as by narrowing or widening, and without obstructing the passage with passive or active inclusions. In this work, we address this problem by leveraging principles from non-Hermitian physics, implemented through active elements seamlessly integrated into the waveguide walls. These elements operate in a real-time feedback loop, introducing local pressure gain and loss, as well as non-local couplings based on pressure integration. By carefully tuning the control couplings, informed by lattice theory and adapted to the waveguide configuration, we achieve a stable parity-time-symmetric regime that governs the wave dynamics. Through numerical simulations and experimental validation in an air-filled waveguide, we demonstrate the accelerated propagation of a wave packet. Additionally, we explore the balance between system stabilization and the extent of achievable velocity enhancement. This study paves the way for innovative approaches to wave transmission in continuous media, enabled by short- and long-range active couplings implemented via embedded real-time feedback mechanisms.