Controlling Sound in Non-Hermitian Acoustic Systems

Although it originated in quantum physics, the concept of $n\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n\ensuremath{-}H\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}y$ (particularly involving a Hamiltonian with $b\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}d$ gain and loss, and thus $P\phantom{\rule{0}{0ex}}T$ symmetry and an exceptional point in frequency space) can also play a key role in classical systems, including those in acoustics. By incorporating suitably engineered gain or loss media, both cavity and scattering acoustic systems can produce a series of intriguing wave phenomena, with prospects for application in the design of innovative functional devices. This review aims to introduce the pedagogical models and recent achievements in this field, in the hope of being useful to a diverse audience.