Manipulating nonlinear sound waves using non-Hermiticity and active control. Nonlinear and Active Sound Absorption

Audible sound is an inevitable physical phenomenon which can be the best or the worst of all things. In its best form, is essential to human communication and entertainment. In its worst form, it appears as a social and health threat through noise in urban and domestic environments, arising from sources such as vehicles, ventilation systems, construction sites and aircraft engines. Both European and international organisations identify noise as a major health issue of modern society and insist on regulating its levels. Some modern noise reduction solutions, using artificial periodic composites and benefiting from the 3D printing techniques do exist, however an important problem regarding sound absorption still remains unsolved and unexplored.High amplitude nonlinear waves cannot be absorbed using the existing conventional techniques and devices. These high amplitude nonlinear waves in fact pose an even larger problem than noise. When they appear in applications such as power plants or gas transport systems they can cause significant environmental, reliability, and safety problems stemming from vibration and structural fatigue.In an original perspective, the NASA project, will utilise recent results from non-Hermitian physics to tame and absorb high amplitude, nonlinear sound waves. The project strives to uncover the main sources of nonlinearity in acoustic isolation solutions, and to use active nonreciprocal scatterers in unison with modern concepts such as topological physics, proposing an unconventional way to efficiently absorb nonlinear acoustic waves. Nonlinearity, which is known to lead into chaos, and randomness, when in conjunction with non-Hermiticity, raises fundamental questions and their synergy may revolutionise our ability to control waves. Within NASA I aim develop novel design principles and tools which will foster the development of efficient, adoptable, and reliable noise reduction technologies particularly in heavy industry and aviation.