Acoustic Diodes Based On Time-modulation

In recent years the interest in acoustic 'diodes' was increasing.An acoustic'diode' is a kind of structure that exhibit unidirectional transmission property.Such a unique property may be realized in structures ultilizing time-modulation of the parameters in linear acoustic systems.Such active isolators have been demonstrated in optics,but in acoustics there was no such an active diode had been proved.Demonstration of an acoustic diode,high transmission of which close to 100%and low transmission close to 0 is still a huge challenge.A linear system ultilizing time-modulation may be a more promising avenue for this.This thesis is focused on the propagation properties in linear acoustic systems with time-modulation of the parameters.We demonstrate a linear acoustic system adopting time-modulation can realize nonreciprocal propagation.We propose and demonstrate,both in simulation and experiment,a basic structure consisting of a rotating blade and the surrounding air can modulate the effective parameters of acoustic metamaterials.The main work of this thesis is to design four acoustic diodes based on time-modulation:1.An acoustic diode based on one modulator.This diode consists of a rotating blade and a highly efficient acoustic filter.We prove,with three methods of simulation and an experiment,that the structure can realize the acoustic unidirectional transmission.This acoustic system is so simple that we can realize easily,however it is not a proper acoustic isolator.The following three kinds of acoustic systems are proper acoustic isolators in theory.2.An acoustic diode based on two tandem modulators.It comprises two series of rotating blades with a 45° difference initial angle separated by a distance.We show,with three methods of simulation,the nonreciprocal propagation property in this acoustic system.3.An acoustic diode based on indirect interband phononic transitions.It is an acoustic waveguide with a modulation region occupying half the waveguide width.It causes unidirectional indirect phonon interband transition and break the time-reversal symmetry.We show,with simulation,the nonreciprocal propagation property in this acoustic system.4.An acoustic diode based on Aharonov-Bohm effect.The diode consists of two modulated waveguide regions with modulation phases 0 and ?/2,respectively,and a central waveguide between them.The Aharonov-Bohm effect has broken time-reversal symmetry.We show,with simulation,the nonreciprocal propagation property in this acoustic system.