As a kind of essential acoustic engineering device,acoustic delay lines are widely used in the fields of audio,radio frequency and surface acoustic wave signal processing.Their main design goals include low insertion loss,wide operating bandwidth,and controllable delay time.In the audio band,topological acoustic delay lines designed using topological boundary states with no backscattering and defect immunity can solve the problem of low loss and time control,however the operating band is narrow and the device volume is excessive.In addition,recently reported slow optical effects in topological unidirectional waveguides also open the possibility of using slow acoustic effects for the design of acoustic delay lines.On the other hand,due to the causal law of real physical systems,acoustic advance lines with opposite effects to acoustic delay lines cannot be realized in passive systems.How to overcome such difficulties and successfully design acoustic advance lines is also a hot issue to be addressed in the study of acoustic time control devices.In this paper,we construct a PT-symmetric acoustic system consisting of a pair of loss and gain admittance metasurfaces and an intermediate acoustically equivalent medium,and tune its operation mechanism by setting the metasurface admittances such that two complementary scattered EP points are realized.The first is called the coherent perfect absorption-laser(CPA-Laser)mode,where the reflection coefficient of the system is 0 and the transmission coefficient is 1,indicating that the sound energy is transmitted lossless in the system,and the transmission phase is positive,so the system can be designed as the previously reported acoustic advance line.The second is the antireflections(ARCs)mode,in which the sound energy can be transmitted lossless,the transmission phase is negative,and the corresponding device is the acoustic delay line.We then use the carbon nanotube(CNT)films to fully model the acoustic admittance metasurfaces,and study the time-domain difference between the output signal and the original signal as the two-sided exponential attenuation envelope signal passes through both modes.The simulation results show that both the CPA-Laser mode and the ARCs mode PT-symmetric systems can perfectly implement the acoustic advance and delay lines,respectively,with the advance/delay time is controlled by the equivalent medium width and CNT film parameters.In general,we have innovatively implemented PT-symmetric acoustic advance/delay lines in this work,which have negligible transmission loss,are suitable for bandwidth acoustic signals,and the advance/delay time is adjustable.Our design approach provides a novel idea for realizing lossless,wide-band and time-adjustable acoustic advance/delay lines using PT symmetry,and we believe that this approach can be extended to various wave fields including optics,mechanics,elastic waves,etc.
PT-symmetric acousticsacoustic metasurfaceCNT filmacoustic advance/delay line
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