Acoustic Field Regulation Based On Phase Gradient Structure

As a kind of synthetic composite material,acoustic metamaterial has unique properties that natural materials do not possess.Using their unique physical properties,a variety of novel acoustic effects can be realized,such as anomalous refraction,asymmetric transmission,acoustic focusing,etc.Phase gradient structure is one of the metamaterial that can form phase gradients and perform various ways of acoustic regulation by periodically arranging units with a specific phase distribution.In this paper,we design acoustic devices composed of phase gradient structure,which can realize the functions such as acoustic unidirectional transmission and adjustable acoustic focusing,etc.The work in this paper consists of the following two main parts:1.We propose to realize angle-dependent asymmetric acoustic transmission by designing a planar structure comprising a gradient-index layer and a layer of homogeneous medium with relatively-lower index.We analytically derive the working frequency and angle range of the device with unidirectional mechanism.And the innovation of the device is that the proposed device gives rise to high-efficiency broadband asymmetric transmission by allowing the acoustic waves on one side to pass within the range of 18500Hz-22500 Hz when the angle of incidence is below 7°,while behaving as an acoustic barrier blocking waves obliquely incoming from both directions as the angle of incidence exceeds a critical angle,thus making it possible to be adapted to different requirements by adjusting the angle of incidence.The dependence of the asymmetric acoustic transmission effect on different parameters is also discussed.Using a gas medium with lower refractive index can broaden the frequency band range,and changing the phase gradient can make the asymmetric transmission effect effective in lower frequency bands.Bearing the advantages of simple design,broad bandwidth and switchable functionality,our scheme opens a route to the design of novel acoustic devices capable of adapting various circumstances,and may find applications in noise control,medical detection,etc.2.We design an adjustable phase gradient structure with acoustic focusing function based on the generalized Snell’s law.The structure consists of several concave cavity channels with different sizes at a certain distance.The phase gradient structure is fixed on the X-type telescopic mechanical structure,and the channel interval can be changed by programming the STM32 to control the expansion and contraction of the mechanical structure,so that the focus position can be adjusted in real time.The broadband focusing function of the structure is verified by simulation and experiment.During the simulation and experiment,the channel interval is increased from 1mm to 3mm and the focal length is changed from 50 mm to 300 mm at 7500 Hz.The broadband focusing effect of the structure with same interval are also considered.The experimental focus position is basically consistent with the simulation results.The novelty of the device lies in the application of the microcontroller to the regulation of the acoustic ultrasurface,which has the advantages of simple regulation and the ability to regulate in real time,realizing a broadband adjustable focus,and has broad application prospects in focused ultrasound therapy and medical ultrasound imaging,etc.