Acoustic Artificial Structure Based On Helmholtz Array For Sound Field Controlling

Acoustic metamaterials are an important branch of artificial structures,which have attracted increasing attention of scholars in the past decades.Compared with ordinary materials in nature,acoustic metamaterials have extraordinary physical properties that natural materials do not have.Acoustic metamaterials can realize arbitrary manipulation of incident acoustic waves and achieve novel physical effects with subwavelength sizes,such as abnormal sound transmission,sound collimation,sound insulation and one-way sound transmission.Therefore,to break the limitations of traditional acoustics for controlling the sound field has great application value and received extensive attention.In this thesis,acoustic metamaterials with Helmholtz arrays are utilized to conduct some research on acoustic beam regulation.The main parts of this article are as follows:1.An artificial structure that is easy to fabricate and can achieve abnormal acoustic transmission is proposed in this part.By engraving the periodic rigid plate with an array of Helmholtz resonators,a collimated sound beam at the resonant frequency can be achieved when a point acoustic source is placed below the center of the structure.and the length of the beam exceeds 20 wavelengths of the incident wave.2.We propose a planar ultrathin artificial structure with air as the background medium.The upper and lower sides of the structure are embedded with Helmholtz resonator groove arrays and a zigzag slit in the center is designed to lower the height of the structure.When the plane acoustic wave penetrates into the structure with the same frequency as the resonant frequency of the HR on both sides and F-P resonance in the center,the HR arrays on both sides of the structure convert the incident plane acoustic waves into surface waves,which converge to the center and couple with the F-P circular waves at the central slit.As a result,a highly efficient collimated beam over 50 incident wavelengths is produced in the output field and the far-field sound pressure radiation angle is limited to within 15 degrees which shows strong directivity.3.Consider that the limitation of the 2D structure in practice,two deep-subwavelength 3D collimation devices are investigated here.The first structure has a helical structure in the center which lengthens the distance that the waves travel,so that the effective refractive index increases to make the working frequency shift to the lower range which decrease the size of the structure virtually.Helmholtz resonator arrays radiating from the center are embedded at the top and bottom to convert the incident waves into surface acoustic waves propagating along the surface of the structure,and then couple with the central transmitted cylindrical waves caused by central F-P resonance to achieve the collimation effect.The parameters of the structure are consciously designed to ensure F-P resonance frequency generated at the central helix and the resonance frequency of the HR be equal to each other.The simulation results show that when the frequency of the incident acoustic wave is close to the resonant frequency,a collimated acoustic beam with a length of more than 5 incident wavelengths can be generated at the transmitted acoustic field at a structural scale lower than 1/5 of the incident wavelength.The second type of three-dimensional ultra-thin collimating structure device has a helical structure in the center,and concentric circle arrays on both sides.By rotating the Helmholtz resonator around the center to form concentric circles,a collimated acoustic device with higher transmittance,and a longer collimating sound beam which exceeds 15 incident wavelengths is achieved.Based on the second structure,we design a structure with unilateral HR arrays to realize the function of one-way acoustic transmission.There exists a same spiral channel as the above3 D structure and the lower side of the structure is embedded by Helmholtz resonator array around the center to form circular rings while the upper side of the structure is smooth.When acoustic waves are incident with an identical frequency with the F-P resonant frequency and HR frequency,most incident acoustic energy converges towards the center and goes through the structure.While the sound wave is incident in reverse,since there is no HR array on the surface of the structure,the incident sound wave is equivalent to encountering a hard boundary and is reflected mostly which leading to near zero transmittance.Numerical simulation results show that almost half of the acoustic waves can transmit at the resonant frequency when the sound wave is incident in one direction and the transmittance is less than 0.01 in the other direction.4.An ultra-thin artificial structure with sound insulation function is put forward in this thesis.The structure is a two-dimensional flat plate array constructed by Helmholtz resonators at the subwavelength scale,which can achieve sound insulation from both sides.At the HR resonant frequency,the incident plane acoustic waves is bound to the surface of the structure,so that few incident acoustic waves can transmit to the other side,thereby blocking the transmission of sound waves and forming a sound insulation barrier.Compare with the traditional sound insulation devices,the subwavelength metasurface based on periodic HR arrays can block the sound from bidirection but has a well-ventilate and light transmitting function.In conclusion,the acoustic metamaterials with Helmholtz arrays are utilized to realize these novel effects.These physical effects not only deepen the understanding of acoustic wave propagation behavior in artificial structures,but also provide a research basis for the design of new acoustic devices in the future.