Investigation On Low-Frequency Sound Absorption Based On Helmholtz Resonator-Type Metamaterials

In recent years,rapid developments have been made in the use of acoustic artificial structures to achieve novel manipulations of sound waves.There is also considerable interest in designing new acoustic functional devices based on acoustic artificial structures.In noise control engineering,as low-frequency noise wavelengths are very long and penetrating,traditional sound-absorbing materials have weak absorption performance and face problems such as thick volume and high prices.The "small size and large wavelength" properties of acoustic metamaterials offer promising recipes for solving these challenges.In application,different scenarios have specific requirements for the construction and function of the sound absorber.Three types of metamaterial absorbers are designed for three different application scenarios/functions,including confined spaces where broadband noise reduction is required,unknown target noise frequencies,and allowing ventilation and heat transfer while absorbing sound.Then,theoretical analysis,simulation and experimental preparation of low-frequency broadband sound absorber,dual-frequency ultra-thin tunable metasurface,multiband asymmetric absorption and reflection system are carried out based on Helmholtz resonator type metamaterials.In the second chapter,a low-frequency broadband sound absorber is proposed based on an array of extended neck Helmholtz structures.A theoretical and numerical model of sound absorption is first established.The low frequency sound absorption performance and the absorption mechanism of the structure are investigated by the reflection coefficient complex frequency method and the finite element method.The low-frequency sound absorption performance and mechanisms of acoustic units are investigated by the reflection coefficient complex frequency method and the finite element method.The design of the array neck provides the structure with improved acoustic impedance tunability,which facilitates fabrication and the formation of broadband sound absorbers.As a result,several acoustic units were connected in parallel to form a broadband coupled absorber with a thickness of only 6 cm.The average absorption coefficient in the 350 Hz-690 Hz range was greater than 0.9 and the accuracy and feasibility of the absorber were experimentally verified.The proposed structure has an ultra-thin thickness,an easy-toimplement way of adjusting the operating frequency,and effectively increases the flexibility of noise control.In the third chapter,based on the Helmholtz resonator,a tunable dual-frequency ultra-thin acoustic metasurface is constructed by combining a perforated plate and two split-tube resonant structures.A physical model was first established based on an acoustic equivalent circuit,and the operating performance of the metasurface in the low-frequency range was investigated by means of the finite element method.Then,the influence of geometric parameters on the absorption performance was investigated.Finally,a broadband coplanar acoustic metasurface is constructed by connecting two different units in parallel.The results show that the sound-absorbing metasurface with a thickness of only 20 mm can be adjusted for two high-efficiency absorption peaks in the range 327Hz-750 Hz simply by rotating the angle of the opening ring.The broadband coplanar acoustic metasurface has an average absorption coefficient greater than 0.9 at 460 Hz-500 Hz at deep sub-wavelength scales.The proposed structures have an ultra-thin thickness,an easy-to-implement way of adjusting the operating frequency and effectively increase the flexibility of noise control.In the fourth chapter,a multiband asymmetric acoustic absorption and reflection system is constructed using ventilated waveguides and multi-order Helmholtz resonators with different radiant modes.A physical model of the system is established based on the coupled mode theory.Theoretical calculations and experimental tests using the transfer matrix method and the two-load method,respectively.Theoretical and experimental results confirm that the coupling of the dark and bright modes at each resonant frequency enables multiband high-efficiency absorption requiring only two functional units.In addition,a hybrid multiband asymmetric system based on a multimode pair is experimentally constructed.One side of the hybrid multiband system almost completely absorbs the lower frequency incident sound waves while reflecting the higher frequency ones;conversely,the other side effectively reflects the lower frequency ones and absorbers the higher frequency ones.Our design showcases the flexibility of customized multiband asymmetric absorption,and also provides an approach for the design of bidirectional wave-manipulation devices.