We have designed a 1D film-type acoustic metamaterial structure consisting of several polymer films directly stacked on each other It is experimentally revealed that the mass density law can be broken by such structures in the low frequency range By comparing the sound transmission loss (STL) curves of structures with different numbers of cycles, materials and incident sound directions, several physical properties of the 1D film-type acoustic metamaterial are revealed, which consist of cyclical effects, surface effects and orientation effects It is suggested that the excellent low frequency sound insulation capacity is influenced by both the cycle number and the stiffness of the film surface Meanwhile, the surface effect plays a dominant role among these physical properties Due to the surface acoustic property, for structures with a particular combination form, the STL dominated by the cyclical effects may reach saturation with increasing number of construction periods Moreover, in some cases, the sound insulation ability is diverse for different sound incidence directions This kind of 1D film-type periodic structure with these special physical properties provides a new concept for the regulation of sound waves

https://iopscience.iop.org/article/10.1088/0022-3727/48/46/465305/pdf

One-dimensional rigid film acoustic metamaterials

Based on one-dimensional lossy Bragg stacks, an asymmetric absorber is realized for low-frequency sound waves, that is, perfect absorption can be obtained when sound waves are normally incident from one side while a small absorption can be obtained from the opposite side. Moreover, the asymmetric absorption persists for a wide incident angle of sound waves in the range from 0° to 42° with the absorptive coefficient larger than 90% from one side while less than 20% from the other side. By changing the thickness of the top sublayer, a series of interesting absorption phenomena such as Fano-resonance type absorption are further investigated.

/pdf/wide-angle-asymmetric-acoustic-absorber-based-on-one-2esz0oxdz5.pdf

Wide-angle asymmetric acoustic absorber based on one-dimensional lossy Bragg stacks.

We investigate a one-dimensional composite with a very low loss coefficient and find that multiple selective absorption peaks reach close to 100% absorption of incident acoustic waves when the acoustic waves propagate through this system. Especially, the unidirectional absorption is demonstrated when the propagation direction of acoustic waves is reversed. These high absorptions stem from the excitations of the strong resonances, which depend on not only the low loss in this composite but also the huge impedance mismatch at the outgoing interface. To further investigate those selective absorption, we construct an equivalent structural model, and our calculations show that these absorption peaks have the extraordinary response to the imaginary part of the effective mass density and bulk modulus. The results exhibit that the imaginary part of the effective mass density and bulk modulus at the frequencies of the absorption peaks abruptly appear, in the meanwhile the real part of those become zero. Based on these theoretical results, an unidirectional absorption device with multiple selection of frequencies can be designed.

Multiple selective absorption peaks of acoustic waves in a one-dimensional composite

In this article, we demonstrate the existence of exotic absorption peaks when acoustic wave propagates through a one-dimensional layered phononic crystal (PC) at frequencies within the band gap into a material of large acoustic impedance mismatch. By analyzing the displacement field distribution, it indicates that the absorption peaks are due to the excitation of the resonant interface states at boundary between the last layer of the PC and emergent material. The number of period and the thickness of last layer of the PC play the decisive role in determining the magnitude and frequency of absorption peaks. And the absorptivity can be nearly unity with a reasonable design of the structure. Specifically, these absorption peaks would almost disappear when the incident direction of acoustic waves is reversed. This property can be employed to design an acoustic wave unidirectional absorption device.

Hongmei Xu

Papers

Exotic absorption peaks of acoustic waves in one-dimensional layered phononic crystal