Broadband Acoustic Ventilation Barriers

Conventional sound barriers impede airflow at the same time. Recent advances in acoustic metasurfaces provide a solution for air-permeable barriers utilizing the Fano-like interference. While the mechanism of Fano-like interference implies that such a realized device serves a narrow working frequency range around every destructive-interference frequency. Considering the fact that noise usually covers a wide frequency range, designing a broadband acoustic barrier is still a challenge. Here, we theoretically design a planar-profile and subwavelength-thickness (approximately $\lambda /8$) acoustic ventilation barrier prohibitive for sound in a broad range. Our design is a metasurface consisting of a central hollow orifice and two surrounding helical pathways with varying pitch. Thanks to the hornlike helical pathways, the response strength from the monopolar and dipolar modes of the system almost keeps balance in the frequency range of interest, leading to an effective blocking of more than $90\mathrm{\%}$ of incident energy in the range of 900–1418 Hz. Experiments are conducted to validate the proposed design, whose results are consistent with the analytical predictions and simulations. The underlying working mechanism ensures the metasurface is capable of handling broadband sound coming from various directions. Our design has potential in air-permeable yet sound-proofing applications, such as simultaneously natural ventilation and noise reduction in green buildings.

Figure 1

(a) Schematic diagram of the binary meta unit placed perpendicular to the direction of wave propagation. The two-colored parts of the meta unit are characterized by two effective mediums with different acoustic properties. (b) Illustration of sound proofing induced from the equal-strength monopole and dipole responses of the meta unit.

Figure 2

(a) Realized meta unit with diameter $D$ and thickness $2h$, which is composed of two parts: the central hollow-out orifice and a surrounding helix. (b) Internal structure of the helix. Two helical blades rotate around the central orifice creating two spiral pathways. The helix consists of three layers: two hornlike helical layers mirror-symmetrically arranged and connected by the fix-pitch helical layer of length $L$ in between. The inner diameter of the hollow orifice is $d$, and the thicknesses of the cylindrical shell and blades are $1$ mm. (c) Monopole and dipole response functions changing with frequency, depicted by red and black dots, respectively. These two curves intersect at $1006$ and $1326$ Hz. (d) Theoretically predicted (red curve) and numerically calculated (dashed black line) transmission loss of the helical meta unit. The frequency step is 1 Hz. Black arrows refer to four frequency points, the two peak frequencies, and a frequency between (beyond) the peak frequencies. (e) Simulated sound-field distributions at the four marked frequencies, where the black lines represent the local velocity streamlines, and the color maps illustrate the acoustic pressure distributions.

Figure 3

(a) Photo of the 3D-printed specimen of the designed helical meta unit. (b) Schematics of the experimental setup where the specimen is clamped in the Brüel and Kjær type-4206 impedance tube. (c) Theoretical (red line), simulated (black dashed line), and measured (green dots) transmission loss of the designed helical meta unit.

Figure 4

Transmission loss varying with (a) the relative diameter of central orifice $d/D$, (b) the effective refractive index at the front and back surfaces $n_2(\pm h)$, and (c) the relative length of fix-pitch layer $L/2h$. In these colormaps, the bright regions represent effectively silenced zones. Stars in the colormap show those points correspond to the selected system parameters and the frequency with a peak transmission (1006 and 1326 Hz) in our design.

Figure 5

(a) Transmission loss of pressure when plane waves come with incident angles of $0^{\circ }$ (blue line), ${30}^{\circ }$ (red line), and ${60}^{\circ }$ (green line), respectively. (b) The simulated sound-field distributions with incidence angles of ${30}^{\circ }$ and ${60}^{\circ }$ at a peak frequency of $1326$ Hz.