Unidirectional Extraordinary Sound Transmission with Mode-Selective Resonant Materials

Unidirectional wave propagation can introduce strong direction-dependent energy responses, and thus, greatly benefits the construction of switching and logic devices. Such an effect has drawn much attention in acoustics, but has not been able to practically address the essential issue of insufficient transmission efficiency in the forward direction. Here, we present a one-way structure that experimentally realizes near-total forward acoustic wave transportation, while still maintaining a broadband high contrast ratio and single-mode property. The experimentally observed rectification is achieved with a comprehensive design of combining acoustic wave intermode transitions, selective-mode filtering, and cavity resonance for strong wave-structure interactions. The demonstrated simplicity and flexibility make this material an appealing integration solution, enabling an alternative control methodology in biomedical ultrasonography, high-performance acoustic sensors, and noise control.

Figure 1

(a) Waveguide material consisting of elements I (yellow), II (blue), and III (pink). (b) Three elements are designed to have geometry parameters P = 26.90 mm, H = 40.34 mm, W = 80.69 mm, S = 67.24 mm, and R = 13.45 mm. (c) Unidirectional extraordinary acoustic wave transmission is due to asymmetric wave-manipulation functions provided by three elements. In forward direction, the even-mode incident plane wave (Green arrow) will be totally converted into odd-mode output through mode conversion in element II, selective blocking in elements I and III, and cavity resonances in all elements. Reverse direction incidents will be blocked by element III due to the band gap. (d) Proposed material can be intuitively regarded as providing distinct direction-dependent additional wave vectors: $q_1$ in forward direction and $q_2$ in reverse direction.

Figure 2

(a) Schematic of the experiment setup and fabricated sample. (b) Simulated transmission contrast (green line) and acoustic energy transmission coefficients in forward (red line) and reverse directions (purple line). Transmission peaks can be observed in forward direction between 2000 and 2800 Hz, with the major peak located at 2290 Hz. (c) Measured forward and reverse direction transmission coefficients and contrast. Major peak in forward direction is at 2280 Hz. Signals measured in reverse direction are mostly background noise, which also cause the contrast change in experimental results. Error bars represent s.d. among five repeat measurements.

Figure 3

(a) Simulated output sound field maps for forward and backward propagations. Incident even-mode waves tunnel through the materials without reflections and are converted into odd modes, while no transmission is observed for the same incident even-mode waves coming from the reverse direction. Waveguide material section is represented by the dotted line to save space. (b) Normalized simulation and (c) experimental measurements of output sound energy field along the marked lines in forward and backward directions. Operating frequency for simulation is 2290 Hz, while experimental data are acquired at 2280 Hz.

Figure 4

(a) Calculated band structures of pure even mode (red line) within element I, hybrid mode (purple line) within element II, and pure odd mode (green line) within element III. (b) To design a unidirectional acoustic resonance material, as presented, all three elements have to be tuned to have cavity resonance peak at 2290 Hz, as marked by the black line, so that near-total transmission through the whole waveguide materials in the forward direction can be realized.