Impurity levels and resonant transmission of acoustic phonons in a double-barrier system

Meta-Structure Hull Design with Periodic Layered Phononic Crystals Theory for Wide-Band Low-Frequency Sound Insolation

The hulls of marine vehicles are generally very effective at attenuating airborne acoustic noise generated by their powertrains. However, conventional hull designs are generally not very effective at attenuating wide-band low-frequency noise. Meta-structure concepts offer an opportunity for the design of laminated hull structures tailored to address this concern. This research proposes a novel meta-structure laminar hull concept using periodic layered Phononic crystals to optimize the sound insolation performance on the air-solid side of the hull structure. The acoustic transmission performance is evaluated using the transfer matrix, the acoustic transmittance, and the tunneling frequencies. The theoretical and numerical models for a proposed thin solid-air sandwiched meta-structure hull indicate ultra-low transmission within a 50-to-800 Hz frequency band and with two predicted sharp tunneling peaks. The corresponding 3D-printed sample experimentally validates the tunneling peaks at 189 Hz and 538 Hz, with 0.38 and 0.56 transmission magnitudes, respectively, with the frequency band between those values showing wide-band mitigation. The simplicity of this meta-structure design provides a convenient way to achieve acoustic band filtering of low frequencies for marine engineering equipment and, accordingly, an effective technique for low-frequency acoustic mitigation.

Enhancement of backward second harmonic generation of acoustic waves in a resonant cavity by using a superlattice

In this paper, the enhancement effect of backward second harmonic generation of acoustic waves in the system, consisting of a linear solid superlattice and a nonlinear liquid resonant cavity, is investigated analytically and numerically. Results show the nonlinear effect in the system can be enhanced drastically due to the existence of defective modes and the large acoustic impedance mismatching between solids and liquids. Because of the latter, the dual-resonance phenomenon can always take place approximately and automatically. These results are different from previous conclusions in optical fields.