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Arrangements of acoustic meta-atoms, better known as acoustic metamaterials, are commonly applied in acoustic cloaking, for the attenuation of acoustic fields or for acoustic focusing. A precise design of single meta-atoms is required for these purposes. Understanding the details of their interaction allows improvement of the collective performance of the meta-atoms as a system, for example, in sound attenuation. Destructive interference of their scattered fields, for example, can be mitigated by adjusting the coupling or tuning of individual meta-atoms. Comprehensive numerical studies of various configurations of a resonator pair show that the coupling can lead to degenerate modes at periodic distances between the resonators. We show how the resonators' separation and relative orientation influence the coupling and thereby tunes the sound attenuation. The simulation results are supported by experiments using a two-dimensional parallel-plate waveguide. It is shown that coupling parameters like distance, orientation, detuning, and radiation loss provide additional degrees of freedom for efficient acoustic meta-atom tuning to achieve unprecedented interactions with excellent sound attenuation properties. [ABSTRACT FROM AUTHOR]

The generic problem of low-frequency acoustic radiation through quiescent air from a circular pipe that is inclined with respect to its exit flange is studied in this work. The exit flange is taken to extend as an infinite plane away from the pipe opening. The analysis implements a hybrid method that combines modal expansions with the boundary element method. The reflection coefficient and pipe end correction for Helmholtz numbers (based on the pipe radius) less than 2.5 are calculated for various inclination angles up to 75°. Calculations are validated using simulations from the finite-element solver of the commercial software package COMSOL. The reflection coefficient and end correction predictions agree closely with the validation simulations yet differ notably from the results available in the literature. The solution obtained from the hybrid method is subsequently used to analyse the acoustic field at the pipe exit and in the downstream space. The key aspects of the governing physics pertaining to practical engineering applications at low frequencies are captured in a low-order approximation, which significantly reduces the degrees of freedom of the problem and provides generally good estimates of the reflection coefficient and end correction, as well as the downstream acoustic field. [ABSTRACT FROM AUTHOR]