Understanding unidirectional and topological wave phenomena requires the unveiling of intrinsic geometry and symmetry for wave dynamics. This is essential yet challenging for the flexible control of near-field evanescent waves, highly desirable in broad practical scenarios ranging from information communication to energy radiation. However, exploitations of near-field waves are limited by a lack of fundamental understanding about inherent near-field symmetry and directional coupling at sub-wavelengths, especially for longitudinal waves. Here, based on the acoustic wave platform, we show the efficient selective couplings enabled by near-field symmetry properties. Based on the inherent symmetry properties of three geometrically orthogonal vectors in near-field acoustics, we successfully realize acoustic Janus, Huygens, spin sources and quadrupole hybrid sources, respectively. Moreover, we experimentally demonstrate fertile symmetry selective directionality of those evanescent modes, supported by two opposite meta-surfaces. The symmetry properties of the near-field acoustic spin angular momenta are revealed by directly measuring local vectorial fields. Our findings advance the understanding of symmetries in near-field physics, supply feasible approaches for directional couplings, and pave the way for promising acoustic devices in the future.

中文翻译：

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近场声学中的对称选择性方向性

理解单向和拓扑波现象需要揭示波动力学的内在几何和对称性。这对于近场渐逝波的灵活控制至关重要，但在从信息通信到能量辐射的广泛实际场景中非常理想。然而，近场波的开发受到缺乏对亚波长的固有近场对称性和定向耦合的基本理解的限制，特别是对于纵波。在这里，基于声波平台，我们展示了由近场对称特性实现的高效选择性耦合。基于近场声学中三个几何正交向量的固有对称性，我们成功地实现了声学 Janus、Huygens、分别为自旋源和四极混合源。此外，我们通过实验证明了这些渐逝模式的丰富对称性选择性方向性，由两个相反的元表面支持。通过直接测量局部矢量场，揭示了近场声自旋角动量的对称性。我们的发现促进了对近场物理学中对称性的理解，为定向耦合提供了可行的方法，并为未来有前途的声学设备铺平了道路。