The emission of waves by a small source is a generic wave problem that has long been thought to be well studied. Here, we report the experimental observation that the energy flux of an outgoing sound wave emitted by a deep-subwavelength-sized source can become negative, which indicates the backward flow of energy. Such a negative transient flux exists, even in a homogenous medium, but is only observable in the time domain and in the extreme near field of the source. By wave-impedance analysis, we show that such a phenomenon is fundamentally rooted in the geometry of the wavefield itself and, hence, is generic. Our findings have implications in the time-dependent emission, absorption, and scattering of waves.

中文翻译：

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亚波长源近场中的负瞬态通量

小源发出的波是一个普遍的波问题，长期以来一直被认为是经过充分研究的。在这里，我们报告了实验观察结果，即由深亚波长大小的源发出的传出声波的能量通量可以变为负值，这表明能量向后流动。这种负瞬态通量存在，即使在均匀介质中，但只能在时域和源极近场中观察到。通过波阻抗分析，我们表明这种现象从根本上植根于波场本身的几何形状，因此是通用的。我们的发现对波的时间相关发射、吸收和散射具有影响。