For the piezoelectric crystal, not only a mechanical source distribution at the surface, but an electrical source distribution also can excite the elastic wave fields.

对于压电晶体，不仅表面力源，而且表面电源分布也能激发弹性波场。

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The earth is full of inorganic raw materials to create mechanical devices and their energy source, historically fossil fuels, was in abundance.

FORBES: Lemin Wu, Economic Growth and Why Humanity Can't Have Nice Things