平面传播时，若介质中体元均按余弦（或正弦）规律运动，叫平面简谐波，是最基本的波动形式。一些复杂的波可视为平面简谐波的叠加。有平面简谐波传播介质中的体元并非简谐振动。有些书中直接称简谐波为余弦或正弦波。
索末菲关于波的展开理论证明，球面波可以展开为平面简谐波的叠加。
Sommerfeld's wave expansion theory demonstrates that a spherical wave can be expanded as the superposition of plane simple harmonic waves.
推导出了具有一般形式的平面简谐波的波动方程，利用此公式求解波动方程简单而且准确。
The wave equation having general forms of planar simple harmonic wave is deduced in this article. It is simple and accurate to solve wave equation by using this formula.
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