这个“明明”有些想当然了。看看RMS是怎么定义的（from Wikipedia）：
the root mean square (RMS or RMS or rms) is defined as the square root of the mean square (the arithmetic mean of the squares of a set of numbers).

所以，重点是the square root of the mean square，主体是square root，而mean square是用来修饰square root的。也就是说这是个“根”，而不是个“方”。“均方根”的翻译正是正常汉语习惯的体现。
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