Root mean square
In mathematics and tuning, the quadratic mean of two frequencies [math]\displaystyle{ f_1 }[/math] and [math]\displaystyle{ f_2 }[/math] is equal to [math]\displaystyle{ \sqrt{\frac{f_1^{2} + f_2^{2}}{2}} }[/math].
Examples
The quadratic mean of 1/1 and 3/2 is √(13/4).
The quadratic mean of 5/4 and 6/5 is √(1201/800).
The quadratic mean of 9/8 and 10/9 is √(12961/10368).