The effect of noise in FM does not remain constant but it increases with the increase in frequency of mod s/g.

Assuming a single noise frequency, that will also modulate the constant carrier Vc, we get a modulation index due to noise as M = Vn/Vo.

$M_{fn} = \frac{\delta}{f_n}$

As modulating s/g frequency increases, modulating index due to mod. s/g decreases.

$Mfs = \frac{\delta}{f_s}$

$\therefore$ The s/g to noise ratio in FM.

$= \frac{S}{N} = \frac{Mfs}{Mfn}$

$\therefore$ As fs $\uparrow$, Mfs $\downarrow$ $\therefore$ S/N $\downarrow$

$\therefore$ A plot of S/N v/s frequency is not uniform rather a triangle. this is called as Noise Triangle.

Pre-emphasis & De-emphasis is performed to avoid this non-uniform S/N.