# Find Stopband Edge Frequency

I tried to find the stopband edge frequency.I know that the signal amplitude falls to -3 dB below the passband edge frequency.I examine a lot of DSP books so i don't any information about how to calculate stopband edge frequency.

One Way the cutoff frequency can be calculated as: consider H(jw) normalized filter transfer function $$H(jw)=\frac{V_0(jw)}{V_i(jw)}$$ Here the 3dB frequency is when H(jw) is 1/sqrt(2) time the max value.Then it is finiding the values omega which satisfies the below equation $$H(jw)=\frac{1}{\sqrt2}$$ This we can see when we take logarithm of 1/sqrt(2) we get approximately -3dB