When observer is stationary and source is moving, observed frequency is

$f_1=f_0\left(\frac{v}{v-u}\right)$

where, $v=$ speed of sound and $u=$ speed of source.

When source is stationary and the observer is moving towards source, then measured frequency is

$f_2=f_0\left(\frac{v+u}{v}\right)$

So, difference in frequencies observed is

$f_2-f_1=f_0\left(\frac{v+u}{v}-\frac{v}{v-u}\right)$

$=-\frac{u^2 f_0}{v(v-u)}$

$\Rightarrow \quad f_2-f_1 < 0 \text { or } f_2 < f_1$