Two persons A and B, each carrying a source of sound of frequency $\upsilon$, are standing a few metres apart in a quiet field.  A starts moving towards B with a speed $u$.  If $V$ is the speed of sound, the number of beats heard per second by A will be

APPARENT FREQUENCY  of source with B received by the person A will be given by $n^{\text{'} }=\frac{n\left(v-v_o\right)}{\left(v-v_s\right)}$

here $v_o=-u and v_s=0$ 

Person A hears the sound of his own source whose frequency $\upsilon$ . He also hears the sound of source carried by person B ${\upsilon }^1=\upsilon \left(1+u/v\right)$                 

Hence,  No. of beats heard ${\upsilon }^1-\upsilon =\frac{\upsilon u}{v}$