A man is watching two trains, one leaving and the other coming in with equal speeds of 4 m/sec. If they sound their whistles, each of frequency 240 Hz, the number of beats heard by the man (velocity of sound in air = 320 m/sec) will be equal to
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Frequency of sound heard by the man from approaching train
$n_a = n \left(\frac{v}{v - v_s}\right) = 240 \left(\frac{320}{320 - 4}\right) = 243 \text{Hz}$ Frequency of sound heard by the man from receding train $n_r = n \left(\frac{v}{v + v_s}\right) = 240 \left(\frac{320}{320 + 4}\right) = 237 \text{Hz}$
Hence, number of beats heard by man per sec
$= n_{a} - n_{r} = 243 - 237 = 6$
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