Solution
Given,
Frequency of the source (f) = 500 Hz,
Velocity of sound (v) = 330 m/s,
Velocity of observer (vo) = 18 m/s,
Velocity of the source (vs) = 0 m/s
Observer B is moving away from the source i.e., observer A. Apparent frequency heard by B is given by,
\(f_1=\left(\frac{v-v_0}v\right)\times f=\frac{330-18}{330}\times500=472.8\) Hz
∴ Beat frequency (B) = f - f1 = 500 - 472.8 = 27.2 Hz ≈ 27 Hz
Hence 27 beats per second are observed by B