=2ms−1 Frequency of source, fs=676Hz Speed of sound in air, v=340ms−1 Let f0 be the frequency heard by each driver. By using Doppler effect for A, (v−vA)fs‌‌=(v+vB)f0 ⇒f0=(‌
v+vA
v−vB
)fs‌‌=(‌
340+2
340−2
)676=‌
342
338
×676=684Hz Now, beat frequency =f0−fs=684−676=8Hz