A small source of sound moves on a circle as shown in the figure and an observer is standing on O. Let n1, n2 and n3 be the frequencies heard when the source is at A, B and C respectively. Then

At point A, source is moving away from observer so apparent frequency n1 < n (actual frequency). At point B source is coming towards observer so apparent frequency n2 > n and point C source is moving perpendicular to observer so n3 =n. Hence n2 > n3 > n1.