A small source of sound moves on a circle with constant speed as shown in fig. (1) and an observer is sitting on O. Let v1 v2 and v3  v1,v2 and v3, be the frequencies heard when the source is at A, B and C respectively, then

At B the source is approaching the observer along the line joining them, so ν2 >ν0 ( frequency of sound emitted by the source ).At A the source is moving away from the observer along the line joining them, so ν1<ν0 .At C direction of velocity of the source is perpendicular to the line joining the source and the observer , so  ν2 >ν3 >ν0 .