Question

Mention two cases in which there is no Doppler effect in sound?​

Answer 1

1) When the source of sound as well as the listener moves in the same direction with the same speed. 2) When one of source | listener is at the centre of the circle and the other is moving on the circle with uniform speed.

Answer 2

1). when source (S) and the listener (L) both are at rest.

2). when S and L move in such a way that distance between them remains constant.

3). when source S and L are moving in mutually perpendicular direction