i) The listener moves towards or away from a stationary source.
ii) The source moves towards or away from a stationary listener.
iii) Both source and listener move towards or away from one other.
iv) The medium moves when both source and listener are at rest.

When ever there is a relative motion between a source and a listener, the frequency of the sound heard by the listener is different from the original frequency of sound emitted by the source. This is known as “Doppler effect”.

An echo is the sound reproduced due to the reflection of the original sound from various rigid surfaces such as walls, ceilings, surfaces of mountains, etc.

One of the famous whispering galleries is in St. Paul’s cathedral church in London. It is built with elliptically shaped walls. When a person is talking at one focus, his voice can be heard distinctly at the other focus. It is due to the multiple reflections of sound waves from the curved walls.

In elliptical surfaces, sound from one focus will always be reflected to the other focus, no matter where it strikes the wall. This principle is used in designing whispering halls. In a whispering hall, the speech of a person standing in one focus can be heard clearly by a listener standing at the other focus.