What physical change occurs when a source of sound moves and listener is at rest?

What physical change occurs when source of sound waves is at rest and the listener moves?

A source of sound moves towards an observe.

With what velocity should a source of sound move towards a listener so that apparent frequency is twice the actual frequency ? Take velocity of sound in air =330m//s

Assertion : The apparent frequency will remain same as the frequency of source of sound if source and listener move at right angles to each other. Reason : Apparent frequency will always increase whether source is moving towards the listener or listener is moving towards the source.

Two sources of sound are in resonance when