Questions in waves

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	What should be the velocity of a sound source moving towards a stationary observer so that apparent frequency is double the actual frequency (Velocity of sound is $v$)
	Two trains are moving towards each other at speeds of 20 m/s and 15 m/s relative to the ground. The first train sounds a whistle of frequency 600 Hz. the frequency of the whistle heard by a passenger in the second train before the train meets is (the speed of sound in air is 340 m/s)
	A small source of sound moves on a circle as shown in the figure and an observer is standing on $O$. Let ${{n}_{1}},\ {{n}_{2}}$ and ${{n}_{3}}$be the frequencies heard when the source is at $A,\ B$ and $C$ respectively. Then
	A source and an observer approach each other with same velocity $50 m/s$. If the apparent frequency is $435 sec^{–1}$, then the real frequency is
	A source emits a sound of frequency of 400 Hz, but the listener hears it to be 390 Hz. Then
	Doppler effect is applicable for
	A source and an observer are moving towards each other with a speed equal to $\frac{v}{2}$ where $v$ is the speed of sound. The source is emitting sound of frequency $n$. The frequency heard by the observer will be
	When an engine passes near to a stationary observer then its apparent frequencies occurs in the ratio 5/3. If the velocity of engine is
	A police car horn emits a sound at a frequency 240 Hz when the car is at rest. If the speed of the sound is 330 m/s, the frequency heard by an observer who is approaching the car at a speed of 11 m/s, is :
	A person carrying a whistle emitting continuously a note of 272 Hz is running towards a reflecting surface with a speed of 18 km/hour. The speed of sound in air is $345m{{s}^{-1}}$. The number of beats heard by him is

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