An observer moving at a velocity of 20 m/s is moving away from a source moving at a speed of 10 m/s towards the observer.
If the frequency of the source is 'f', then find the frequency observed by the observer is: ( $v_{sound} = 330 m/s$ )

1. $\frac{32}{31}f$

2. $\frac{47}{23}f$

3. $\frac{31}{32}f$

4. f

A train moves towards a stationary observer at a speed of 34 m/s. The train sounds a whistle and its frequency registered is $f_1$. If the train's speed is reduced to 17 m/s, the frequency registered is $f_2$. If the speed of sound is 340 m/s , then the ratio $\frac{f_1}{f_2}$ is

1. $\frac{18}{19}$

2. $\frac{1}{2}$

3. 2

4. $\frac{19}{18}$

The driver of a car travelling at a speed of 30 m/s towards a hill sounds a horn of frequency 600 Hz. If the velocity of sound in air is 330 m/s, the frequency of reflected sound as heard by the driver is:

1. 550 Hz

2. 555.5 Hz

3. 720 Hz

4. 500 Hz

A speeding motorcyclist sees a traffic jam ahead of him. He slows down to 36 km/hour. He finds that traffic has eased and a car moving ahead of him at 18 km/hour is honking at a frequency of 1392 Hz. If the speed of sound is 343 m/s, the frequency of the honk as heard by him will be:

1. 1332 Hz

2. 1372 Hz

3. 1412 Hz

4. 1454 Hz

A source of sound S emitting waves of frequency 100 Hz and an observer O are located at some distance from each other. The source is moving with a speed of 19.4 ms^-1 at an angle of ${60}^0$ with the source-observer line as shown in the figure. The observer is at rest. The apparent frequency observed by the observer (velocity of sound in air 330 ms^-1), is: 
              

1. 100 Hz

2. 103 Hz

3. 106 Hz

4. 97 Hz

A siren emitting a sound of frequency \(800\) Hz moves away from an observer towards a cliff at a speed of \(15\) ms^-1. Then, the frequency of sound that the observer hears in the echo reflected from the cliff is:
(Take, the velocity of sound in air = \(330\) ms^-1)
1. \(800\) Hz
2. \(838\) Hz
3. \(885\) Hz
4. \(765\) Hz

Two cars moving in opposite directions approach each other at speeds of 22 m/s and 16.5 m/s, respectively. The driver of the first car blows a horn with a frequency of 400 Hz. The frequency heard by the driver of the second car is [assume velocity of sound to be 340 m/s]:

1. 361 Hz

2. 411 Hz

3. 448 Hz

4. 350 Hz
 

A man sitting on a moving train hears the whistle of the engine. If the frequency of the whistle is 600 Hz, then:

1. the apparent frequency as heard by him is smaller than 600 Hz.

2. the apparent frequency is larger than 600 Hz.

3. the frequency as heard by him is 600 Hz.

4. None of the above

A policeman buzzes a whistle of frequency 420 Hz. A car with an audio recorder is moving towards the policeman with a speed of 72 km/h, recording a frequency of ${\mathrm{v}}_1$. If, after crossing, it records the frequency of ${\mathrm{v}}_2$, then ${\mathrm{v}}_1$- ${\mathrm{v}}_2$  is: (Velocity of sound is 300 m/s)

1. $\sqrt{2}$ Hz

2. 20 Hz

3. 28 Hz

4. 56 Hz

A train moves towards a stationary observer at 50 m/s. The train sounds a whistle and its frequency is registered by the observer as ${\mathrm{f}}_1$. If the train's speed is increased by 25 m/s, the frequency registered is ${\mathrm{f}}_2$. If the speed of sound is 300 m/s, then ${\mathrm{f}}_1$:${\mathrm{f}}_2$ is:

1. 3:5

2. 5:7

3. 7:9

4. 9:10