Two loudspeakers M and N are located 20 m apart and emit sound at frequencies 118 Hz and 121 Hz , respectively. A car is initially at a point P , 1800 m away from the midpoint Q of the line M N and moves towards Q constantly at 60 km h - 1 along the perpendicular bisector of M N . It crosses Q and eventually reaches a point R , 1800 m away from Q . Let ν t represent the beat frequency measured by a person sitting in the car at time t . Let ν p , ν Q and ν R be the beat frequencies measured at locations P , Q and R , respectively. The speed of sound in air is 330 m s - 1 . Which of the following statement(s) is (are) true regarding the sound heard by the person?

The plot below represents schematically the variations of beat frequency with time

Two vehicles, each moving with speed u on the same horizontal straight road, are approaching each other. The wind blows along the road with velocity w . One of these vehicles blows a whistle of frequency f 1 . An observer in the other vehicle hears the frequency of the whistle to be f 2 . The speed of sound in still air is v . The correct statement (s) is (are):

If the wind blows from the source to the observer, f 2 > f 1 .

If the wind blows from the observer to the source, f 2 < f 1 .

If the wind blows from the source to the observer, f 2 < f 1 .

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Two sound sources are moving away from a stationary observer in opposite directions with velocities $V_{1}$ $V_{1}$ and $V_{2} (V_{1} > V_{2})$ . The frequency of both the sources is $900 Hz$ . $V_{1}$ and $V_{2}$ are both quite less than speed of sound, $V = 300 m / s$ . Find the value of $(V_{1} - V_{2})$ so that beat frequency observed by observer is $9 Hz$ . (in $m / s$ .).

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Important Questions on Sound Waves and Doppler Effect

MEDIUM

Physics

IMPORTANT

PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 8 - Sound Waves and Doppler Effect>Exercises>Q 6

Loudness of sound from an isotropic point source at a distance of

70 cm

20 dB

. What is the distance (in

m

) at which it is not heard.

MEDIUM

Physics

IMPORTANT

PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 8 - Sound Waves and Doppler Effect>Exercises>Q 7

An ambulance sounding a horn of frequency

264 Hz

is moving towards a vertical wall with a velocity of

5 {ms}^{- 1}

. If the speed of the sound is

330 {ms}^{- 1}

, how many beats per second will be heard by an observer standing a few meters behind the ambulance?

MEDIUM

Physics

IMPORTANT

PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 8 - Sound Waves and Doppler Effect>Exercises>Q 8

In a car race sound signals emitted by two cars are detected by the detector on the straight track at the end point of the race. Frequency observed is

330 Hz

and

360 Hz

and the original frequency is

300 Hz

of both cars. Race ends with the separation of

100 m

between the cars. Assume both cars move with constant velocity and velocity of sound is

330 m / s

. Find the time taken (in sec) by wining car.

MEDIUM

Physics

IMPORTANT

PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 8 - Sound Waves and Doppler Effect>Exercises>Q 9

Airport authority has made the regulations that maximum allowable intensity level detected by a microphone situated at the end of

1630 m

long runway can be

100 dB

. An aeroplane when flying at a height of

200 m

produces an intensity level of

100 dB

on ground. While taking off, this aeroplane makes an angle of

30 °

with horizontal. Find the maximum distance (in

m

) this aeroplane can cover on the runway, so that the regulations are not violated (assume no reflection).

HARD

Physics

IMPORTANT

PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 8 - Sound Waves and Doppler Effect>Exercises>Q 10

The sound level at a point is increased by

30 dB

. By what factor is the pressure amplitude increased?

MEDIUM

Physics

IMPORTANT

PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 8 - Sound Waves and Doppler Effect>Exercises>Q 11

Two tuning forks $A$ and $B$ are vibrating at the same frequency $256 Hz$ . A listener is standing midway between the forks. If both tuning forks move to the right with a velocity of $5 m / s$ , find the number of beats heard per second by the Tistener (speed of sound in air is $330 m / s$ ).

Question Image

MEDIUM

Physics

IMPORTANT

PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 8 - Sound Waves and Doppler Effect>Exercises>Q 12

A driver in a stationary car horns which produces monochromatic sound waves of frequency $n = 1000 Hz$ , normally towards a reflecting wall. If the wall approaches the car with a velocity $u = 3.3 m / s$ , calculate the frequency of sound reflected from wall and heard by the driver. What is the percentage change of sound frequency on reflection from the wall?

Question Image

MEDIUM

Physics

IMPORTANT

PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 8 - Sound Waves and Doppler Effect>Exercises>Q 13

A source of sound of frequency

256 Hz

is moving towards a wall with a velocity of

5 m / s

. How many beats per second will be heard by an observer

O

standing in such a position that the source

S

is between

O

and wall?

(c = 330 m / s)

Important Questions on Sound Waves and Doppler Effect

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