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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IMPORTANT

Earn 100

For a sound wave travelling towards $+ x$ direction, sinusoidal longitudinal displacement $ξ$ at a certain time is given as a function of $x$ (see figure). If bulk modulus of air is $B = 5 \times 10^{5} N / m^{2}$ , the variation of pressure excess will be

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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 44

Consider a source of sound

S

, and an observer/detector

D .

The source emits a sound wave of frequency

f_{0}

The frequency observed by

D

is found to be
(i)

f_{1}

, if

D

approaches

S

and

S

is stationary
(ii)

f_{2}

, if

S

approaches

D

and

D

is stationary
(iii)

f_{3}

, if both

S

and

D

approach each other with the same speed. In all three cases, relative velocity of

S

wrt

D

is the same. For this situation which is incorrect?

MEDIUM

Physics

IMPORTANT

PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 8 - Sound Waves and Doppler Effect>Exercises>Q 45

When source and detector are stationary but the wind is blowing at speed

v_{w}

, the apparent wavelength

λ^{'}

on the wind side is related to actual wavelength

λ

by [take speed of sound in air as

v]

MEDIUM

Physics

IMPORTANT

PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 8 - Sound Waves and Doppler Effect>Exercises>Q 46

The driver of a car approaching a vertical wall notices that the frequency of the horn of his car changes from

400 Hz

450 Hz

after being reflected from the wall. Assuming speed of sound to be

340 m / s

, the speed of approach of car towards the wall is

MEDIUM

Physics

IMPORTANT

PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 8 - Sound Waves and Doppler Effect>Exercises>Q 47

The difference between the apparent frequency of a source of sound as perceived by the observer during its approach and recession is

2 %

of the frequency of the source. If the speed of sound in air is

300 m s^{- 1}

, the velocity of the source is

MEDIUM

Physics

IMPORTANT

PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 8 - Sound Waves and Doppler Effect>Exercises>Q 48

The frequency of a radar is

780 MHz

. After getting reflected from an approaching aeroplane, the apparent frequency is more than the actual frequency by

2.6 kHz

. The aeroplane has a speed of

HARD

Physics

IMPORTANT

PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 8 - Sound Waves and Doppler Effect>Exercises>Q 49

A train moves towards a stationary observer with speed of

34 m s^{- 1}

. The train sounds a whistle and its frequency registered by the observer is

f_{1}

. If the train speed is reduced to

17 m s^{- 1}

, the frequency registered is

f_{2}

. If the speed of sound is

340 m s^{- 1}

, then the ratio

\frac{f_{1}}{f_{2}}

HARD

Physics

IMPORTANT

PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 8 - Sound Waves and Doppler Effect>Exercises>Q 50

A siren placed at a railway platform is emitting sound of frequency

5 kHz

. A passenger sitting in a moving train

A

records a frequency of

5.5 kHz

, while the train approaches the siren. During his return journey in a different train

B

, he records a frequency of

6.0 kHz

while approaching the same siren. The ratio of the velocity of train

B

to that of train

A

HARD

Physics

IMPORTANT

PHYSICS FOR JOINT ENTRANCE EXAMINATION WAVES AND THERMODYNAMICS>Chapter 8 - Sound Waves and Doppler Effect>Exercises>Q 51

A person speaking normally produces a sound intensity of $40 dB$ at a distance of $1 m$ . If the threshold intensity for reasonable audibility is $20 dB,$ the maximum distance at which he can be heard clearly is

For a sound wave travelling towards +x direction, sinusoidal longitudinal displacement ξ at a certain time is given as a function of x (see figure). If bulk modulus of air is B=5×105 N/m2, the variation of pressure excess will be

Important Questions on Sound Waves and Doppler Effect

Learn and Prepare for any exam you want !

For a sound wave travelling towards $+ x$ direction, sinusoidal longitudinal displacement $ξ$ at a certain time is given as a function of $x$ (see figure). If bulk modulus of air is $B = 5 \times 10^{5} N / m^{2}$ , the variation of pressure excess will be