Apparent Frequency:Source Moving

Q.

Two whistles A and B produces notes of frequencies 660 Hz and 596 Hz respectively. There is a listener at the mid-point of the line joining them. Now the whistle B and the listener start moving with speed 30 m/s away from the whistle A. If speed of sound be 330 m/s, how many beats will be heard by the listener

1. 2

2. 6

3. 8

4. 4

Q.

Match the items of Column A with the corresponding items of Column B.

Column 1	Column 2
Sound waves of frequency less than $20Hz$	$20Hz\text{to}20,000Hz$
Seconds pendulum	wavelength
Audible range of frequency	cycle per second
Distance between two successive compressions	about $340m{s}^{-1}$
Unit of frequency	infrasonic waves
Speed of sound waves in air	time period is $2s$

Q.

A car is moving with a speed of $72km{h}^{-1}$ towards a roadside source that emits sound at a frequency of $850Hz$. The car driver listens to the sound while approaching the source and again while moving away from the source after crossing it. If the velocity of sound is $340m{s}^{-1}$, the difference between the two frequencies, the driver hears is

1. $50Hz$

2. $85Hz$

3. $100Hz$

4. $150Hz$

Q. A train, standing at the outer signal of a railway station blows a whistle of frequency 400 Hz in still air. (i) What is the frequency of the whistle for a platform observer when the train (a) approaches the platform with a speed of 10 m s¯¹, (b) recedes from the platform with a speed of 10 m s¯¹? (ii) What is the speed of sound in each case ? The speed of sound in still air can be taken as 340 m s¯¹

Q. A tuning fork producing sound of frequency $380\text{Hz}$ is moving towards a wall with a velocity of $4{\text{ms}}^{-1}$. The number of beats per second heard by a stationary listener produced due to direct and reflected sounds will be
[speed of sound in air $=340{\text{ms}}^{-1}$]

1. $6$
2. $0$
3. $7$
4. $5$

Q. A police car horn emits a frequency 250Hz when it is at rest. What frequency does a stationary observer hear if the police car sounds its horn while approaching at a speed of $27m{s}^{-1}$ ? What frequency is heard if the horn sounded as the car is leaving at $27m{s}^{-1}$ ? Velocity of sound $=340m{s}^{-1}$?

1. 271 Hz, 231 Hz
2. 271 Hz, 252 Hz
3. 271 Hz, 225 Hz
4. 271 Hz, 261 Hz

Q. In the figure shown, a source of sound of frequency 510 Hz moves with constant velocity = 20 m/s in the direction shown. The wind is blowing at a constant velocity = 20 m/s towards an observer who is at rest at point B. Corresponding to the sound emitted by the source at initial position A, the frequency detected by the observer is equal to (speed of sound relative to air is 330 m/s)

1. 510 Hz
2. 500 Hz
3. 525 Hz
4. 550 Hz

Q. In the figure shown, a source of sound of frequency 510 Hz moves with constant velocity = 20 m/s in the direction shown. The wind is blowing at a constant velocity = 20 m/s towards an observer who is at rest at point B. Corresponding to the sound emitted by the source at initial position A, the frequency detected by the observer is equal to (speed of sound relative to air is 330 m/s)

1. 510 Hz
2. 500 Hz
3. 525 Hz
4. 550 Hz

Q. Your clock radio awakens you with a steady and irritating sound of frequency 600Hz. One morning, you drop the clock radio out of you fourth – story dorm window, 15.0m from the ground. Assume the speed of sound is 343m/s. As you listen to the falling clock radio, what frequency do you hear just before you hear it striking the ground?

1. 472 Hz
2. 572 Hz
3. 628 Hz
4. 728 Hz

Q. A receiver and a source of sonic oscillations of frequency $2000\text{Hz}$ are located on the x-axis. The source swings harmonically along x-axis with circular frequency $\omega$ and amplitude $50\text{cm}$. At what approximate value of $\omega$ will the frequency bandwidth (difference of maximum and minimum frequency) registered by stationary receiver be equal to $200\text{Hz}$? (The velocity of sound in air is $340\text{m/s}$ and assume that the velocity of source is small compared to the velocity of sound)

1. $20\text{rad/s}$
2. $34\text{rad/s}$
3. $48\text{rad/s}$
4. $62\text{rad/s}$

Q. A train approaching a platform at a speed of 54 km/h sounds a whistle. An observer on the platform finds its frequency to be 1620 Hz. The train passes the platform keeping the whistle on and without slowing down. What frequency will the observer hear after the train has crossed the platform? The speed of sound in air = 332 m/s.

1. 1620 Hz
2. 1600 Hz
3. 1480 Hz
4. None of these

Q. The driver of a car approaching a vertical wall notices that the frequency of the horn of his car changes from 400 Hz to 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

1. 10 m/s
2. 20 m/s
3. 30 m/s
4. 40 m/s

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

1. $97Hz$
2. $100Hz$
3. $103Hz$
4. $106Hz$

Q. A stationary observer receives a sound of frequency 2000 Hz. The variation of apparent frequency and time is shown. Find the speed of source; if velocity of sound is 300 m/s:

1. 66.6 m/s
2. 33.3 m/s
3. 27.3 m/s
4. 59.3 m/s

Q.

Two tuning forks have frequencies 450 Hz and 454 Hz respectively. On

sounding these forks together, the time interval between successive

maximum intensities will be

[MP PET 1989; MP PMT 2003]

1. sec

2. sec

3. 1 sec

4. 2 sec

Q. A stationary source emits sound of frequency $f_0=492Hz.$ The sound is reflected by a large car approaching the source with a speed of $2m{s}^{-1}.$ The reflected signal is received by the source and superposed with the original. What will be the beat frequency of the resulting signal in Hz? (Given that the speed of sound in air is 330 $m{s}^{-1}$ and the car reflects the sound at the frequency it has received.)

Q.

A whistle giving out 450 Hz approaches a stationary observer at a speed of 33 m/s. The frequency heard by the observer (in Hz) is :

1. 409

2. 429

3. 517

4. 500

Q. A train approaching a platform at a speed of 54 km/h sounds a whistle. An observer on the platform finds its frequency to be 1620 Hz. The train passes the platform keeping the whistle on and without slowing down. What frequency will the observer hear after the train has crossed the platform? The speed of sound in air = 332 m/s.

1. 1620 Hz
2. 1600 Hz
3. 1480 Hz
4. None of these

Q. A sound source is performing a uniform circular motion with speed $v_0$ and radius $r$. An observer is at point $A$ $(r<<OA)$ as shown in the figure. The apparent frequency heard by the observer is maximum and minimum at respectively points

1. $P$ and $R$
2. $R$ and $P$
3. $S$ and $Q$
4. $Q$ and $S$

Q. The frequency of a car horn is 400 Hz. If the horn is honked as the car moves with a speed $u_s$ = 34 m/s through still air towards a stationary receiver, the wavelength of the sound passing the receiver is [velocity of sound is 340 m/s]

1. 0.765 m
2. 0.850 m
3. 0.935 m
4. 0.425 m

Q. A man standing on a platform hears the sound of frequency 605 Hz coming from 550 Hz train whistle moving towards the platform. If the velocity of sound is 330 m/s. What is the speed of train?

1. 30 m/s
2. 35 m/s
3. 40 m/s
4. 45 m/s

Q.

A train moves towards a stationary observer with speed 34 m/s. The train sounds a whistle and its frequency registered by the observer 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. $\frac{19}{18}$

Q.

A train moves towards a stationary observer with speed 34 m/s. The train sounds a whistle and its frequency registered by the observer 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. $\frac{19}{18}$

Q. A source of sound of frequency $256H_z$ is moving rapidly towards a wall with a velocity of $5m{s}^{-1}$ how many beats per second will be heard if sound travels at a speed of $330m{s}^{-1}$?

1. 5 Hz
2. 6 Hz
3. 7 Hz
4. 8 Hz

Q. Two trains move towards each other with the same speed. Speed of sound is $340{\text{ms}}^{-1}$. If the pitch of the tone of the whistle of one when heard on the other changes by $\frac{9}{8}$ times, then the speed of each train is:

1. $2{\text{ms}}^{-1}$
2. $40{\text{ms}}^{-1}$
3. $20{\text{ms}}^{-1}$
4. $100{\text{ms}}^{-1}$

Q. A source of sound S is moving with a velocity 50 m/s towards a stationary observer. He measures the frequency of the source as 1000 Hz. What will be the apparent frequency of the sound when it is moving away from the observer after crossing him? The velocity of the sound in the medium is 350 m/s.

1. 750 Hz
2. 857 Hz
3. 1143 Hz
4. 1333 Hz

Q. A siren producing a sound of frequency $750\text{Hz}$ moves away from an observer at rest towards a cliff at a speed of $30\text{m/s}$. The frequency of echo heard by the observer reflected from the cliff is
[speed of sound in air $=330\text{m/s})$

1. $800\text{Hz}$
2. $750\text{Hz}$
3. $775\text{Hz}$
4. $825\text{Hz}$

Q. The frequency of a car horn is 400 Hz. If the horn is honked as the car moves with a speed $u_s$ = 34 m/s through still air towards a stationary receiver, the wavelength of the sound passing the receiver is [velocity of sound is 340 m/s]

1. 0.765 m
2. 0.850 m
3. 0.935 m
4. 0.425 m

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

1. $97Hz$
2. $100Hz$
3. $103Hz$
4. $106Hz$

Q. 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

1. $f_1\ne f_2\ne f_3$
2. $f_1<f_2$
3. $f_3<f_0$
4. $f_1<f_3<f_2$