Apparent Frequency:Source Moving

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 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. 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. Two cars moving in opposite directions approach each other with speed of $22m/s$ and $16.5m/s$ respectively. The driver of the first car blows a horn having a frequency $400Hz$. The frequency heard by the driver of the second car is (Take velocity of sound $340m/s$)

1. $448Hz$
2. $350Hz$
3. $361Hz$
4. $411Hz$

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. The difference between the apparent frequencies of a source of sound as perceived by a stationary observer during its approach and recession is 2% of the actual frequency of the source. If the speed of sound is 300 m/s, the speed of source is

1. 1.5 m/s
2. 3 m/s
3. 6 m/s
4. 12 m/s

Q. A car approaching a crossing at a speed of $50\text{m/s}$ blows a horn of frequency $500\text{Hz}$ when $30\text{m}$ away from the crossing. The speed of sound in air is $330\text{m/s}$. What frequency will be heard by an observer at rest at $40\text{m}$ from the crossing?

1. $540\text{Hz}$
2. $550\text{Hz}$
3. $560\text{Hz}$
4. $570\text{Hz}$

Q. The frequency changes by 10% as the source approaches a stationary observer with constant speed Vs. What should be the percentage change in frequency as the source recedes from the observer with the same speed? Given that Vs ≪ v(v is the speed of sound in air).

1. $14.3\%$
2. $20\%$
3. $16.7\%$
4. $10\%$

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. A car approaching a crossing at a speed of $50\text{m/s}$ blows a horn of frequency $500\text{Hz}$ when $30\text{m}$ away from the crossing. The speed of sound in air is $330\text{m/s}$. What frequency will be heard by an observer at rest at $40\text{m}$ from the crossing?

1. $540\text{Hz}$
2. $550\text{Hz}$
3. $560\text{Hz}$
4. $570\text{Hz}$

Q. A source of the sound of frequency $f_1$ is placed on the ground. A detector placed at a height is released from rest on this source. The observed frequency f(Hz) is plotted against time t(sec). The speed of sound in air is 300 m/s. Find $f_1(g=10m/s^2)$.

1. $0.5\times {10}^{3}Hz$
2. $1\times {10}^{3}Hz$
3. $0.25\times {10}^{3}Hz$
4. $0.2\times {10}^{3}H$

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 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 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 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. A source of sound produces waves of wavelength 60 cm when it is stationary. If the speed of sound in air is 320 m/s and source moves with speed 20 m/s, the wavelength of sound in the forward direction will be approximately:

1. 56.2 cm
2. 60.8 cm
3. 64.4 cm
4. 68.6 cm
   

Q.

The horn of a car emits sound with a dominant frequency of 2400 Hz. What will be the apparent dominant frequency heard by a person standing on the road in front of the car if the car is approaching at 18.0 km/h? Speed of sound in air = 340 m/s.

1. 2436 Hz

2. 2400 Hz

3. 2374 Hz

4. None of these

Q.

A boat is travelling in a river with a speed of 10m/s along the stream flowing with a speed 2m/s. From this boat, a sound transmitter is lowered into the river through a rigid support. The wavelength of the sound emitted from the transmitter inside the water is 14.45mm. Assume that attenuation of sound in water and air is negligible.
a. What will be the frequency detected by a receiver kept inside downstream?
b. The transmitter and the receiver are now pulled up into air. The air is blowing with a speed of 5m/s in the direction opposite the river stream. Determine the frequency of the sound detected by the receiver.
(Temperature of the air and water is ${20}^{\circ }C$; density of river water is ${10}^{3}kg/m^3$ ; bulk modulus of the water is $2.088\times {10}^9$ Pa, gas constant R = 8.31J/mol – K; mean molecular mass of air is for air is $28.8\times {10}^{-3}kg/mol:\frac{c_p}{c_v}$ for air is 1.4.)

1. 100545 Hz, 103646 Hz
2. 100100 Hz, 103103 Hz
3. 100696 Hz, 103040 Hz
4. 100425 Hz, 103954 Hz

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 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. A whistle revolves in a circle with a constant angular speed of $20\text{rad/s}$ connected with a string of length $50\text{cm}$. If the frequency of the sound from the whistle is $385\text{Hz}$, then what is the minimum frequency heard by an observer at rest who is far away from the centre?
(velocity of sound is $370\text{m/s}$)

1. $385\text{Hz}$
2. $375\text{Hz}$
3. $394\text{Hz}$
4. $333\text{Hz}$

Q. 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

1. 2 km/s
2. 1 km/s
3. 0.5 km/s
4. 0.25 km/s

Q. Two cars moving in opposite directions approach each other with speed of $22m/s$ and $16.5m/s$ respectively. The driver of the first car blows a horn having a frequency $400Hz$. The frequency heard by the driver of the second car is (Take velocity of sound $340m/s$)

1. $448Hz$
2. $350Hz$
3. $361Hz$
4. $411Hz$

Q. A motorcycle starts from rest and accelerates along a straight line at $2.2m/s^2.$ The speed of sound is 330 m/s. A siren at the starting point remains stationary. When the driver hears the frequency of the siren at 90% of when the motorcycle is stationary, the distance travelled by the motorcyclist is

1. 123.75 m
2. 247.5 m
3. 495 m
4. 990 m