Apparent Frequency:Source Moving

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. $411Hz$
4. $361Hz$

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. Given that sound travels in air at $340\text{m/s}$. In air, a sound source is producing a wave of wavelength $34\text{cm}$. If the same source is put in a water tank, what would be the wavelength of sound wave in water?

$[$Speed of sound in water is $1360\text{m/s}]$

1. $0.34\text{m}$
2. $1.36\text{m}$
3. $3.4\text{m}$
4. $13.6\text{m}$

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

Q. A train moves towards a stationary observer with a speed of 34 m/s. The train sounds a whistle and its frequency registered by the observer is f₁. If the train’s speed is reduced to 17 m/s, the frequency registered is f₂. If the speed of sound is 340 m/s, then the ratio $\frac{f_1}{f_2}$ is

1. 
2. 
3. 
4. 

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

Q. A source emitting a sound of frequency f is placed at a large distance from an observer. The source starts moving towards the observer with a uniform acceleration ‘a’. Find frequency heard by the observer corresponding to the wave emitted just after the source starts. The speed of sound in medium is v.

1. 
   
2. 
3. 
   
4. 

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

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. 100696 Hz, 103040 Hz
2. 100545 Hz, 103646 Hz
3. 100100 Hz, 103103 Hz
4. 100425 Hz, 103954 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. $100{\text{ms}}^{-1}$
2. $40{\text{ms}}^{-1}$
3. $20{\text{ms}}^{-1}$
4. $2{\text{ms}}^{-1}$

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. 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.

In Quincke’s experiment the sound detected is changed from a maximum to a minimum when the sliding tube is moved through a distance of$2.50cm$. Find the frequency of sound if the speed of sound in air is$340m{s}^{-1}$.

Q. A road runs midway between two parallel rows of buildings. A motorist moving with a speed of $36km{h}^{-1}$ sounds the horn. He hears the first echo one second after he has sounded the horn. Speed of sound in air is $330m{s}^{-1}$ . Then The distance between the two buildings and the time after which he hears second echo are

1. 360 m, 2.3 s
2. 300.2m, 1.5 s
3. 350 m, 2.5 s
4. 329.8 m, 2s

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. 500

3. 429

4. 517

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. 
2. 
3. 
4. 

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. $7$
3. $0$
4. $5$

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. 
2. 
3. 
4. 

Q.

A car moving at 108 km $h^{-1}$ finds another car in front of it going in the same direction at 72 km $h^{-1}$. The first car sounds a horn that has a dominant frequency of 800 Hz. What will be the apparent frequency heard by the driver in the front car? Speed of sound in air =$330m{s}^{-1}$

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

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 train has just completed a U-curve in a track which is a semi-circle. The engine is at the forward end of the semi-circular part of the track while the last carriage is at the rear end of the semi-circular track. The driver blows a whistle of frequency 200 Hz. Velocity of sound is 340 m/s. Then the apparent frequency as observed by a passenger in the middle of the train, when the speed of the train is 30 m/s, is

1. 219 Hz
2. 188 Hz
3. 200 Hz
4. 181 Hz

Q. A whistle emitting a sound of frequency 440 Hz is tied to a string of 1.5 m length and rotated with an angular velocity of 20 rad/s in the horizontal plane. Then the range of frequencies heard by an observer stationed at a large distance from the whistle will be (v = 330 m/s)

1. 400.0 Hz to 480.0 Hz
2. 403.3 Hz to 484.0 Hz
3. 400.0 Hz to 484.0 Hz
4. 403.3 Hz to 480.0 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 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.

Sound waves from the loudspeaker spread fairly equally in all directions, if the wavelength of the sound is substantially larger than the diameter of the loudspeaker,

(a) Calculate the frequency for which the wavelength of sound in air is ten times the diameter of the speaker if the diameter is $20cm$.

(b) Sound is essentially transmitted in the forward direction if the wavelength is much shorter than the diameter of the speaker. Calculate the frequency at which the wavelength of the sound is one-tenth of the diameter of the speaker described above. Take the speed of sound to be $340m{s}^{-1}$.

Q.

A source of sound $S$ and a detector D are placed at some distance from one another. A big cardboard is placed near the detector and perpendicular to the line SD as shown in figure (below). It is gradually moved away and it is found that the intensity changes from a maximum to a minimum as the board is moved through a distance of $20cm$. Find the frequency of the sound emitted. Velocity of sound in air is $336m/s$.