Apparent Frequency:Listener Moving

Q.

The driver of a bus approaching a big wall notices that the frequency of his bus’s horn changes from $420Hzto490Hz$ when he hears it after it gets reflected from the wall. Find the speed of the bus if the speed of the sound is $330m{s}^{-1}$.

1. $81km/h$

2. $91km/h$

3. $71km/h$

4. $61km/h$

Q. Two sources P and Q produces notes of frequency $990\text{Hz}$ each. A listener moves from P to Q with a speed of $1{\text{ms}}^{-1}$. If the speed of sound is $330\text{m/s}$, then the number of beats heard by the listener per second is

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

Q. The frequency of a tuning fork is $384\text{Hz}$ and velocity of sound in air is $352{\text{ms}}^{-1}$. How far sound has travelled when fork completes 36 vibrations?

1. $33\text{m}$
2. $16.5\text{m}$
3. $11\text{m}$
4. $22\text{m}$

Q. A man standing on a platform observes that the frequency of the sound of a whistle emitted by a moving train is less by $140\text{Hz}$ that the frequency emitted when the train is stationary. If the velocity of sound in air is $330\text{m/s}$ and the speed of the train is $70\text{m/s}$, the frequency of the whistle is

1. $571\text{Hz}$
2. $800\text{Hz}$
3. $400\text{Hz}$
4. $260\text{Hz}$

Q. The engine of a train sounds a whistle at frequency $\left(f\right)$. The frequency heard by a passenger is

1. $\frac{1}{f}$
2. $>f$
3. $f$
4. $<f$

Q. Two cars moving in opposite directions approach each other with speeds of $40{\text{ms}}^{-1}$ and $50{\text{ms}}^{-1}$ respectively. The driver of the first car blows a horn of frequency $400\text{Hz}$. The frequency heard by the driver of the second car is
[speed of sound $=340{\text{ms}}^{-1}$]

1. $515\text{Hz}$
2. $375\text{Hz}$
3. $520\text{Hz}$
4. $450\text{Hz}$

Q. Two tuning forks with natural frequencies $340\text{Hz}$ each move relative to stationary observer. One fork moves away from the observer while the other moves towards him at the same speed. The observer hears beats of frequency $3\text{Hz}$. Find the speed of the tuning fork (velocity of sound in air is $340\text{m/s}$).

1. $1.5\text{m/s}$
2. $3\text{m/s}$
3. $4.5\text{m/s}$
4. $6\text{m/s}$

Q. A stationary observer receive sound from two identical tuning forks, one of which approaches and the other one recedes with the same speed (much less than the speed of sound). The observer hears $2$ beats per second. The oscillation frequency of each tuning fork is $f_0=1400\text{Hz}$ and the velocity of sound in air is $350{\text{ms}}^{-1}$. The speed of each tuning fork is close to:

1. $\frac{1}{2}{\text{ms}}^{-1}$
2. $1{\text{ms}}^{-1}$
3. $\frac{1}{4}{\text{ms}}^{-1}$
4. $\frac{1}{8}{\text{ms}}^{-1}$

Q.

Figure (16-E111)shows a person standing somehere in between two identical tuning forks, each vibrating at

512 Hz. If both the tuning forks move towards right at a speed of 5.5 m s^{-1}, find the number of beats heard by the listener. Speed of sound in air =$330m{s}^{-1}$.

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

1. A plot below represents schematically the variation of beat frequency with time
   
2. The rate of change in beat frequency is maximum when the car passes through $Q$
3. ${\nu }_P+{\nu }_R=2{\nu }_Q$
4. The plot below represents schematically the variation of beat frequency with time
   

Q. An observer move towards a stationary source of sound with a velocity equal to one fifth of the velocity of sound. The apparent change in frequency is

1. $35\%$
2. $30\%$
3. $25\%$
4. $20\%$

Q. An observer travelling with a constant velocity of $20m{s}^{-1}$ passes close to a stationary source of sound and notices that there is a change of frequency of $50H_Z$ as he passes the source. What is the frequency of the source? Speed of sound in air = 340 m/s.

1. 450 Hz
2. 400 Hz
3. 425 Hz
4. 500 Hz

Q.

A traffic policeman sounds a whistle to stop a car-driver approaching towards him. The car-driver does not stop and takes the plea in court that because of the Doppler shift, the frequency of the whistle reaching him might have gone beyond the audible limit of 20 kHz and he did not hear it. Experiments showed that the whistle emits a sound with frequency close to 16 kHz. Assuming that the claim of the driver is true, how fast was he driving the car ? Take the speed of sound in air to be $330m{s}^{-1}$. Is this speed practical with today's technology ?

Q. A train standing at a certain distance from a railway platform is blowing a whistle of frequency 500 Hz. If the speed of sound is $340m{s}^{-1}$, the frequency and wavelength of the sound of the whistle heard by a man running towards the engine with a speed of $10m{s}^{-1}$ respectively are

1. 500 Hz, 0.7 m
2. 500 Hz, 0.68 m
3. 486 Hz, 0.7 m
4. 515 Hz, 0.68 m

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

[Assume, medium is stationary ]

1. $\frac{18}{19}$
2. $\frac{1}{2}$
3. $2$
4. $\frac{19}{18}$

Q.

Two identical tuning forks vibrating at the same frequency 256 Hz are kept fixed at some distance apart. A listener runs between the forks at a speed of 3.0 m/s so that he approaches one tuning fork while receding from the other (figure). Find the beat frequency observed by the listener. Speed of sound in air = 332 m/s.

1. 4.6

2. 5.6

3. 5.2

4. 4.2

Q.

A bullet passes past a person at a speed of 220 m/s. Find the fractional change in the frequency of the whistling sound heard by the person as the bullet crosses the person. Speed of sound in air =330 m/s.

Q. A police car with a siren of frequency $9\text{kHz}$ is moving with uniform velocity $75\text{km/hr}$ towards a tall building which reflect the sound wave. The speed of sound in air is $340\text{m/s}$. The frequency of siren heard by the car driver is

1. $8\text{kHz}$
2. $10\text{kHz}$
3. $2\text{kHz}$
4. $14\text{kHz}$

Q. A source moves towards a stationary observer with speed $40\text{m/s}$. The source sounds a whistle and its frequency registered by the observer is $n_1$. If the source's speed is reduced to $20\text{m/s}$, the frequency registered is $n_2$. If the speed of sound is $340\text{m/s}$, then the ratio $\frac{f_1}{f_2}$is

[Assume, medium is stationary]

1. $\frac{5}{4}$
2. $\frac{15}{16}$
3. $\frac{16}{15}$
4. $\frac{4}{5}$

Q. Two sound sources shown in the figure vibrate in phase. By moving $S_1$ along $P{S}_1$ consecutive minima are heard when $L_1-L_2$ has values, $20\text{cm},60\text{cm}$ and $100\text{cm}$. If the frequency of sound source is $\frac{1700}{n}\text{Hz}$. Then find the value of $n$ : [Speed of sound is $340\text{m/s}$ ]

Q. Two bikes moving in opposite directions approach each other with speed of $40\text{m/s}$ and $20\text{m/s}$ respectively. The first bike sounds a horn having a frequency $380\text{Hz}$. The frequency heard by the biker of the second bike is
[velocity of sound in air is $340\text{m/s}]$

1. $450\text{Hz}$
2. $460\text{Hz}$
3. $456\text{Hz}$
4. $465\text{Hz}$

Q.

A traffic policeman standing on a road sounds a whistle emitting the main frequency of 2.00 kHz. What could be the appparent frequency heard by a scooter-driver approaching the policeman at a speed of 36.0 km/h ? Speed of sound in air = 340 m/s.

Q. A fixed source of sound emitting a certain frequency appears as $f_a$ when the observer is approaching the source with speed $v_0$ and $f_r$ when the observer recedes from the source with the same speed. The frequency of the source is

1. $\frac{f_r+f_a}{2}$
2. $\frac{f_r-f_a}{2}$
   
3. $\sqrt{f_a{f}_r}$
   
4. $\frac{2f_r{f}_a}{f_r+f_a}$

Q.

An observer moves towards a stationary source of sound, with a velocity ${\frac{1}{5}}^{th}$ of the velocity of sound. What is the $\%$ increase in the apparent frequency?

1. Zero

2. $0.5\%$

3. $5\%$

4. $20\%$

Q. A train is travelling at $120\text{kmph}$ and blows a whistle of frequency $1000\text{Hz}$. The frequencies of the sound heard by a stationary observer, if the train is approaching him and moving away from him are:

[(Velocity of sound in air $=330\text{m/s}$ and assume medium is stationary]

1. $91\text{Hz}-111\text{Hz}$
2. $908\text{Hz}-1112\text{Hz}$
3. $82\text{Hz}-110\text{Hz}$
4. $820\text{Hz}-1080\text{Hz}$

Q.

A person riding a car moving at 72 km/h sounds a whistle emitting a wave of frequency 1250 Hz. What frequency will be heard by another person standing an the road (a) in front of the car (b) behind the car ? Speed of sound in air = 340 m/s.

Q. A driver in a car, approaching a vertical wall notices that the frequency of his car horn, has changed from $440\text{Hz}$ to $480\text{Hz}$ when it gets reflected from the wall. If the speed of sound in air is $345\text{m/s}$, then speed of the car is

1. $\text{54 km/hr}$
2. $\text{36 km/hr}$
3. $\text{18 km/hr}$
4. $\text{24 km/hr}$

Q. A source of sound emits sound waves at frequency $f_0$. It is moving towards an observer with fixed speed $v_s$ ($v_s<v$, where v is the speed of sound in air). If the observer were to move towards the source with speed $v_0$, one of the following two graphs (A and B) will give the correct variation of the frequency f heard by the observer as $v_0$ is changed.

The variation of f with $v_0$ is given correctly by:

1. graph A with slope = $\frac{f_0}{(v+v_s)}$
2. graph A with slope = $\frac{f_0}{(v-v_s)}$
3. graph B with slope = $\frac{f_0}{(v-v_s)}$
4. graph B with slope = $\frac{f_0}{(v+v_s)}$

Q. A source of sound S has a frequency $f$. Wind is blowing from source to observer O with velocity $u$. If speed of sound with respect to air is $C$, the wavelength of sound detected by O is:

1. $\frac{C+u}{f}$
2. $\frac{C(C+u)}{(C-u)f}$
3. $\frac{C-u}{f}$
4. $\frac{C}{f}$

Q. A source of sound producing sound of frequency $f$ is at rest. An observer starts moving toward it at $t=0$ with a constant small acceleration. Which of the following graph best represents the variation of observed frequency $f^{\prime }$ recorded by the observer with time $t$?

1. 
2. 
3. 
4.