Apparent Frequency:Change in Wavelength

Q.

An observer moves towards a stationary source of sound of frequency n. The apparent frequency heard by him is 2n. If the velocity of sound in air is 332 m/sec, then the velocity of the observer is

1. 166 m/sec

2. 664 m/sec

3. 332 m/sec

4. 1328 m/sec

Q. Speed of sound wave is v. If a reflector moves towards a stationary source emitting waves of frequency f with speed u, the wavelength of reflected waves will be

1. 
2. 
3. 
4. 

Q. When source and detector are stationary but the wind is blowing at speed , the apparent wavelength ${\lambda }^{\prime }$ on the wind side is related to actual wavelength $\lambda$ by [take speed of sound in air as v]

1. 
2. 
3. 
4. 

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

1. 500 Hz
2. 525 Hz
3. 512 Hz
4. 485 Hz

Q. A source of sound emits waves of frequency $f_0=1200\text{Hz}.$ The source is travelling with a velocity $v_1=30\text{m/s}.$ towards east. There is a large reflecting surface in front of the source which is travelling with a velocity $v_2=60\text{m/s}$ towards west. Speed of sound in air is $v=330\text{m/s}$. Find the ratio of wavelength $\left({\lambda }_1\right)$ of sound emitted by source when travelling towards the reflecting surface to the wavelength $\left({\lambda }_2\right)$ of sound approaching the source after getting reflected.

1. $\frac{9}{13}$
2. $\frac{7}{15}$
3. $\frac{13}{9}$
4. $\frac{15}{7}$

Q. An observer moves towards a stationary source of sound with a speed (1/5)th of the speed of sound. The wavelength and frequency of the source emitted are λ and f, respectively. The apparent frequency and wavelength recorded by the observer are, respectively,

1. 
2. 
3. 
4. 

Q. There are four possible relative motions between the source of a sound and the listener
1) source moves towards stationary listener
2) source moves away from stationary listener
3) listener moves towards stationary source
4) listener moves away from stationary source
In which of the cases, the change in frequency is the same. The magnitude of velocity of source or listener being the same.

1. 1 and 2
2. 2 and 3
3. 1 and 4
4. 3 and 4

Q.

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

Q. A bat moving at $10m{s}^{-1}$ towards a wall sends a sound signal of 8000 Hz towards it. On reflection it hears a sound of frequency f. The value of f in Hz is close to (speed of sound $=320m{s}^{-1}$)

1. 8258
2. 8000
3. 8516
4. 8424

Q. The frequency changes by 10% as a sound source approaches a stationary observer with constant speed vs. what would be the percentage change in frequency as the source recedes the observer with the same speed. Given that $v_s<v$. (v = speed of sound in air)

1. 14.3 %
2. 20 %
3. 10.0 %
4. 8.5 %

Q. A source of sound moves towards a stationary listener with the velocity of sound. If the actual frequency of the sound produced by the source be $f,$ then change in frequency will be

1. $\frac{f}{4}$
2. $\frac{f}{2}$
3. $f$
4. none of these

Q. Speed of sound in air is 320 m/s. A pipe closed at one ends has a length of 1 m and there is another pipe open at both ends having a length of 1.6 m. Neglecting and corrections, both the air columns in the pipes can resonate for sound of frequency :

1. 80 Hz
2. 240 Hz
3. 320 Hz
4. 400 Hz

Q. An observer moves towards a stationary source of sound with one-fifth of the speed of sound. The wavelength and frequency of the source omitted are $\lambda$ and $f$ respectively. The apparent frequency and wavelength recorded by the observer are

1. $0.85f,0.8\lambda$
2. $1.2f,1.2\lambda$
3. $1.2f,\lambda$
4. $f,1.2\lambda$

Q. A whistle whose air column is open at both ends has a fundamental frequency of $5100Hz$. If the speed of sound in air is $340{ms}^{-1}$, the length of the whistle, in $cm$, is:

1. $10/3$
2. $20/3$
3. $5$
4. $5/3$

Q. Let $V_s$ be the speed of the source emitting sound waves, $n$ the actual frequency of the source of sound, $V$ the speed of the sound in the medium and $n_1$ the frequency of sound waves as perceived by a stationary observer to whom the source of sound is approaching. The formula for calculating $n_1$ is :

1. $n_1=n(1-V_s/V)$
2. $n_1=n\left(\frac{V}{V-V_s}\right)$
3. $n_1=n/(1+V_s/V)$
4. $n_1=n$

Q. The frequency changes by $10\%$ as a sound source approaches a stationary observer with constant speed $V_S.$ What would be the percentage change in frequency as the source recedes the observer with the same speed. Given that $V_S$< V

1. $1.5\%$
2. $4.5\%$
3. $10.5\%$
4. $8.5\%$

Q. A sound source is moving towards a stationary observer with one-tenth of the speed of sound. The ratio of apparent to real frequency is

1. $(11/10{)}^2$
2. $(9/10{)}^2$
3. $10/9$
4. $11/10$

Q. A sound is moving towards a stationary listener with $\frac{1}{10}th$ of the speed of sound.The ratio of apparent to real frequency is

1. $(\frac{10}{9}{)}^2$
2. $\frac{10}{9}$
3. $(\frac{11}{10}{)}^2$
4. $\frac{11}{10}$

Q. A wall is moving with velocity u and a source of sound moves with velocity u/2 in the same direction as shown in the figure. Assuming that the sound travels with velocity 10u, the ratio of incident sound wavelength on the wall to the reflected sound wavelength by the wall is equal to

1. 9:11
2. 11:9
3. 4:5
4. 5:4

Q. An observer moves towards a stationary source of sound with a speed $1/5^{th}$ of the speed of sound. The wavelength and frequency of the source emitted are $\lambda$ and $f$ respectively. The apparent frequency and wavelength recorded by the observer are respectively :

1. $1.2f,1.2\lambda$
2. $f,1.2\lambda$
3. $1.2f,\lambda$
4. $0.8f,0.8\lambda$

Q. When source and detector are stationary but the wind is blowing at speed , the apparent wavelength ${\lambda }^{\prime }$ on the wind side is related to actual wavelength $\lambda$ by [take speed of sound in air as v]

1. ${\lambda }^{\prime }=\lambda$
2. ${\lambda }^{\prime }=\frac{v_w}{v}\lambda$
3. ${\lambda }^{\prime }=\frac{v_w+v}{v}\lambda$
4. ${\lambda }^{\prime }=\frac{v}{v-v_w}\lambda$

Q. A source of sound $A$ emit waves of frequency 1800 Hz is falling towards ground with a terminal speed $v$. The observer $B$ on the ground directly beneath the source receives waves of frequency $2150Hz$. The source $A$ receives waves, reflected from ground, of frequency nearly :
(Speed of sound $=$ 343 m/s)

1. $2150Hz$
   
2. $2500Hz$
   
3. $1800Hz$
   
4. $2400Hz$
   

Q. At what speed should a source of sound move so that stationary observer finds the apparent frequency equal to half of the original frequency [RPMT 1996]

1. $\frac{v}{2}$
2. $\frac{v}{4}$
3. $2v$
4. $v$

Q. In given gas, sound travels at a speed of $300metres/sec$. Find the order of the root-mean-square velocity of the molecules of this gas:

1. $41m/sec$
2. $410m/sec$
3. $4100m/sec$
4. $4m/sec$

Q. Open organ pipe when filled with water partially then its fundamental frequency

1. Increases
2. Decreases
3. Remain same
4. Either $a$ or $b$

Q. A fixed source of sound emitting a certain frequency appears as $f_a$ when the observer is approaching the source with speed v and frequency $f_r$ 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. $\frac{f_a-f_r}{2}$
3. $\sqrt{f_a{f}_r}$
4. $\frac{2f_r{f}_a}{f_r+f_a}$

Q. If the wavelength of reflected waves is $\frac{x}{10}m$. Find $x$

Q. A vehicle, with a horn of frequency n is moving with a velocity of 30 m/s in a direction perpendicular to the straight line joining the observer and the vehicle. The observer perceives the sound to have a frequency $(n+n_1)$. If the velocity of sound in air is 300 m/s, then

1. $n_1=10n$
2. $n_1=0$
3. $n_1=0.1n$
4. $n_1=-0.1n$

Q. If the wavelength of sound in air incident on reflecting surface is $\frac{x}{10}m$. Find $x$.

Q. A driver in an automobile hears a siren behind her and pulls over to let a fire engine pass. The sound of the siren changes as the fire truck approaches, is beside her, and passes her. Which of the following is/are true under these conditions?
I. The pitch of the sound increases as the fire truck approaches.
II. The wavelength of the sound increases as the fire truck approaches.
III. The wavelength of the sound increases after the fire truck passes.

1. I only
2. II only
3. I and III only
4. II and III only
5. I, II, and III only