Q. No.A source of sound moves towards an observer.
1. The frequency of the source is increased
2. The velocity of sound in the medium is increased
3 The wavelength of sound in the medium towards the observer is decreased
4. The amplitude of vibration of the particles is increased
A listener is at rest with respect to the source of sound. A wind starts blowing along the line joining the source and the observer. Which of the following quantities do not change?
(a) Frequency
(b) Velocity of sound
(c) Wavelength
(d) Time period
Choose the correct option:
1. (a) and (b)
2. (b) and (c)
3. (c) and (d)
4. (a) and (d)
A small source of sound moves on a circle as shown in the figure and an observer is sitting at O. Let v1, v2, v3 be the frequencies heard when the source is at A, B and C respectively.
1. v1 > v2 > v3
2. v1 = v2 > v3
3. v2 > v3 > v1
4. v1 > v3 > v2
The change in frequency due to Doppler effect does not depend on
1. the speed of the source
2. the speed of the observer
3. the frequency of the source
4. separation between the source and the observer
The engine of a train sounds a whistle at frequency v. The frequency heard by a passenger is
1. > v
2. < v
3. \(=\frac{1}{v}\)
4. =v
| Assertion (A): | When sound waves are emitted by a moving source, their wavelength decreases in the direction of motion of the source compared to when the source is at rest. |
| Reason (R): | This is due to the fact that the speed of sound in the medium changes due to the motion of the source. |
| 1. | (A) is true but (R) is false. |
| 2. | (A) is false but (R) is true. |
| 3. | Both (A) and (R) are true and (R) is the correct explanation of (A). |
| 4. | Both (A) and (R) are true but (R) is not the correct explanation of (A). |
| Assertion (A): | Intensity of sound wave does not change when the listener moves towards or away from the stationary source. |
| Reason (R): | The motion of the listener causes the apparent change in wavelength. (assuming the source is at rest.) |
| 1. | Both (A) and (R) are true and (R) is the correct explanation of (A). |
| 2. | Both (A) and (R) are true but (R) is not the correct explanation of (A). |
| 3. | (A) is true but (R) is false. |
| 4. | Both (A) and (R) are false. |
A rocket is moving at a speed of \(200\) ms–1 towards a stationary target. While moving, it emits a wave of frequency \(1000\) Hz. Some of the sound reaching the target gets reflected back to the rocket as an echo. The frequency of the sound as detected by the target and the frequency of the echo as detected by the rocket respectively are: (speed of sound = \(330\) m/s)
1. \(4080\) Hz and \(2540\) Hz
2. \(1000\) Hz and \(1000\) Hz
3. \(2540\) Hz and \(4080\) Hz
4. \(2540\) Hz and \(2540\) Hz
An observer moves towards a stationary source of sound with a speed of 1/5th 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. 1.2f, 1.2λ
2. 1.2f, λ
3. f, 1.2λ
4. 0.8f, 0.8λ
A whistle revolves in a circle with an angular speed ω = 20 rad/sec using a string of length 50 cm. If the frequency of sound from the whistle is 385 Hz, then what is the minimum frequency heard by an observer which is far away from the centre? (Vsound = 340 m/s)
1. 385 Hz
2. 374 Hz
3. 394 Hz
4. 333 Hz
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