A whistle revolves in a circle with an angular speed of 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 in the same plane ? (v = 340 m/s)

(1) 333 Hz

(2) 374 Hz

(3) 385 Hz

(4) 394 Hz

Concept Questions :-

Doppler effect
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A Siren emitting sound of frequency 800 Hz is going away from a static listener with a speed of 30 m/s, frequency of the sound to be heard by the listener is (take velocity of sound as 330 m/s)

(1) 733.3 Hz

(2) 644.8 Hz

(3) 481.2 Hz

(4) 286.5 Hz

Concept Questions :-

Doppler effect
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A car sounding a horn of frequency 1000 Hz passes an observer. The ratio of frequencies of the horn noted by the observer before and after the passing of the car is 11 : 9. If the speed of sound is v, the speed of the car is 

(1) 110v

(2) 12v

(3) 15v

(4) v

Concept Questions :-

Doppler effect
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A small source of sound moves on a circle as shown in the figure and an observer is standing on O. Let n1, n2 and n3 be the frequencies heard when the source is at A, B, and C respectively. Then

(1) n1 > n2 > n3

(2) n2 > n3 > n1

(3) n1 = n2 > n3

(4) n2 > n1 > n3

Concept Questions :-

Doppler effect
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A source emits a sound of frequency of 400 Hz, but the listener hears it to be 390 Hz. Then 

(1) The listener is moving towards the source

(2) The source is moving towards the listener

(3) The listener is moving away from the source

(4) The listener has a defective ear

Concept Questions :-

Doppler effect
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A person carrying a whistle emitting continuously a note of 272 Hz is running towards a reflecting surface with a speed of 18 km/hour. The speed of sound in air is 345 ms–1. The number of beats heard by him is 

(1) 4

(2) 6

(3) 8

(4) 3

Concept Questions :-

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If the pressure amplitude in a sound wave is tripled, then the intensity of sound is increased by a factor of

(1) 9

(2) 3

(3) 6

(4) 3

Concept Questions :-

Energy of waves
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If the amplitude of the sound is doubled and the frequency reduced to one-fourth, the intensity of sound at the same point will be :

(1) Increased by a factor of 2

(2) Decreased by a factor of 2

(3) Decreased by a factor of 4

(4) Unchanged

Concept Questions :-

Energy of waves
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Intensity level of a sound of intensity I is 30 dB. The ratio II0 is (Where I0 is the threshold of hearing) 

(1) 3000

(2) 1000

(3) 300

(4) 30

Concept Questions :-

pressure wave in sound
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Quality of a musical note depends on :

(1) Harmonics present

(2) The amplitude of the wave

(3) Fundamental frequency

(4) The velocity of sound in the medium

Concept Questions :-

Standing waves
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