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Physics MCQs for NEET — Practice Questions with Answers

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If source and observer both are relatively at rest and if speed of sound is increased then frequency heard by observer will :

Increases

Decreases

Can not be predicted

Will not change

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Explanation

No change in frequency.

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A source and an observer move away from each other with a velocity of 10 m/s with respect to ground. If the observer finds the frequency of sound coming from the source as 1950 Hz, then actual frequency of the source is (velocity of sound in air = 340 m/s)

1950 Hz

2068 Hz

2132 Hz

2486 Hz

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Explanation

$n^{'} = n (\frac{v - v_{O}}{v + v_{S}}) = n (\frac{340 - 10}{340 + 10}) = 1950$

$\Rightarrow n = 2068 H z$

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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)

333 Hz

374 Hz

385 Hz

394 Hz

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Explanation

Minimum frequency will be heard, when whistle moves away from the listener.

$n_{\min} = n (\frac{v}{v + v_{s}})$ where $v = r ω = 0 .5 \times 20 = 1 m / s$

⇒ $n_{\min} = 385 (\frac{340}{340 + 10}) = 374 H z .$

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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)

733.3 Hz

644.8 Hz

481.2 Hz

286.5 Hz

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Explanation

$n' = n (\frac{v}{v + v_{S}}) = 800 (\frac{330}{330 + 30}) = 733 .33 H z$ .

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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

$\frac{1}{10} v$

$\frac{1}{2} v$

$\frac{1}{5} v$

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

The listener is moving towards the source

The source is moving towards the listener

The listener is moving away from the source

The listener has a defective ear

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Explanation

Since apparent frequency is lesser than the actual frequency, hence the relative separation between source and listener should be increasing.

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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

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Explanation

According the concept of sound image

$n' = \frac{v + v_{person}}{v - v_{person}} . 272 = \frac{345 + 5}{345 - 5} \times 272 = 280$ Hz

$Δ n =$ Number of beats =280 – 272 = 8 Hz

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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

$\sqrt{3}$

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Explanation

Intensity ∝ (Amplitude)²

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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 :

Increased by a factor of 2

Decreased by a factor of 2

Decreased by a factor of 4

Unchanged

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Explanation

$I = 2 π^{2} a^{2} n^{2} v ρ$ ⇒ $I \propto a^{2} n^{2}$ ⇒ $\frac{I_{1}}{I_{2}} = {(\frac{a_{1}}{a_{2}})}^{2} \times {(\frac{n_{1}}{n_{2}})}^{2}$

$= {(\frac{1}{2})}^{2} \times {(\frac{1}{1 / 4})}^{2} \Rightarrow I_{2} = \frac{I_{1}}{4}$

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Intensity level of a sound of intensity I is 30 dB. The ratio $\frac{I}{I_{0}}$ is (Where I₀ is the threshold of hearing)

3000

1000

300

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Explanation

$L = 10 \log_{10} (\frac{I}{I_{0}}) = 30 \Rightarrow \frac{I}{I_{0}} = 10^{3}$

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