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Waves MCQs for NEET — Physics Questions with Answers

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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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Quality of a musical note depends on :

Harmonics present

The amplitude of the wave

Fundamental frequency

The velocity of sound in the medium

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Explanation

The quality of sound depends upon the number of harmonics present. Due to different number of harmonics present in two sounds, the shape of the resultant wave is also different.

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If T is the reverberation time of an auditorium of volume V then :

$T \propto \frac{1}{V}$

$T \propto \frac{1}{V^{2}}$

$T \propto V^{2}$

$T \propto V$

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Explanation

Reverberation time $T = \frac{k V}{α S}$ ⇒ T ∝ V.

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The intensity level due to two waves of the same frequency in a given medium are 1 bel and 5 bel. Then the ratio of amplitudes is :

1 : 4

1 : 2

1 : 10⁴

1 : 10²

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Explanation

By using $L = \log_{10} \frac{I}{I_{0}}$

$L_{2} - L_{1} = \log_{10} \frac{I_{2}}{I_{0}} - \log_{10} \frac{I_{1}}{I_{0}}$

$5 - 1 = \log_{10} \frac{I_{2}}{I_{1}}$ ⇒ $4 = \log_{10} \frac{I_{2}}{I_{1}}$ ⇒ $\frac{I_{2}}{I_{1}} = 10^{4}$

⇒ $\frac{a_{2}^{2}}{a_{1}^{2}} = 10^{4}$ ⇒ $\frac{a_{2}}{a_{1}} = \frac{10^{2}}{1}$ ⇒ $\frac{a_{1}}{a_{2}} = \frac{1}{10^{2}}$

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Of the following the one which emits the sound of a higher pitch is :

Mosquito

Lion

Man

Woman

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Explanation

Pitch of mosquito is higher among all given options.

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A point source emits sound equally in all directions in a non-absorbing medium. Two points P and Q are at distances of 2m and 3m respectively from the source. The ratio of the intensities of the waves at P and Q is :

9 : 4

2 : 3

3 : 2

4 : 9

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Explanation

Intensity $\propto \frac{1}{{(Distance)}^{2}}$

⇒ $\frac{I_{1}}{I_{2}} = {(\frac{d_{2}}{d_{1}})}^{2} = {(\frac{3}{2})}^{2} = \frac{9}{4}$ .

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