Sound

41.	Which of the following is a sound of frequency 20 Hz to 20,000 Hz?
	A. Ultrasound
	B. Infrasonics
	C. Hypersonic
	D. Audible sound Option “D” is correct. Sound waves between 20Hz to 20,000Hz are audible sounds.

42.	The relation between velocity (v), frequency (n) and wavelength (λ) of a wave is given –
	A. v=nλ
	B. v=n/min
	C. v=n-min
	D. v=n+l Option “A” is correct.

43.	A big explosion on the Moon cannot be heard on the Earth. Which of the following statement is the correct explanation for this?
	A. The explosion produces high frequency sound waves which are inaudible.
	B. Sound waves require a material medium for propagation.
	C. Sound waves are absorbed in the moon’s atmosphere.
	D. Sound waves are absorbed in the earth’s atmosphere. Option “B” is correct. Sound waves require a material medium for propagation. Sound waves require a material medium for propagation. There is vacuum between Earth and Moon. In vacuum, there are no molecules (material medium) through which sound waves produced by the mechanical vibrations of the explosion can propagate. Therefore, big explosion on the Moon cannot be heard on the Earth.

44.	Bats can fly in the dark because-
	A. He can see clearly in the dark
	B. The pupil of his eye is very large
	C. They are guided by ultrasonic waves produced by them
	D. Any bird can do that Option “C” is correct.

45.	Assertion (A) : Bats fly at night without colliding with any object Reason (R) : Infrasonic waves are emitted by bats.
	A. Both A and R are correct and R is the correct explanation of A.
	B. Both A and R are correct, but R is not the correct explanation of A.
	C. A is correct but R is incorrect.
	D. A is wrong, but R is correct. Option”C” is correct

46.	The velocity of sound in air is taken as 332 m/s. If the pressure of a gas is taken double, then what will be the velocity of sound in air?
	A. 664 m/s
	B. 332 m/s
	C. 166 m/s
	D. 100 m/s Option “B” is correct. speed of sound will vary with variation in temperature, humidity and elasticity of a medium. Whereas, The velocity of sound is independent of the pressure of the gas because as pressure increases, the density of gas also increases And thus in this case velocity of sound will remain the same although its pressure becomes double

47.	Sound travels fastest in the following substances
	A. In steel
	B. In the air
	C. In a vacuum
	D. In water Option “A” is correct. The speed of sound depends on the density of the medium in which it is travelling and increases when the density increases. For example, in solids sound travels faster than in liquid and even faster than in gas, and the density is highest in solids, lower in liquids and lowest in gas. Hence, sound travels fastest in solids.

48.	The thunder of a cloud is heard later compared to flash of lightning because :
	A. speed of sound is greater than speed of light
	B. speed of light is greater than speed of sound
	C. speed of sound is the same as speed of light
	D. sound is obscured by the rain Option “D” is correct. The light travels much faster than the sound. The thunder travels at the speed of sound while the light from the flash travels at the speed of the light. The speed of light is about 340 m/s through air, while the speed of light is about 300 million m/s.So the thunder is heard after the flash of lightening.

49.	The approximate speed of sound in air is equal to :
	A. 33.2 $m s^{-1}$
	B. 220 $m s^{-1}$
	C. 332 $m s^{-1}$
	D. 232 $m s^{-1}$ Option “C” is correct.

50.	Sound waves travel fastest__
	A. In solids
	B. In liquid
	C. Gas
	D. In the wind Option “A” is correct.

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