Sound

11.	Speed of sound in different medium-
	A. Is equal
	B. Varies and highest in solid
	C. Varies and lowest in liquid
	D. Varies and highest in gas Option “B” is correct. The speed of sound is maximum in a solid medium because the molecules are closer together, allowing sound waves to travel more quickly through it.The speed of sound is maximum in solid, then it is in liquid and it is the slowest in gases. The speed of sound remains almost the same for all frequencies in a given medium under the same physical conditions.

12.	In which medium will the speed of sound be maximum at a temperature of about 20°C?
	A. Air
	B. Granite
	C. Water
	D. Iron Option “D” is correct. The correct answer is Aluminium. In aluminium, the speed of sound is maximum at 25°C. Out of the following, the speed of sound is maximum in Aluminium. The speed of sound in Aluminium at 25 deg Celsius is around 6420 m/s. The matter or substance through which sound is transmitted is called a medium. It can be solid, liquid, or gas. Sound is a mechanical wave and needs a material medium for its propagation. It cannot travel through a vacuum. Sound travels at different speeds depending on what it is travelling through. Of the three mediums (gas, liquid, and solid) sound waves travel the slowest through gases, faster through liquids, and fastest through solids. Temperature also affects the speed of sound. For Example, at 0° Celcius, sound travels through air at 331 meters per second, but, at 20°C, room temperature, sound travels at 343 meters per second.

13.	Which one of the following statements is not correct
	A. The velocity of sound in air increases with increase in temperature
	B. The speed of sound in air does not depend on the pressure
	C. The velocity of sound in air decreases as the humidity increases.
	D. The velocity of sound in air is not affected by the change in amplitude and frequency. Option “C” is correct

14.	Speed of sound in air__
	A. Decreases with increase in temperature
	B. Increases with decrease in temperature
	C. Does not depend on temperature
	D. Decreases with decrease in temperature Option “D” is correct. Temperature is also a condition that affects the speed of sound. Heat, like sound, is a form of kinetic energy. Molecules at higher temperatures have more energy, thus they can vibrate faster. Since the molecules vibrate faster, sound waves can travel more quickly. The speed of sound in room temperature air is 346 meters per second. This is faster than 331 meters per second, which is the speed of sound in air at freezing temperatures. Hence, the speed of sound increases with temperature.

15.	Assertion (A) : The steam coming out of the train engine is seen first and heard after the whistle. Reason (R): The speed of sound waves is very less as compared to the speed of light waves.
	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 “A” is correct.

16.	Supersonic planes fly
	A. At the speed of sound
	B. Subsonic
	C. Faster than the speed of sound
	D. At the speed of light Option “C” is correct. A supersonic aircraft is an aircraft capable of supersonic flight, that is, flying faster than the speed of sound.

17.	The ratio of the speed of a body to the speed of sound is called:
	A. Durability number
	B. Lamellas number
	C. Octane number
	D. Mach number Option “D” is correct. Mach number of a body is the ratio of its speed to that of local sound. The local speed of sound, and thereby the Mach number, depends on the condition of the surrounding medium, in particular the temperature and pressure. The Mach number is primarily used to determine the approximation with which a flow can be treated as an incompressible flow.

18.	Mach number is used in connection with the speed of
	A. sound
	B. watercraft
	C. aircraft
	D. spacecraft Option “C” is correct. Mach number is defined as the ratio of speed of object to the speed of sound in air. Objects having Mach number greater than 1 are termed as supersonic objects. Mach number is generally used to define the speed of aircrafts.

19.	Noise or noise pollution is measured by-
	A. Hertz
	B. Fathoms
	C. Decibels
	D. Nanometres Option “C” is correct. Noise pollution is caused by the incresed levels of sound. It is measured in decibels (dB) A. Hertz is the SI unit of frequency. B. Fathom is the unit of length in the imperial and the US customary systems. 1 fathom equals to 1.8 metres. C. Nanometre is the unit of length. 1 nm equals to 1X10^-9 m. D. Decibels is the unit of sound. 1 dB equals 10 times the common logarithm of the power ratio.

20.	The sound intensity of normal conversation is
	A. 20-30 decibels
	B. 30-60 decibels
	C. 50-60 decibels
	D. 90-100 decibels Option “B” is correct. The intensity of sound is measured in decibels (dB). 30 dB – 60 dB is for normal conversation, 90 dB- truck traffic, 100 dB – from construction site, 115 dB – sandblasting, loud rock concert, auto horn, 120 dB – 150 dB from jet engine. The undesired high level of sound is called as noise which causes pain and can injure unprotected ears. Thus, the correct answer is option B. So, the correct answer is ‘Decibels’

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