Sound (Physics)

Total Questions: 56

31. Which one of the following is the effect of the flight of supersonic jet? [M.P.P.C.S. (Pre) 1993]

Correct Answer: (c) Depletion in ozone layer
Solution:

A jet engine is a machine for turning fuel into thrust. The thrust is produced by action and reaction also known as Newton's third law of motion.

The force (action) of the exhaust gases pushing backward produces an equal and opposite force (reaction) called thrust, that powers the vehicle forward. Those jet planes which are able to fly faster than the speed of sound are called supersonic jet planes.

Jet engines are responsible for depletion of ozone. Flying at stratosphere height, they emits nitrogen oxide which has the potential to destroy significant quantities of ozone in the stratosphere.

32. Assertion (A): A jet aircraft moving at mach number equal to 1 travels faster at an altitude of 15 km than while moving at mach number equal to 1 near the sea level. [I.A.S. (Pre) 2007]

Reason (R): The velocity of sound depends on the temperature of the surrounding medium.

Code:

Correct Answer: (d) (A) is false, but (R) is true.
Solution:

Speed of supersonic bodies is indicated by the mach number. The mach number of an airplane is the ratio of the speed of a body to the speed of sound in a particular medium, usually the atmosphere of the earth.

As we know that the speed of sound increases with increase in temperature, the speed of sound at higher altitude (15 km above sea level) decreases due to decrease in temperature.

Thus, statement (1) is false. The velocity of a sound wave travelling through the air does not vary with the air pressure but depends on the temperature of the air. Thus reason (R) is correct.

33. Decibel is used to measure: [M.P.P.C.S. (Pre) 2005]

Correct Answer: (c) Sound in atmosphere
Solution:

'Decibel' (dB) is used to measure the intensity of sound in atmosphere.

34. Decibel unit is used to measure: [U.P.P.C.S. (Mains) 2011]

Correct Answer: (b) Sound intensity
Solution:

'Decibel' (dB) is used to measure the intensity of sound in atmosphere.

35. Decibel unit is used to measure the: [Uttarakhand Lower Sub. (Pre) 2010]

Correct Answer: (c) Intensity of sound
Solution:

'Decibel' (dB) is used to measure the intensity of sound in atmosphere.

36. What is the decibel level of sound produced by two persons in conversation? [R.A.S/R.T.S. (Pre) 2003]

Correct Answer: (c) About 30 Decibel
Solution:

Intensity of Sound (in Decibel)

Source of SoundIntensity (in decibel)
Whisper15–20
Normal Conversation30–60
Anger Conversation70–80
Truck / Motorcycle90–95
Instrument Factory100–110
Jet Plane140–150

37. A noise level of 100 decibel would correspond to: [I.A.S. (Pre) 2000]

Correct Answer: (d) Noise from a machine shop
Solution:

Intensity of Sound (in Decibel)

Source of SoundIntensity (in decibel)
Whisper15–20
Normal Conversation30–60
Anger Conversation70–80
Truck / Motorcycle90–95
Instrument Factory100–110
Jet Plane140–150

38. Which of the following represents the decibel level of rustling of tree leaves in normal circumstance? [U.P.P.C.S. (Pre) 2018]

Correct Answer: (b) 20 dB
Solution:

In normal circumstances, the decibel level of the rustling of tree leaves is about 20 decibel. A decibel (dB) is 1/10 of a bel, which is a unit of sound intensity named in the honour of Alexander Graham Bell.

39. As per the WHO, the safe noise level for a city is- [U.P.P.C.S. (Mains) 2010]

Correct Answer: (a) 45 dB
Solution:

As per the WHO the safe noise level for a city is 45 decibel (dB).

40. In the context of permissible noise levels match List-I with List-II and select the answer from the code given below the lists. [U.P.P.C.S. (Pre) 2022]

AreaPermissible Noise Level
A. Residential area1. 50 dB
B. Silent zone2. 55 dB
C. Industrial area3. 65 dB
D. Commercial area4. 70 dB

Code:

ABCD
(a)3421
(b)1234
(c)2134
(d)2143
Correct Answer: (d)
Solution:

As per the Noise Pollution (Regulation and Control) Rules, 2000 of India, ambient air quality standards in respect of noise (permissible noise levels) in given areas are as follows:

Area

Permissible Noise Level (dB)
Day Time

Night Time

Residential area

55

45

Silent zone

50

40

Industrial area

75

70

Commercial area

65

55