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(Aktu Btech) Air and Noise Pollution Control Important Unit-5 Noise Pollution

We’re here to look at the most essential important, repeated questions and notes regarding Air and Noise Pollution Control that might appear in your upcoming exams, such as Btech and others. Unit-5 Noise Pollution

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Important Questions For Air and Noise Pollution Control:
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Q1. Define noise and explain as to how and why it should be regarded as an environmental pollutant?


  • 1. Sound in the environment is created by vibrations in the air (or some other medium) reaching human ears and stimulating a hearing sense.
  • 2. Noise is created when a sound becomes too loud, disagreeable, or unwelcome.
  • 3. Because unwanted sound (i.e., noise) has various negative consequences on human body health, it might be classified as an environmental pollutant.
  • 4. Noise is listed as an air contaminant under India’s Air (Prevention and Control of Pollution) Act of 1981.
  • 5. Noise can thus be characterised as an undesired sound pollution that causes negative physiological and psychological impacts in an individual by interfering with one’s social activities such as work, rest, recreation, sleep, and so on.
  • 6. It should also be highlighted that when noise is defined as undesirable sound, it becomes a subjective definition, independent of the loudness of the sound, because a certain loudness may not be enjoyed by one person while being quite pleasant to another.
  • 7. A particular sound liked by one individual can thus become a source of noise pollution for another.
  • 8. Noise, as an air pollution, differs from other pollutants in that it is transient in nature, rather than a continual or persistent phenomena.
  • 9. When noise pollution stops, the environment is clear of this pollutant, unlike other pollutants such as gases and particulate matter, which persist in the air after entering the atmosphere. 

Q2. Discuss the behaviour of point, line and plane sound sources. 

Ans. A. Point Sources: 

  • 1. The SPL from an ideal point source radiator decreases at a rate of 6 dB per distance doubling.
  • 2. The inverse square of the distance reduces the intensity of sound from a point source. This is referred to as the inverse square law.
  • 3. The energy emitted by a point source is dispersed evenly across the surface of an expanding sphere.
  • 4. The sphere’s surface area is inversely proportional to the distance (sphere’s radius) squared.

B. Line Sources: 

  • 1. The SPL from an infinitely long line source decreases at a rate of 3 dB per distance doubling.
  • 2. This is due to the fact that the energy distribution is now over the surface of a cylinder, rather than a sphere as with the point source.
  • 3. Because the expanding cylinder’s surface area is inversely proportional to distance rather than distance squared, the energy density declines simply with distance from the source rather than distance squared.

C. Plane Sources: 

  • 1. Consider an indefinitely large flat surface radiating sound.
  • 2. The SPL from an infinitely large plane sound source is constant as one moves away from it.
  • 3. The energy distribution from the source is now over the surface of another plane some distance away.
  • 4. As the wave travels, it does not expand, but rather continues to pass through the same area as the source.
  • 5. As a result, the energy density at any location in space equals the energy density at the source plane.
  • 6. In the presence of a plane radiator, the SPL remains constant.

Q3. Discuss the various sources of noise pollution.

Ans. Sources of Noise Pollution: 

The occurrence of noise will dependent or independent in nature. According to their source of generation, noise pollution can be classified into two categories:

Discuss the various sources of noise pollution. Air and noise Pollution Control
  • A. Natural Sources: Natural noise pollution is caused by natural events such as severe rain, lightning, volcanic explosions, cloud thundering, and so on, but their effects are limited to a specific period and location and do not disturb the majority of people.
  • B. Human-made Sources: This type of noise pollution affects vast. Loud music, sound of vehicles, noise produced by industries is under it. Some causes of these are as follows: 
    • 1. From Household Equipments: Noise pollution is caused by household equipment such as mixers, hoover cleaners, washing machines, coolers, music systems, dog barking, and so on.
    • 2. From Social Events: Such social group programmes where most people create a lot of noise. Examples include group worship, discos, parties, market places, marriage functions, and so on.
    • 3. From Industrial Activities: The noise created by industrial units is quite dangerous and noisy, for example, manufacturing industries, printing machines, construction sites, and so on.
    • 4. From Transportation Means: Nowadays, wherever we go, we can hear the loud noise of vehicles that make the most noise, such as trains, buses, aeroplanes, taxis, agricultural machines (tractors, crashers), and so on.

Q4. Write a short not on : 

A. Psychoacoustic of noise pollution. 

B. Noise criteria of noise pollution.

Ans. A. Psychoacoustics of Noise Pollution: 

  • 1. Psychoacoustics is the scientific study of sound perception, that is, the psychological and physiological responses associated with sound, including speech and music.
  • 2. Hearing consists of mechanical wave propagation as well as a sensory and perceptual event.
  • 3. When sound enters the ear, it is converted into a neuronal action potential.
  • 4. The ear has a non-linear response to sounds of varying intensities; this non-linear response is referred to as loudness.

B. Noise Criteria of Noise Pollution:

  • 1. The human ear can hear sounds in the frequency range of 20 Hz (0.02 kHz) to 20,000 Hz (20 kHz).
  • 2. The top limit decreases with age; most adults cannot hear above 16 kHz.
  • 3. The body’s sense of touch can detect tones between 4 and 16 Hz.
  • 4. The minimum threshold at which a sound can be heard varies with frequency. A frequency dependent absolute threshold of hearing (ATH) curve can be derived and used to find the lower limits by measuring this minimum intensity for testing tones of various frequencies.

Q5. Discuss the different techniques used to control the noise pollution in industries.

Ans. Noise Reduction (Control) in Industries: Following are different techniques used to control the noise pollution in industries: 

  • 1. Noise reduction from process exhaust stacks.
  • 2. Noise control for high-pressure steam, natural gas, and other vents.
  • 3. Acoustic blankets/warps reduce noise generated by the equipment casing.
  • 4. Acoustic treatment of roof exhausters, ventilation systems, and towers.
  • 5. Utility enclosure and barrier wall openings must be closed.
  • 6. Noise reduction method using damping in chutes, hoppers, machine guards, conveyors, panels, and other areas.
  • 7. Choosing efficient equipment, such as efficient fans, compressors, PD blowers, and engines, contributes greatly to noise reduction.
  • 8. Because fan speed is directly related to fan speed, little tweaks can have a big impact.
  • 9. Acoustic absorbent duct lining ventilation solutions, such as foam or fibreglass, will effectively reduce airborne noise.
  • 10. One of the most effective strategies for reducing noise pollution is to muffle noises at the source.
  • 11. In general, when noise levels in industry surpass 85 dB(A), protective equipment is required.

Q6. Write a short note on: 

1. Ultrasound.

2. Infrasound.

3. Sonic Boom. 

Ans. 1. Ultrasound: 

  • i. Ultrasound is a type of sound wave. At frequencies greater than human hearing’s perceptible top limit.
  • ii. In terms of physical attributes, ultrasound is identical to “regular” (audible) sound, except that humans cannot hear it.
  • iii. This limit varies from person to person, but in healthy young people it is around 20 kilohertz (20,000 hertz).
  • iv. Ultrasonic machines use frequencies ranging from 20 kHz to many gigahertz.
  • v. Ultrasound is employed in a wide range of applications. Ultrasonic devices are used to detect and measure objects and distances. 
  • vi. Ultrasonic imaging, often known as sonography, is commonly utilised in medicine.
  • vii. Ultrasound is used in nondestructive testing of products and structures to detect invisible flaws.
  • viii. Ultrasound is utilised in industry for cleaning, mixing, and speeding chemical processes.
  • ix. Ultrasound is used by animals such as bats and porpoises to locate prey and barriers. 

2. Infrasound:

  • i. Infrasound, often known as low-frequency sound, describes sound waves having frequencies lower than the lower limit of audibility (typically 20 Hz).
  • ii. Because hearing gets less sensitive as frequency falls, the sound pressure must be sufficiently high for i. humans to perceive infrared.
  • iii. The ear is the primary organ for sensing infrasound, but at higher levels, infrasound vibrations can be felt in other areas of the body.
  • iv. The study of such sound waves is known as infrasonics, and it covers sounds below 20 Hz down to 0.1 Hz and, in exceptional cases, 0.001 Hz.
  • People use this frequency range to monitor earthquakes and volcanoes, to record rock and petroleum formations beneath the ground, and to investigate the mechanics of the heart in ballistocardiography and seismocardiography.

3. Sonic Boom: 

  • i. A sonic boom is the sound associated with shock waves created when an object travels faster than the speed of sound through air.
  • ii. Sonic booms produce massive amounts of sound energy, resembling an explosion or a thunderclap to the human ear.
  • A sonic boom in miniature is the crack of a supersonic bullet passing overhead or the crack of a bullwhip.
  • iv. Sonic booms from big supersonic planes can be very loud and alarming, causing individuals to awaken and causing slight damage to some structures.
  • v. They resulted in the ban of routine supersonic flight over land.
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