Noise has been defined as a sound without agreeable quality or as unwanted or undesirable sound

Transcription

1 Unit 3 d: Control of environmental noise Lecture Objectives I. Describe the potential sources of environmental noise and their consequences II. Explain the short and long term effects of noise III. Describe the methods available for the control of environmental noise. IV. Outline the legislation in Trinidad and Tobago used to control noise Noise has been defined as a sound without agreeable quality or as unwanted or undesirable sound Noise problem can be divided in two categories. First is the community noise wherein one is concerned with annoyance caused to people in their indoor environment. It is the intrusion of noise inside the dwellings above the allowable level mainly from vehicular traffic. The other is the intense sound that exists in some workplaces like factories and traffic islands where there is a real danger of permanent physical damage. Prolonged exposure to excessive noise produces adverse physiological effects such as hypertension, annoyance, disturbance in sleep and deafness etc. LEVELS OF NOISE The sound level (L) is thus represented as, L = log10 Q/Q0 (Bels) Where Q = Measured quantity of sound pressure or sound power or sound intensity. Q0 = Reference standard quantity of sound pressure. L = Sound level in Bels. A Bel is a large unit so for convenience it is divided into 10 subunits called decibels (db). So when sound level is expressed in decibels. L = 10 log10 Q/Q0 (db). According to World Health Organization (WHO) a level of 45 db is considered a safe noise level in a city. However, a noise level of 65 db can be tolerable as per the international standards.

2 THE NATURE OF SOUND Sound is the sensation caused by a vibrating medium acting on the ear, but the term is usually applied to the vibration itself. The source of sound is most often some vibrating solid body like a string or a sheet which generates vibrations in the air, but it may be generated by vibrations of a gaseous medium, as such the air in a whistle or flute. The medium conveying it to the ear can be a gas (air) or a liquid. In which the vibrations are transmitted as a longitudinal wave motion, i.e successive compressions and rarefactions of the molecules. SENSITIVITY OF SOUND The average person can hear frequencies from about 29 to Hz depending upon the age and other subjective factors. The lowest intensity perceived as a sound is W/m2 and this is known as the standard threshold of audibility. The upper limit is the threshold of pain at 1 W/m2. Vibration above this intensity would cause pain and can damage the human ear.

3 EFFECTS OF NOISE There is a bad effect of noise on human health depending upon the noise level, exposurentime and the health conditions. The effects of noise can be classified in the following ways: (i) Psychological and physiological effects.:- The noise of different levels has different psychological and physiological effects. The following table shows some of them. Noise level Effects Noise level 65 db. Effects Noise may create annoyance up to this level, but it is only psychological (nervous effect). Above this level physiological effects such as mental and physical fatigue may occur 90 db Many years of exposure to such noise level would cause permanent hearing loss. 100 db With short period of exposure to this noise level the aural acuity may be impaired temporarily and prolonged exposure is likely to cause irreparable damage to the auditory organ. 120 db Short exposure cause pain and other damages to human beings. 150 db. Causes instantaneous loss of hearing ADVERSE EFFECTS OF NOISE Effects on Human Body Function Exposure to noise is likely to bring about activation of sympathetic nervous system in a similar way as heat, cold, pain etc. One of the short time reactions is the change in blood circulation. The other effects on brain and other organs are the increased release of insulin in pancreas, increase in the secretion of oxitaxin and ADH from the posterior and it also decreases the detoxifying function of the liver including aromatic substances with carcinogenic effects. Long time noise can produce stomach ulcer, reduced flow of gastric juice and change in acidity neurosis, allergies and circulatory disease, abortion and other congenital defects in children, deafness etc.

4 Sleep Interference The main annoying effect of noise is the interruption on sleep. Sleep is a must and even one nights missed sleep may disturb us. One can become short tempered and weary. Noise can interfere with sleep even when the sleeping person is not awakened. Effect on Working Efficiency It has been proved that the working efficiency reduces with increase in noise. Reduction in noise from 96 db to 87 db has increased the performance by 12%. Reduction of noise either by sound proofing or putting sound obstructing barriers improves the quality of work. It also reduces the industrial accidents. The noise affects communication signals, so reduction of noise increases the efficiency. Effect on Wild Life Noise produces physiological effects on human as well as animal health. It has been surveyed that noise has adversely affected the wildlife of the country. It has been observed by the zoo authorities that animals particularly deer, lions, rhino etc. are the worst affected ones by the traffic noise. They become dull, inactive, lesser reproductive and ill. Even the migratory birds are reduced if there is noise. In the forests because of the human activity the noise increases and the animals like lion, tiger and elephants feel segmentation and their reproduction is automatically reduced. Effects on Non-living Things Noise affects even the non-living things. High intensity noise may produce cracks into buildings. The noise and vibrations from machinery result in shattering of foundations, loosening of plaster and cracks in walls and house hold crockery.

5 CONTROL OF NOISE POLLUTION Though the noise cannot be totally eliminated but can be reduced by adopting certain measures. The basic principles of noise control are as follows. 1. Noise Control at Source In an industry to control the noise at source the machinery should be equipped by effective silencers, properly installed on the designed foundation and well maintained. Use of shock absorbing material, efficient flow technique, reducing fluid jet velocities, restricting sound producing area, reducing peak accelerations are some of the techniques of noise control at source. Use of guards, covers, enclosures and muffler systems, sealing all openings, use of proper cutting speeds and feed rate are other ways of noise reduction. The proper maintenance of machines by lubrication and timely replacement of bearings is the main thing required for the control of noise at source. 2. Noise Control along the Path The modification of sound path includes: i. Use of solid high barriers to interrupt the direct transmission. Solid fences and high earth beams can be provided up to attenuation along the highways. ii. Increasing distance between the source and the receiver. iii. By planting trees. Trees are very good absorber of the sound and otherwise also very much useful as described earlier. 3. NOISE CONTROL AT RECEIVING END To control the noise at receiver level following measures are adopted. Double glazing windows in the building for improved sound proofing. Gasketing and sealing door and windows openings. Providing additional sound insulation for roofs and walls. Isolate the operator from the noise. By controlling hours of exposure to noise. Use of personal protective devices like ear plugs, ear defenders, disposable ear plugs etc.

6 3. OTHER WAYS OF NOISE CONTROL Forming legislation for noise levels for various types of equipment, land use pattern, declaration of silence zone, restricting the use of loudspeakers are some of the ways of noise reduction. Educating people about hazards of noise pollution and developing awareness at all levels can solve the problem. REVIEW QUESTIONS 1. Apart from ppe, describe other methods of control against noise. 2. If the speed of a sound wave is 300 m/s, with a frequency of 60 Hz, what is its wavelength? 3. Describe two ways in which noise can be attenuated, giving one example of a material used in each case. 4. In which material does sound travel the fastest? 5. What is the approximate noise level of conversational speech? 6. Give some examples of impulse noises. 7. Which weighted scale is normally used to measure noise to determine the hearing hazard present? 8. In the far field, with each doubling in distance from the source, name the factor which will determine that sound will diminish in intensity. 9. Distinguish between noise and sound giving an example in each case. 10. Describe the noise levels allowed under the noise pollution rules for General areas and discuss the impact on Carnival activities. 11. Define Noise and give the factors that can affect the velocity of sound. 12. Label the following diagram of the human ear.

7 13. Write short notes on the following: Steady Level Noise, Mixed Noise and Impact Noise. 14. For the purposes of noise pollution, Trinidad and Tobago has been divided into three areas wherein prescribed standards are to be applied. Complete the following using the standards (Noise Pollution Rules, 2000). Maximum Permissible Sound Pressure Levels for Zones I, II & III Time of Day Anytime 8.00am- 8.00pm 8.00pm- 8.00am Industrial Areas Environmentally Sensitive General Area Areas Continuous (dba) Instantaneous (db) Continuous (dba) Instantaneous (db) Continuous (dba) Instantaneous (db) To be left blank dba To be left blank To be left blank above background 3 dba To be left To be left above blank blank background

8 15. What is the approximate noise level of a jet liner taking off on a runway? 16. Give examples of impulse noise. Explain how the noise emanating from this portable generator can be reduced.