Here is a compilation of notes on Noise. After reading these notes you will learn about: Meaning of Noise 2. Level of Noise 3. Classification 4. Measurement.
Contents:
- Notes on the Meaning of Noise
- Notes on the Level of Noise
- Notes on the Classification of Noise
- Notes on the Measurement of Noise
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Note # 1. Meaning of Noise:
If we view through an oscilloscope, noise waves make an uneven and jagged track on the screen in sharp contrast to the following track made by a harmonious sound.
Sound is a form of energy which is emitted by a vibrating body and on reaching the ear it causes the sensation of hearing through nerves of ear. The frequency limits of audibility are from 20 HZ to 20,000 HZ. Sound of frequencies less than 20 HZ are called infrasonics and greater than 20,000 HZ are called ultrasonics.
A noise problem generally consists of three inter-related elements the source, the receiver and the transmission path. This transmission path is usually the atmosphere through which the sound is propagated, but can include the structural materials of any building housing the receiver (Fig. 3).
In layman’s terminology, noise is just unpleasant sound.
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The quality of unpleasantness of sound waves has been found to depend upon one or more of the following factors:
(i) The intensity of the sound waves,
(ii) The frequency of the sound waves,
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(iii) The time of exposure to sound waves, and
(iv) Intermittence of sound waves.
Broadly speaking, intensity is the energy of sound waves (A big bell will produce more intense sound waves than small bell) and frequency is the number of times per second the sound waves hit the ear of man. This is the reason that a high note on the violin will carry more frequency than a low note.
On the basis of the above definition, the loudness which is more commonly understood source of noise, is in fact, a combination of intensity and frequency and can be measured in decibels.
Note # 2. Level of Noise:
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Decibel (dB) is the unit of sound, named after Alexander Graham Bell.
Computation of Noise:
If the sound pressure level, L1 in dB is measured at r1 meter, then the sound pressure level, L2 in dB at r2 meter is given by,
L2 = L1 – 20log10 (r2/r1) … (1)
If the sound levels are measured in terms of pressure, then, sound pressure level, LP is given by
LP = 20log10 (P / P0) dB … (2)
The LP is measured against a standard reference pressure, P0 = 2 × 10-5 N/m2 which is equivalent to zero decibel. The sound pressure is the pressure exerted at a point due to a sound producing source.
Day-night Equivalent Noise Level (LDN):
The day hours in respect to assessment of noise levels, are fixed from 6 AM – 9 PM (i. e., 15 hrs) and night hours from 9 PM – 6 AM (i. e., 9 hrs). A sound level of 10 dB is added to Ln due to the low ambient sound levels during night for assessing the Ldn values.
The day-night equivalent noise levels of a community can be expressed as:
Ldn, dB = 10 × log10 [15/24(10Ld/10) + 9/ 24(10(Ld+10)/10)] … (3)
where, Ld = day-equivalent noise levels (from 6 AM – 9 PM), dB
Ln = night equivalent noise levels (from 9 PM – 6 AM), dB
Addition of Sound Levels:
The effective sound levels from two or more sources cannot be simply added algebraically. For example, the effective sound level (L) from two air conditioners 60 dB (L1 = L2 = 60) each, say is not 60 + 60 = 120 dB but 60 + 3 = 63dB table 3.
The frequency analysis allows to separate the main components of the signals by dividing the frequency range into smaller frequency bands using a set of filters.
Noise can be differentiated into two types; one that consists of regularly repeated or periodic sounds and the other that consists of a periodic sound.
The simplest periodic sound is a pure tone i.e., a pressure disturbance that fluctuates sinusoidally at a particular frequency. The lower the frequency, the longer is the wave length (wavelength = velocity of sound/frequency).
The noise produced by most of the sources such as automobiles or aircraft engines are examples of periodic sounds.
An octave band is a frequency band with upper and lower frequencies having a ratio of 2. The frequencies of 707 HZ and 1414 HZ define an octave band, whose band centre frequency is 1000 HZ and would be referred to as the 1000 HZ octave band.
Frequency analysers can be divided into two group’s viz. constant band width analyser and constant bandwidth percentage analyser. In the constant bandwidth analyser the filter bandwidth is kept constant throughout the frequency range while in the constant percentage bandwidth analyser, the bandwidth is proportional to the centre frequencies.
The nine preferred centre frequencies for noise level measurement are 31.5, 63, 125, 250, 500, 1000, 2000, 4000 and 8000 HZ.
Frequency of a sound wave is the number of times it repeats itself in each second (i.e., the rapidity, with which the pressure fluctuations occur). Human beings generally have the ability to hear sounds in the frequency range 20 to 20,000 Hz (1 Hz = 1 cycle per second).
Although individuals differ considerably in their ability to hear sounds of a given frequency Fig. 2 shows the threshold of hearing for young healthy ears as a function of frequency. The point at which the limit of hearing the threshold i. e. 0 dB, the sound pressure is equal to the standard reference pressure of 2 × 10-5 N/m2.
The curve in Fig. 1 indicates that the human hearing is most sensitive to frequencies in the range 500 to 6000 Hz and is less sensitive both at lower and higher frequencies. As a person ages there is a slow, natural deterioration in hearing sensitivity known as presbycusis which is especially marked at the higher frequencies.
Note # 3. Classification of Noise:
Noise can be classified into three parts:
(i) Work place noise
(ii) General noise in the city and
(iii) Jet engine noise, or Aircraft noise
(i) The Work Place Noise:
The work place noise or industrial noise due to constant exposure can lead to hearing loss. The Indian Standard Institute (ISI) has set the standard that 90 decibel noise level should not be exceeded in industrial establishments as beyond that workers are likely to suffer hearing loss.
(ii) General noise in the city:
The large scale and small scale units in residential areas cause greater annoyance to the neighbouring residents by carrying on operations late in the night and emitting noise all the time. According to NPL experts, who conducted noise survey in Delhi 25 years ago. “With the growth of city there has been a horizontal spread of noise to the newer areas and there has not been much of vertical growth”.
In congested areas of the city, there are noise zones where 100 decibel of noise is a common punctuation in the day. People had learnt to live with noise and have got adjusted to the high noise levels although it causes the skin pales, mucous membranes drain. In other word, the whole biological system is disturbed.
The internal wreckage caused by shriek of siren or the roar of a jet engine includes gastric ulcers and thymus gland atrophy. With the sonic boom (shock waves produced by an object moving faster than the speed of sound) at our threshold, mankind is threatened with the most awesome noise pollution.
According to prof. Garret J. Hardin, “Sonic boom is much worse than noise. Experiencing it is like living inside a drum beaten by an idiot at insane intervals”.
In fact the advanced technology has caught us in the midst of a dilemma so that to stop in our tracks is as suicidal as to continue going on. In the opinion of an Editor — There is now a need to create a responsible technology which does not rampage through the society. There should be a technology which filters only safe and desirable technology to progress.
We want an ‘environment screen’ against noise intrusions and other irritants so that we may re-establish the potential dignity of humanity. Let us hope the human destiny to seek order and see that purpose prevails before the noise and din of technology succeeds in stalemating it.
(iii) Aircraft Noise:
The noise of aircraft is described in terms of Perceived Noise Levels (PNL). It is expressed in pNdB. The pNdB value for a noise is approximately 13 units greater than the dB value for the noise.
Another term used for aircraft noise is the Effective Noise Level expressed in EPNdB. It is used to describe the noise of a single aircraft activity. In order to describe the noise at an airport, the EPNdB values calculated by information like the number of flights of each aircraft, the flight paths that the aircraft uses and the time of day at which the aircraft takes off.
The result of these measurements are expressed in terms of Noise Exposure Forecast (NEF) contours, which are intended to represent the long-term average noise exposure in communities around airports.
Note # 4. Measurement of Noise:
Noise measurement provides us assessment of adverse impacts of noise and adopting suitable control techniques for noise reduction. Sound level meter is used in laboratories to measure noise pollution.
A sound level meter is an instrument which has a microphone amplifier and weighting networks and an indicating meter which gives a reading in dB relative to 2 × 10-5 N/m2.
The microphone responds directly to the pressure variations in the sound field and its output is amplified to give a reading of sound pressure level directly on the meter. The weighting networks superimpose a frequency response on the amplifier similar to that of the human ear. The networks most widely used are the ‘A’, ‘C’ and ‘D’ scales shown in Fig. 4.
The ‘A’ network is very similar to the inverted threshold curve which is shown in Fig. 3 and measurements on the A’ scale in dB (A) are now the most widely used and correlate well with subjective noise ratings and also with risk of hearing damage from exposure to continuous noise.
The B network is now hardly ever used and the ‘C’ network gives a reasonably flat response over the frequency range 31.5 Hz to 8 Khz and is generally used for overall objective sound pressure level measurements. The ‘D’ network is used almost exclusively for the measurement of aircraft noise, although there are indications that it may be applicable to other types of noise as well.
The time constants used for the sound level meter standards are:
S (Slow) = 1 second
F (Fast) = 125 milli second
Steady sounds are measured by the “fast” time constants and unsteady soundly using “slow” time constants. The noise level is not always steady and may vary considerably, in an irregular way over the measurement period. This uncertainty can be measured by the continuous equivalent level.
It can be defined as the constant sound pressure level which produces, the same total energy as the actual level over the given time. It is denoted as Leq. The Leq facility is also available in certain models of sound level meters.
Other equipment’s and their specific areas of use are given in table 4.
For sampling of noise levels from industrial sources, noise levels in the different octave bands are measured by a sound level meter in conjunction with octave – band filters at the workers ear level or at about a distance of one meter from the source of noise.
It has been noticed that at levels above 90 dBA there are indications that working efficiency deteriorates and the conclusion may, therefore, be drawn that in factories where the noise is continuous, care in respect of hearing conservation will also ensure good working efficiency.
Laboratory Guidelines:
The Minister of Environment and Forest (MEF) have declared Indian standards for ambient noise level which are given in table 5. MEF classified 6 a.m. – 9 p.m. as day hours and 9 p.m. – 6 a.m. as night hours. The workers facing noise level > 90 dB in factories are prescribed for protection against noise by State Pollution Control Board.
Silence Zone:
Up to 100 meter around hospitals, educational institutions and courts. The zones are to be declared by competent authority. Use of vehicle horns, loud speakers and bursting of crackers shall be banned in these zones.
A standard safe time limit has been given by Occupational Safety and Health Administration (OSHA) for exposure to various noise levels (table 6). Exposure to a noise beyond this safe time limit for more than a year continuously will lead to hearing loss.
Speech Interference:
The most apparent effect of noise on work is through speech interference. The maximum speech interference levels and acceptable noise limits are given in Table 7 and 8 respectively.
Note:
The speech interference Level is equal to the arithmetic average of the octave band noise levels (in dB) in the three octave bands centred on 1000, 2000 and 4000 Hz.
Effects:
Community response to industrial noise and hence the setting of acceptable limits for community areas is difficult to establish precisely because of the variety and complexity of different factors involved. At high noise levels there is a correlation between annoyance and absolute noise level.
At lower noise levels, however, this correlation becomes more tenuos and instead annoyance seems to depend on difference in noise level. The estimated community response to noise and tolerated noise levels is given in tables 9 and 10.
In fact the example of recent Swiss proposals is shown in Table 10.
A decibel can be defined as an abstract unit. We must remember that the threshold at normal hearing is 20-25 decibels and of normal conversation is 60 decibels. It has also been noticed that speech interference occurs at 75 decibel and definite annoyance begins at 80 decibel.
The motor activity is disturbed at 90 decibel and physiological disturbance occurs beyond 120 decibel. It has been observed that music of disco dance occurs at 120-140 decibels. The effect of ‘rock music’ on human begins is also not enviable.
According to Kudesia:
It is quite possible that due to disco dance and rock ‘n’ roll music we are raising a nation of teen-agers who will be hard of hearing before they reach the age of 35. Those who listen to songs from radio on loud pitch or engage themselves in Kirtan Mandli fall in the same line.
Many complain of singing in their ears after a disco dance, rock session and singing is considered by experts as a warning sign of potential hearing loss. Definite pain occurs at 140 decibel and though no human being has reportedly died of noise, experimental results have shown that mice died at 175 decibel of noise.
According to the All India Institute of Medical Sciences — working or living in noisy environment may lead to gradual hearing loss which is mostly irreversible. On the other hand, exposure to blast and the like may produce sudden hearing loss which requires immediate medical attention.
It has been experimentally proved that loud noise is bad particularly for those suffering from high tension, diabetese. The roadside noise may not only wake up a person but may also alter his heart beat and respiration. Noise also produces startling effects on babies and they may even develop a fear psychosis as a result of sudden and sharp noise.
Some scientists of U.S.A. have discovered that high pitched noise leads to road accidents.
In India also this correlation resulted in a number of fatal accidents as follows:
A pressure home produces 120 decibel of noise whereas from 80 decibel onwards hearing impairment begins. In a way drivers of smaller vehicles and other road users get stocked on the sudden blowing of pressure horns by heavy vehicles.
The motorcyclists with silencers removed or cut go zooming past in residential areas enjoying a thrill by making all that racket, oblivious to the annoyance they are causing to other citizens.
In Delhi the rapid growth of vehicles from 2.5 lakh to 6.8 lakh in last 10 years has added to noise pollution by a much higher level. The multiplicity of high types of vehicles and the failure to segregate fast and slow moving traffic results in high noise emission as vehicles use brakes and then accelerate frequently.