Paragraph on Noise: Sources and Measurement!
Sources of Noise:
In the residential areas, the chief generators of outdoor noise are road vehicles, trains, road repair work, construction site work, children playing, noisy local activities, service deliveries, aircraft, helicopters, etc. In industrial areas, noise may also be caused by the factories. Factory noise is primarily generated “indoors”. But if it is radiated from the buildings concerned, it is classified as “outdoor” noise.
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Noise sources themselves may be classified as “outdoors” or “indoors”. Similarly, the observer may be indoors or outdoors. The main difference between these two is that the observer who is indoors has the benefit of attenuation of outside noise by the structure of the building.
Single Noise Sources:
Let us consider a single source of noise such as a machine. Assuming that the noise is radiated uniformly in all directions over flat ground, the sound level L received from such a single source is given by
L = Lw – 20 Log10 R-8 dB(A), …(1)
where Lw = A-weighted sound power level of the source and R = distance of the single source in metres.
The sound level L at a distance R from the single source may also be estimated from the relation
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L = Lw – Q dB (A), …(2)
where Q, the amount to be subtracted from the A-weighted power level, Lw, can be obtained from Fig. 1. It should be noted in connection with Fig. 1 that this graph is unreliable at distances less than thrice the major dimension of the source, or if the source radiates sound in a markedly directional manner.
The actual sound level received at a distance R from the single source may be influenced by other factors too. For example, if the point under consideration is at a building facade, an addition 3 dB (A) should be added to allow for the effect of sound reflections from the facade.
To allow for ground and atmosphere attenuation, moreover, further corrections may be necessary, although these are generally ignored for general estimates and calculations. If the noise at the point under consideration is coming from a single source, the reduction in noise level can be achieved by one or more of the following direct treatments:
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(1) Increasing the distance R:
This will have the effect of reducing the sound level received at the point by about 6 dB (A) for each doubling of the distance. This reduction can be directly estimated from Fig. 1.
(2) Screening:
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This usually offers a reduction of about 7-10 dB(A), but possibly up to 15 dB (A) as a maximum.
(3) Sound reduction at source:
Enclosure of the noise source could give a reduction of up to 50 dB (A), but usually less than one half this value. Silencing in the case of machines could give a reduction of about 5-20 dB (A).
It is evident from the consideration of the three methods given above that the most effective treatment for noise control is reduction of sound at the source.
Multiple Sources of Noise:
When noise from two different sources is received simultaneously, the total sound level resulting from this is obtained by adding to the higher sound level a quantity related to the difference between the two levels. This quantity is given in Table 2 for various difference between the two sound levels.
As an example, if the sound level of one source is 90 dB (A) and that of the other is 85 dB (A), then the difference is 5 dB (A). We find from Table 2 that, for this difference, the amount to be added to the higher sound level is 1 dB (A). Thus the combined sound level from these two sources will be 90 + 1 = 91 dB (A).
When more than two separate sources of sound are operating simultaneously, the process described above is repeated to get the resulting combined level. Thus the first two sources are combined as above. Then the third is combined with the resultant sound level of the first two sources; and so on.
Measurement of Noise:
Aircraft and traffic noise may be measured and/or monitored by various methods recording both actual peak levels and number of noise-producing incidents (e.g., NNI for aircraft noise and L10 for traffic noise). Construction site noise and the industrial noise generated by factories is now normally monitored in terms of Leq but the legislation for these generally remains incomplete and relatively ineffective.
The measurement of neighbourhood noise from the site may be complicated by the presence of ambient noise, since a measuring device (a sound level meter, for example) cannot distinguish between noise sources. Consequently, such measurements represent either the ambient noise alone (from sources other than the site) or the neighbourhood noise (from the site) and the ambient noise combined.
Where the total measured sound level exceeds that of the ambient noise by 10 dB (A) or more, the total value is equal to the level of neighbourhood noise. Where the total measured level exceeds the level of ambient noise by less than 10 dB (A), the corrections given in Table 1 should be applied to the total level in order to obtain the level of neighbourhood noise only.
Noise Exposure Levels:
There are several methods to measure and express noise exposure levels. The most-widely used method among these makes the use of the index Leq (equivalent continuous sound level) to express noise exposure levels.
Among many countries using this method, the Noise Advisory Council in the United Kingdom has recommended the adoption of Leq as a single measurement of all environmental noise, e.g., measurement of the noise of road traffic, aircraft noise, industrial noise, etc.
Leq is the level of a notational steady sound which, at a given position and over a definite period of time, would have the same A-weighted acoustic energy as the fluctuating noise. Thus Leq is essentially itself a noise scale.
Noise exposure level resulting from the exposure to a single event is expressed by using another index, LAX (single-event noise exposure level). LAX is defined as the sound level which, if maintained constant for a period of one second, would cause the same A-weighted sound energy to be received as is actually received from the noise events.
Where there are a number of different sounds contributing to the overall noise level, LAX values for each source can be used separately to produce the value of Leq due to each source over the same total period.
However, it is impractical to express background noise in terms of a value of LAX hence the suggested use of a notational Leq due to background noise. This is then combined with the value of Leq due to the noise sources as a measurement of total environmental noise.
Some Indices of Noise:
In the United Kingdom and some other European countries, the most important indices to assess the noise nuisance are:
(1) L10 (18 hours) (for road traffic noise);
(2) Noise and Number Index (for aircraft noise); and
(3) Corrected Noise Level (for noise in industrial premises).
Each of these noise indices is associated with a particular day-time period. For example, L10 (18 hours) relates to the period 0600 to 2400 hours. The Noise and Number Index (NNI), on the other hand, relates to a 12-hours day-time period 0600 to 1800 hours.
Similarly, Corrected Noise Level (for industrial noise) is related to the period from 0800 to 1800 hours. In the conversions discussed below, the value of L is that for the same time period as the one specified for the particular index. Another point to bear in mind in this connection is that all these conversions are approximate.
For traffic noise, the values of L10 (18 hours) are defined as the arithmetic average of the values of L10 for each of the one-hour period from 0600 to 2400 hours. It may so happen that, in some cases, L10 may not be the only feature of the traffic noise distribution available.
If the original distribution of noise levels is available in histogram form, Leq may be obtained directly. On the other hand, if other parameters of the cumulative distribution (e.g., L10 and L90) are known, then L. may be estimated by assuming a Gaussian distribution of noise levels and using the empirical equation
It is possible to estimate L even if only L10 values are known. For most busy traffic locations, the arithmetic mean of hourly L10 values will give a value of L10 (18 hours) very close to the value obtained from direct measurements of L10over the total 18-hours period, provided the hour-by-hour variations in traffic patterns are not extreme.
This means that a fairly close estimate of Leq for the 18-hours period 0600 to 2400 hours could be made by subtracting 3 dB from the L10 (1 hour) index value.