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Consultation on proposals for transposition and implementation of Directive 2002/49/EC of the European Parliament and of the Council of 25 June 2002 relating to the assessment and management of environmental noise

Annex G: Noise Indicators

1 Annex I of the END discusses and defines the noise indicators that are to be used in the noise mapping to describe the external noise environment. This Annex provides some background information relating to noise and how it is described.

2 A noise indicator is an objective descriptor of noise that is intended to reflect how people react to noise. Research over many years has shown that people react to different noise in different ways and it is not just the level (or loudness) of the noise that affects the reaction. Furthermore, different noises have different characteristics. A noise might be:

• continuous at a constant level (e.g. the noise from a fan);
• continuous, but varying in level (e.g. the noise from a busy road);
• intermittent (e.g. the noise from a local railway line).

3 In addition, the noise might be a general or broad-band noise, containing both low and high-pitched sounds, or it might be just low pitched or it might be a discrete pitch so that it is tonal in quality (i.e. it sounds like a single musical note).

4 Thus, a noise indicator must attempt to reflect all these different features of noise in a way that relates to how people react to these different features.

5 The human ear is sensitive to a wide range of levels of sound and measuring noise in terms of sound pressure would involve a very large range of numbers. To simplify the descriptions of the magnitude of sound, the decibel (dB) unit is used. This describes the sound level as a ratio of the sound energy being heard to the energy of sound that is audible to those with good hearing. Mathematically, this is expressed by:

Sound Level equals ten times the logarithm of the Sound Energy (E) divided by the Reference Energy (E0)

or

Sound Level = 10*log(E/E0)

6 As these levels are expressed in decibels, care must be taken when combining more that one sound level. For example, if the sound level at a location from one source is 60dB, the occurrence of one additional identical source also producing 60dB at the receiver location means that the total is 63dB (and not 120dB). This is because it is the energy (E) that is added and not the decibel values. Thus if each of the sources produce energy of E, the total level would be:

Total Sound Level =10*log((E+E)/E0), or

=10*log(2E/E0), which is

=10*log(E/E0) + 10*log(2), which is

=Original Sound Level +3

7 Thus when manipulating decibels, it is necessary to use mathematics to reach the corresponding energy values. This involves dividing the sound level by 10 and then anti-logging the result. Mathematically, anti-logging can be shown as:

10^(Sound Level / 10) or 10 (Sound Level/10)

8 The human ear also responds differently to different pitches or frequencies of sound. On average, more sound energy is needed at lower frequencies than at higher frequencies to stimulate the same response in the human ear.

9 In order to reflect this feature of hearing, many indicators include a frequency weighting that corresponds to the way the human ear responds to different frequencies. The most commonly used weighting is the so-called 'A' weighting. Sound levels that include this weighting are either described in terms of dB(A) (i.e. an 'A' weighted decibel) or the letter 'A' appears in the name of the indicator.

10 The level of most environmental noise varies over time, and some form of statistical analysis is required to reach a single value to describe that varying sound. One of the most commonly used methods involves averaging the sound energy that is heard over a particular time period. The indicator is called the equivalent continuous level or Leq. This is the notional continuous level that , over a particular time period, would contain the same sound energy as the actual varying sound. If the sound has been 'A' weighted, then the equivalent continuous level of this sound is expressed as L_Aeq.

11 The END has identified two indicators to use to provide a strategic description of the noise environment at a particular location. These are the L_den and L_night. Both these indicators are based on the L_Aeq as follows:

12 The L_den considers an average 24 hour period into a year and divides that 24 hour period into 3 parts, namely day, evening and night. Hence the 'den' in the subscript of its name. In the UK, day is defined as the 12 hour period from 0700-1900 for an average 24 hour period in the year. In the END, this is known as L_day. Similarly, Leve is the L_Aeq averaged over the hours 1900-2300 for an average 24 hour period in the year; and L_night is the L_Aeq averaged over the hours 2300-0700 for an average 24 hour period in the year.

13 The L_den indicator also take account of the fact that many people are more sensitive to noise occurring in the evening rather than in the day, and even more sensitive to noise that occurs at night. Thus a penalty is added to the Leve and L_night values of 5dB and 10dB respectively. This is not the same as adding an additional noise source, but simply a penalty that increases the value of the noise indicator, i.e. Leve and L_night.

14 Thus within L_den there are three separate sound levels over three different time periods, two of which have attracted a penalty. As the indicator is seeking a single number averaged over a 24 hour period, these three sound levels have to be combined to take account of the differing periods and the penalties that are added.

15 As indicated above, when combining decibels in this way, it is necessary to manipulate the corresponding sound energy rather than the decibel values. Hence the following are required:

Day energy =12 (the number of hours in the day) times 10^(L_day divided by 10)

=12*10(L_day/10)

Evening energy = 4 (the number of hours in the evening) times 10^((Leve +5 (the evening penalty)) divided by 10)

= 4*10 ((Leve+5)/10)

Night energy =8 (the number of hours in the night) times 10^((L_night +10 (the night penalty)) divided by 10)

= 8*10((L_night+10)/10)

16 The day, evening and night energy are added and averaged over the 24 hour period as follows:

Average Energy =(12*10(L_day/10) + 4*10((Leve+5)/10) + 8*10((L_night+10)/10) ) divided by 24.

17 This value needs to be converted back into decibels to obtain the L_den as follows:

L_den = 10*log((12*10(L_day/10)+ 4*10((Leve+5)/10) +8*10 ((L_night +10)/10) )/24)

Or

Small changes in the way we perform everyday tasks can have huge impacts on Scotland’s environment.

Walking short distances rather than using the car, or being careful not to overfill the kettle are just two positive steps we can all take.

This butterfly represents the beauty and fragility of Scotland’s environment. The motif will be utilised extensively by the Scottish Executive and its partners in their efforts to persuade people they can do a little to change a lot.

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