5. Assessment of some indicators within an impact
5.5. Example chain: Noise – annoyance to humans
5.5.4. A review of the main noise indicators
The first important aspect in the analysis of the noise–annoyance relationship is the choice of proper noise indicators to describe disturbance to people.
In general, different energetic noise indicators have been used to describe the noise emission depending on the source analysed. Those indicators are focused on the ability to describe the acoustic emission of the sound events or the noise sources and, in general, do not take into account the relationship with the annoyance to the humans exposed to noise.
In this section an overview of the most common noise indicators used for acoustical environmental monitoring is reported. Moreover, some typical
applications of the indicators in the noise–annoyance relationship are given to understand the aspects on which recent and past research has focused.
5.5.4.1. General acoustical noise indicators
First of all there are some basic indicators that are used in acoustics to generally describe the noise and which are used as a basis to build other indicators.
In particular we have:
• the “equivalent level Leq”. As defined in ISO 1996/1-1982, it is a basic energetic indicator used to describe a noise varying over time. It represents the average noise level changing its pressure level during a period T of observation of the emission. For the formula and details, see Annex 9. To describe the meaning of the indicator, a typical representation of noise measure called “time history”, representing the time evolution of the emission, is illustrated in Figure 19. Precisely, Figure 19 shows a time history of a noise emission in a street. The blue line depicts the instantaneous pressure level while the red line shows the “running Leq”
that represents the equivalent level integrated from the beginning of the measurement to the selected point on the curve. A characteristic of Leq is its influence on the higher values of noise; in fact, even if during the night period the instantaneous level decreases sensibly, the corresponding Leq, calculated on all night period, has a minimum decrease.
Figure 19. Example of time history of road traffic noise
The Leq is useful to have a synthetic evaluation of the noise but it is easy to understand that different types of noise, emitted by different sources in different ways (constant or impulsive), are differently perceived but they could record the same Leq. It is sometimes useful to have the value of Leq on a specific time, for example every hour. In that case the indicator is written Leq,h. In general, the noise emitted by traffic is not impulsive.
However, there are some circumstances, for example during night or in a street with low traffic, where it is possible to define the emission of a
single transit instead of a global traffic noise. Using Leq to describe such situations is not enough to shed light on the real impact. Furthermore, in urban areas there are some places where the traffic is discontinuous, e.g.
near intersections and at traffic lights. At those points the Leq is not effective in describing the noise variation (Can et al., 2008)
• the “maximum level Lmax”: It is the maximum value of the noise level recorded during the measurement time
• the “minimum level Lmin”: It is the minimum value of the noise level recorded during the measurement time
• the “statistical level Lxx”: As defined in ISO 1996/1-1982, it represents the pressure level that is exceeded or the “xx” % of the measurement time. It is measured in dB(A). The statistical levels usually considered are L5, L10, L50, L90, L95. The last two indicators, L90, L95, are typically used to describe the “background noise”. In fact they represent, respectively, the level exceeded for 90 and 95 % of the measurement time
• the “sound exposure level SEL” or “LAE” or “LAX”: It is defined in ISO 1996/1-1982. It is used to describe the energetic emission of a single noise event in a particular context, e.g. a passage of a single vehicle in an empty street or a passage of a train. For the formula and details, see Annex 9.
5.5.4.2. Road traffic noise indicators
The most common indicators used specifically to evaluate road traffic noise emission include:
• The “traffic noise index TNI”: It was proposed by Griffiths and Langdon (1968) as cited by Schultz (1972). For the formula and details, see Annex 9. The indicator was developed in the UK but is little used because it becomes representative only when the traffic is fluent. Some examples of the use of TNI are presented in Langdon (1976, part I and II).
• The “noise pollution level NPL”: This indicator was developed by Robinson (1969, cited by Schultz, 1972). For the formula and details, see Annex 9.
The indicator is composed of two terms: the first one, the “Leq”, is the
“average” level of the noise, the “energetic mean”; the second, the “σ”, represents the fluctuation of the level during the emission time. Moreover, the parameter σ is influenced by the background noise: in fact if we have a lower background noise, the fluctuation and the variability of the events are higher. The above indicator has not had a good success because of the difficulty to correctly define the parameter σ. Some examples of its application are presented in Rice (1975), Langdon (1976, part I and II) and Hall and Taylor (1977).
• The “CRTN Indicators L10,18h”: The most common noise indicator of traffic noise used at present in the UK and in Ireland is the LA10,18h. This indicator comes from the use of the Calculation of Road Traffic Noise (CRTN) prediction method. This indicator is the arithmetic average of eighteen LA10,1h values (i.e. the noise level exceeded for 10 % of the hourly period)
from 06:00 to midnight. It is used in the UK for the purpose of national Insulation Regulations; in that case the value of noise contains a correction factor of +2.5 dB for the reflection from a façade (O’Malley et al., 2009).
• The “statistical level L50“: This indicator, calculated on a specific period of the day (e.g. day or night or 24h), was used to evaluate the road traffic noise (Hall and Taylor, 1977) but now it is in disuse. In some cases the L10 over 24, 18 and 12 hours has been used to predict annoyance in residences (Langdon, 1976, part I and II) but the result shows a low correlation between the variables.
5.5.4.3. Railway noise indicators
For railway traffic, there are few specific noise indicators. The most common indicators are:
• The “transit exposure level TEL”: it is an index used to describe the noise emitted by rail. Its formulation is given by the EN ISO 3095:2005 (EN ISO, 2005). For the formula and details, see Annex 9.
• The “sound exposure level SEL” or “LAE” or “LAX”: see above for its definition.
5.5.4.4. Aircraft noise indicators
For aircraft traffic noise, the most common specific indicators are (for formulas and details, see Annex 9):
• The “perceived noise level PNL”: Developed by Kryter (1959), this indicator is used to describe the noise emitted by a single aircraft flying over.
• The “effective perceived noise level EPNL”: It is an evolution of the PNL (Bishop and Horonjeff, 1967 as cited by Schultz,1972). This indicator takes into account the evolution of the PNL during the time with an increase of the level depending on the duration of the high level.
• The “noise number index NNI”: Its basic measure is the perceived noise level (HMSO, 1963, cited by Schultz, 1972 and DORA, 1981). The index was developed during a social survey in 1961 in the vicinity of the London (Heathrow) Airport.
• The “noise exposure forecast NEF”: This indicator is proposed by the US Federal Aviation Administration for the noise emitted by aircrafts (Bolt Beranek and Newman, 1964-1965, cited by Schultz, 1972).
• The “weighted noise exposure forecast WECPNL”: This indicator is an evolution of the indicator EPNL proposed by International Civil Aviation Organisation, as mentioned by Changwoo et al. (2007). There are different computations of the index. In general the WECPNL represents a unique index to describe the noise emitted in a time period by different numbers of flights.
• The “indicator LVA”: This indicator, used in Italy, is described in the Italian norm D.M. 31/10/1997.
5.5.4.5. General environmental noise indicators
There are some noise indicators used to describe noise emitted by different noise sources (e.g. road, railway and aircraft traffic) that take into account the period of the emission during the day. These common indicators are (for the formula and details, see Annex 9):
• The “Day-Night equivalent level LDN or DNL”: It is an indicator used for different noise sources: road, railway and aircraft. This indicator was proposed by the U.S. Environmental Protection Agency (EPA, 1974, cited in Langdon, 1976, Part II). The LDN is an A-weighted average noise level that takes into account the different impact of the noise according to the period of the day when noise is emitted.
• The “Day-Evening-Night equivalent level Lden or DENL”: It is an A-weighted average level of the noise emitted in the three periods of the day, with a penalty of 5 dB(A) for the evening period and a penalty of 10 dB(A) for the night period. It is an indicator proposed in the European Directive 49/2002/EC.
• The “Night level Lnight”: It is an annoyance indicator proposed in the European Directive 49/2002/EC. It represents the equivalent level during the “night period”.