The Mayor's Ambient Noise Strategy - Greater London Authority

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The Mayor’s Ambient Noise Strategy Mayor of London 219 of the pressure variation of that sound to a reference level and is measured in decibels (dB). The logarithmic unit dB has the advantage of compressing the numerical range, from 0 dB up to, for example, 120 dB for a nearby pneumatic drill. Also, the human ear responds roughly logarithmically rather than linearly to sound. That is, the ear becomes (logarithmically) less sensitive to increments of additional sound energy. Sound can be described by its sound pressure level in each frequency band. However, the human ear typically notices higher frequency sounds more readily than low frequencies. A-weighting is commonly used to reflect this. The A-weighting scale attaches a lower weighting to lower frequencies relative to higher frequencies. The A-weighted sound pressure level is denoted ‘dB(A)’. C-weighting may be used when evaluating very loud sounds, or those with a lot of energy at lower frequencies. Other weightings may be used. With modern equipment, sound can readily be measured across a wide frequency spectrum and subsequently analysed. Sound levels in the environment typically vary through time. For example, free flowing traffic on a busy motorway may be continuously audible, while overflying aircraft may be individual events separated by a return to pre-existing background levels. A variety of descriptors have been used to aggregate sound through time, for different sources and in different circumstances. Two commonly used descriptors are: ■ ■ Equivalent continuous A-weighted sound pressure level - This is the notional A-weighted level which would deliver the same energy as the actual fluctuating source if it were to be delivered continuously over the defined period. Notation for this measure is L Aeq, time period . The time period must always be stated. Percentile parameters - This is the value exceeded for a given percentage of the measurement period. L A90 is the dB(A) level exceeded for 90% of the measurement period, whereas L A10 is the level exceeded for 10% of the measurement period. L A10 has been widely used in the UK to describe road traffic noise. L A90 is commonly used to describe background noise. Railway noise, and, increasingly, other environmental noise, is commonly described using L Aeq . A number of indices are used in different countries to describe aircraft noise. The UK Government has moved from using NNI (Noise and Number Index) to using L Aeq , which is the commonest international measure. L Amax and SEL (see glossary) are also commonly used in describing railway and aircraft noise events. The European Environmental Noise Directive will require mapping and reporting of a composite day-evening-night index, L den . L den combines the L Aeq for a 12 hour day period, with L Aeq for a 4 hour
220 Mayor of London The Mayor’s Ambient Noise Strategy evening period, using a weighting of +5 dB(A), and with L Aeq for the 8 hour night period which is weighted by + 10 dB(A). The 8 hour night period, L night , is also to be mapped and reported separately. Combining sound levels Sound pressure levels from different sources cannot be added or subtracted arithmetically - it a logarithmic scale. Thus, if the first vehicle gives a sound pressure level of 70 dB(A), the addition of a second identical vehicle results in a level of 73 dB(A), not 140 - see table. Table 8 Combining Sound Levels Difference between levels to be combined, dB Add to higher level, dB 0 or 1 3 2 or 3 2 4 to 9 1 10 or more 0 A sound can be masked or partially masked by another. Reducing one annoying noise can thus make another more audible. Local combinations of continuous and intermittent noise sources are likely to affect the level of annoyance. In urban environments with high, but changing background noise levels, it may be difficult to predict human response to changes, even when the physical changes can be accurately predicted. Introducing a new noise source, or eliminating an old one, may have different impacts from those expected. Human response to noise Human response to noise can take many forms (see Chapter 2, adverse impacts). In terms of subjective reactions, like bother or annoyance, much depends on context - what people think about the sound, as well as its loudness or other physical characteristics. Sound has meaning. It is not just pressure fluctuation. The likelihood of an adverse human response to noise in a given situation appears to depend on a large number of factors. These include: ■ ■ ■ ■ ■ ■ ■ Absolute level of the noise; Margin by which the noise exceeds the background noise level; How continuous or intermittent the noise is; Nature of the noise, e.g. pitch, tonality, clatter, hum; Time of day, week and in some cases, year; Trends in local noise (e.g. is ‘party night’ getting louder each week); Perception as to whether or how far the noise is ‘avoidable’;
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