The Mayor's Ambient Noise Strategy - Greater London Authority

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The Mayor’s Ambient Noise Strategy Mayor of London 223 it. Sound is detected by hearing. Vibration is transmitted to other parts of the body. Vibration may occur at a single frequency, or more commonly, there are a number of different frequency components. A particle may vibrate in any one of three axes (vertical, longitudinal and transverse), or in combination. The perception range for vibration, 1 to 80 Hz, is much narrower than for audible sound, 20 to 20 kHz. Vibration can be quantified in terms of three parameters: ■ ■ ■ Acceleration - the rate of change of velocity over time; Velocity - the rate at which displacement varies with time; Displacement, or amplitude - the distance moved from a fixed reference position. Ground-borne vibration is typically measured in terms of velocity (millimetres per second) or acceleration (metres per second per second). For impulsive or intermittent sources, peak particle velocity or acceleration is measured, this being the maximum value recorded during the event. BS 7385 Part 1 1990 gives advice on the measurement of vibration in buildings. Peak particle velocity is the preferred unit for assessing the risk of building damage. Either velocity or acceleration are used for assessing effects on people. BS 7385 Part 2 1993 gives guidance on acceptable vibration levels to avoid vibration-induced building damage. Vibration can be felt by people at levels much lower than those which could cause structural damage. BS 6472:1992 provides guidance on satisfactory magnitudes of vibration in terms of human response. It defines Vibration Dose Values (VDV) which are expected to be acceptable, although a wide range of individual sensitivity is found in practice. Assessing changes in noise A wide range of actions can be taken to reduce noise. Many of them individually deliver modest improvements. A change in sound level of 10 dB is generally perceived as a subjective doubling or halving of loudness. Under controlled laboratory conditions, a sound level change of 1 decibel (dB) has been found to be just perceptible when the sound consists of a single frequency, or limited range. Observers typically find it more difficult to detect small changes in sounds which include a broad range of frequencies. In the latter case, a 2 dB change in sound level may be more likely to be noticeable. A 3 dB change in sound level has often been taken as the threshold at which
224 Mayor of London The Mayor’s Ambient Noise Strategy changes in complex sounds become noticeable to the average listener, over a period of time. However, people can notice changes of under 1 dB in terms of a ‘noise index’, such as L Aeq (as distinct from ‘sound level’). Examples are where changes in noise are the result of changes in the number of events, rather than the loudness of each event, or where there is a change in the distribution of noise over the relevant measurement period 4 . The ‘Design Manual of Roads and Bridges’ (Volume 11, Section 3, Part 7, Chapter 3, Highways Agency, August 1994) states that, in the period following an abrupt change in traffic flow, people may find benefits or disbenefits from changes in the noise index of as low as 1 dB(A). Noise needs to be considered across the whole ‘footprint’ spreading out from the source. A change that is only just perceptible within the footprint may reduce its extent. Noise may become indistinguishable from background, or inaudible as a noise event, nearer to the source than before. Small noise reductions could still be worth making as part of a cumulative programme of work to reduce noise. However, caution needs to be exercised in prominently advertising any particular measure in advance as ‘noise reducing’. Action needs to be sustained and coordinated across a wide range of fronts. This is reinforced by the need to take inter-relationships between different noise sources into account. Reducing one noise can make apparent other noises that were previously masked. Historically, the non-linear nature of the human response to sound has enabled large relative increases in sound energy to be put into the environment, with a less than proportionate adverse human reaction. However, moving the other way means, in many cases, removing large amounts of sound energy relative to the likely positive human perception. At the same time, most machines are only using very small amounts of their total energy to produce sound - typically one millionth 5 . Often, sound is being produced from many different sources on the machine, and as it relates with its environment. Reductions need to be sequenced optimally in social cost-benefit terms. Attitudes to noise are highly related to context. Limitations on applying evidence gained from one situation to another need to be better understood. Implicit bias towards the attitudes, values or behaviour of the younger, fitter or economically stronger need to be avoided. However, there is no agreed, readily available way of identifying a psychologically ‘noise-sensitive’ population group 6 . Establishing an average cost effectiveness threshold in £ per decibel reduction might appear to be
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