Environmental noise pollution, what causes it, how it affects us, how we measure it and what is being done about it.
A previous article in this publication (June 2010) explored issues associated with the measurement and calculation of noise levels, aspects of monitoring and some of the considerations associated with measurements were discussed. This article attempts to broaden previously discussed concepts in order to examine environmental noise pollution from the perspective of what it is, what causes it, how it affects us, how we measure it and what is being done about it.
In order to understand sound and noise it is important to understand the physical parameters and descriptors. Sound may be formally defined as:
• A disturbance in pressure that propagates through a compressible medium. More generally, sound can refer to any type of mechanical wave motion, in a solid or fluid medium, that propagates via the action of elastic stresses and that involves local compression and expansion of the medium • The auditory sensation produced by transient or oscillatory pressures acting on the ear, or by mechanical vibration of the cranial bones at audio frequencies1
Acoustic pressure oscillates about the mean or atmospheric pressure and in the audible pressure range is between 0.000002pa and 200pa. In order to compress the scale (for meaningful use) and because the ear responds to sound pressure waves in a non linear, logarithmic way, the decibel scale is applied to sound measurements. Table 1 gives typical noise levels and their acoustic pressures.
There are various mechanisms by which sound is produced, but probably the most often encountered in environmental noise is radiation by vibrating surfaces, essentially acting like loudspeakers, and aerodynamic noise caused by turbulence and air flow, such as fans or exhausts.
The decibel describes the logarithm of a power or intensity ratio, and when applied to acoustics considers the ratio of the sound pressure being measured to that of the threshold of hearing (po) thusdB = 20 log(p / po ) = 10 log(p² / po²)
The dB scale exhibits some convenient properties. A doubling of source strength (such as a doubling of traffic flow) generates an increase of 3dB, whatever the starting level, thus two sounds each of 60 dB will add to give 63dB. Where one sound is dominant over another, the increase will be small, eg 60dB + 50 dB = 60.4dB.
Subjective loudness also follows non-linear rules, e.g. an increase of 1dB would be just noticeable under ideal conditions, an increase of 3dB is generally considered to be noticeable while a 10 dB increase is required to appear subjectively twice as loud.
In addition to the non-linear response to sound amplitude, the ear also responds in a non-linear way to frequency (Hertz). The lower limit of human hearing is 20Hz and the upper limit is 20000Hz (20kHz) but the sensitivity of human hearing varies with frequency. Equal loudness contours relate perceived loudness to frequency, with reference to that at 1kHz. From the original equal loudness curves in what is known as Fletcher-Munson curves3, it may be seen that humans are most sensitive to frequencies between 1 kHz and 5 kHz and less sensitive to frequencies below this range. Hence a filter circuit is applied to the output of measuring instruments to simulate this response. This characteristic, which is now generally used for environmental noise, is known as an ‘A’ weighting, and is generally referred to with the suffix dB(A).
Noise is the link between what we hear and what we perceive, thus may be described as our subjective interpretation of a sound, particularly when it is undesired. Hence the often quoted definition “Noise is considered as any unwanted sound that may adversely affect the health and well-being of individuals or populations,”4 and “Environmental noise is defined as noise emitted from all sources except noise at the industrial workplace.”5.
Human reaction to noise varies widely, and depends on characteristics within the sound such as its magnitude, exceedance over the background level, frequency spectrum and character, duration/intermittency, time of day/night, attitude toward the source and may also be related to other environmental factors in the area. Apart from being undesirable, exposure to noise may be the cause of adverse health effects such as hearing loss; interference with speech; physiological factors; sleep disturbance and annoyance.
When attempting to quantify environmental noise for the purposes of environmental assessment, which requires determination of ‘significant effects’6, it is conventional to use established relationships between noise and annoyance, known as ‘dose-response’ relationships7. Annoyance is a complex human response that is affected by various contextual and personal factors. These indicate a correlation between noise level and annoyance, although the correlation differs between noise sources.
These curves are only suitable for long term, steady state noise levels. Studies have measured changes in perceived noise nuisance associated with changes in traffic exposure. These studies have found that nuisance ratings change more than would be predicted from the ‘steady state’ relationship. Such a relationship is exploited in assessing traffic noise, where the UK methodology requires an evaluation of both steady state annoyance and changes in annoyance resulting from alterations in traffic noise.8
In order to meaningfully quantify a rapidly fluctuating time varying signal such as sound pressure, it is often necessary to manipulate it in some way to derive single figure descriptors, and in order to understand environmental noise it is important to appreciate the different descriptors available, and what information can be derived from their results. Figures 1 and 2 demonstrate the relationship between descriptors.
• The LAeq Index The equivalent continuous sound level LAeq is the level of a notional steady sound which, at a given position and over a defined period of time, would deliver the same A-weighted acoustic energy as the fluctuating noise. An increase of 3dB may be achieved by a doubling of the acoustic energy of contributing sources, or alternatively by doubling the number of events
• LDEN The LAeq level determined over a 24 hour period with a 5dB weighting applied for evening and a 10dB weighting applied for the night (defined over 4 and 8 hours respectively). LDEN is defined in Directive 2002/49/EC9 and is intended to be used for the purposes of implementing ‘assessment and management of environmental noise’ across member states
• LAmax The maximum sound level sampled during the specified measurement period. In normal use, sound level meters may be set to sample at 0.0125s intervals (fast) or 1s intervals (slow)
• Statistical Sound Level Indices, LAN Noise from road traffic and other environmental sources fluctuates continuously, both on a short and long term basis. It is therefore necessary to use indices which involve averaging over the appropriate time period. The A-weighted level LAN is that level exceeded for N% of the time
• LA90 The background noise level is commonly quoted using the LA90 index as it represents the noise level exceeded for 90% of the time >
• LA10 LA10 is the sound level in dB, which is exceeded for 10% of the measurement period and is used almost exclusively for traffic noise in the UK. It is a useful indicator of freely-flowing traffic noise as it excludes short term noise levels of duration less than 10% of the measurement period. For freely-flowing traffic noise the LAeq is usually lower than, and within 3dB of, the LA10
Time history and frequency
Typical noise sources may be measured and examined in terms of both time history and frequency distribution. Frequency distribution may be shown as octave bands or third-octave bands, where the audible frequency range divided into 8 or 24 frequency bands. Alternatively, finer resolutions are readily available down to resolutions approaching 1 Hz. Full spectrum analysis is particularly useful for picking out tonal components, characterised by prominent spikes in the spectra. The degree of annoyance caused by tonal noise generally depends upon the frequency relationship between tones in the noise spectrum and the exceedence in decibels of the tone over neighbouring frequency bands.
Environmental noise sources
“In the European Union countries about 40% of the population are exposed to road traffic noise with an equivalent sound pressure level exceeding 55dB(A) daytime and 20% are exposed to levels exceeding 65dB(A). Taking all exposures to transportation noise together, about half of the European Union citizens are estimated to live in zones which do not ensure acoustical comfort to residents. More than 30% are exposed at night to equivalent sound pressure levels exceeding 55dB(A) which are disturbing to sleep.”5
Noise levels at a given location depend on three fundamental characteristics:
• Noise at source, a function of the noise generating characteristics • Propagation, function of distance, propagation height, intervening ground type, barriers (including buildings) and any other topographic features which influence propagation • Site layout
Road traffic noise
Road traffic noise is probably the most widespread and invasive cause of environmental pollution.
Traffic noise is a function of traffic volume, composition, speed, road characteristics (surface and gradient). At low speeds the main source of traffic noise is the engine and exhaust system. As speeds increase aerodynamic noise increases, but the main source is associated with the interaction of vehicle tyre and the road surface. A number of mechanisms such as vibration of tyre walls and compression and expansion of air within tyre treads and the road surface texture occur. The nature of the road surface is critical. There are a number of different types of road surface – in the UK the two most commonly used are hot-rolled asphalt and thin surface courses. Thin surface courses tend to be quieter than traditional hot-rolled asphalt, and there is scope to vary the extent of noise reduction. The influence of the road surface varies with speed, and as would be expected, is greatest when noise due to interaction between tyre/road surface predominates.
Noise arising from passage of railway vehicles is generated by a number of different sources, the main ones being propulsion systems and from the vibration of the wheels and rails generated by their rolling contact. Aerodynamic noise becomes a significant source in the case of high speed trains.
Transient noise impacts may also result from impacts at points and crossings, wheel or flange squeal on curves, braking and from audible warning devices. Auxiliary equipment associated light rail and tram vehicles, such as roof-mounted cooling systems, can also contribute to vehicle noise during pass-by and when stationary.
As air traffic increases aircraft noise affects a greater number of people. Noise from aircraft and associated operations is divided into air noise and ground noise. Air noise from aircraft depends on elements such as engine thrust, elevation or atmospheric conditions.
Ground noise can be a significant issue for people living or working close to both the airport and the surface access routes. Ground noise may be defined as the noise generated during operation of airport infrastructure, or airport-related infrastructure other than aircraft which are either in flight, taking off or landing.
Unlike noise from a road carrying freely flowing traffic, perceived noise levels due to construction may fluctuate widely due to of the following factors:
• The particular construction activity and whether it produces a constant or varying noise level over time • The particular plant used during each construction activity and whether it is operated in a steady state or transient condition, and whether it is operated under full or part load • The time of day, evening or night that the activity occurs and the prevailing background noise level at the receptor • The duration of the activity (which could vary from only a few seconds to many months) • The overall area of the works and whether the activity stays predominantly in one location (e.g. installing a sign post) or moves across the area (e.g. resurfacing a road) • The absolute noise level at the receptor (e.g. during the daytime a relatively high absolute noise level may be acceptable at a receptor; during the night a lower absolute noise level may be required to prevent sleep disturbance) • Variations in ground conditions, quality of materials, quality of plant and the skill and expertise of operatives • Weather conditions
Industrial noise may be caused by a whole range of installations from the largest of factories to domestic air conditioning units. A common cause of complaint is low frequency noise, or noise having tonal components. Heating, ventilation and air conditioning plants, compressors, fans, pumps and exhaust stacks are just some examples of causes of complaint. Many items of plant operate continuously and it is frequently at night when general background noise levels are lower that problems occur.
Guidelines and legislation
Most countries have noise limits incorporated into their planning processes, but the following are important European-wide documents.
World Health Organisation: 1999 Guidelines for Community Noise5
“The scope of WHO’s effort to derive guidelines for community noise is to consolidate actual scientific knowledge on the health impacts of community noise and to provide guidance to environmental health authorities and professionals trying to protect people from the harmful effects in non-industrial environments.”
The WHO guideline values are organised according to specific environments. Many areas already exceed guideline values which are intended to form part of an overall noise management process.
The guideline values are for steady, continuous sound but do allow a degree of interpretation for intermittent sound by taking into account the number of events and the relationship to the background sound level.
The European Commission and the World Health Organisation’s ‘Night Noise Guidelines (NNGL) for Europe’ 2008 can be seen as an extension of the WHO Guidelines for Community Noise (2000)
“The aim of this document is to present the conclusions of the World Health Organization (WHO) working group responsible for preparing guidelines for exposure to noise during sleep. The need for ‘health-based’ guidelines originated in part from the European Union Directive 2002/49/EC relating to the assessment and management of environmental noise (commonly known as the Environmental Noise Directive and abbreviated as END) which will compel European Union Member States to produce noise maps and data about night exposure from mid-2007.”
Directive 2002/49/EC, relating to the assessment and management of environmental noise – known as the Noise Directive Requires member states to monitor the environmental problem by drawing up ‘strategic noise maps’ for major roads, railways, airports and agglomerations. These maps are to be used to assess the number of people annoyed and sleep-disturbed respectively throughout Europe.
Further requirements are to inform and consult the public about noise exposure, its effects, and the measures considered to address noise. Action plans are to be produced to reduce noise where necessary and maintain environmental noise quality where it is good. The directive does not set any limit value, nor does it prescribe the measures to be used in the action plans, which remain at the discretion of the competent authorities.
The ultimate aim of the Directive is to develop a long-term EU strategy, “which includes objectives to reduce the number of people affected by noise in the longer term, and provides a framework for developing existing Community policy on noise reduction from source.”
The above provides a brief overview of aspects of environmental noise pollution. The subject is extensive and embraces a number of engineering disciplines and an attempt has been made to demonstrate how they fit together, from the acquisition of meaningful data to the application of social studies based on subjective interpretation of physical evidence. n
1. C. Morfey, ‘ The Dictionary of Acoustics’ (2000) 2. The Environment Agency, Horizontal Guidance for Noise Part 2 Noise Assessment and Control, (2002) 3. H. Fletcher, and W.A Munson, ‘Loudness, its definition, measurement and calculation,’ J. Acoust. Soc. Am. 5, 82-108 (1933) 4. World Health Organisation, ‘Environmental Health Criteria,’ (1980) 5. World Health Organisation ‘Guidelines for Community Noise’ (1999) 6. Directive 85/337/EEC ‘on the assessment of the effects of certain public and private projects on the environment, (1985). And subsequent amendments. 7. EU’s Future Noise policy, WG2 – Dose/Effect ‘Position paper on dose response relationships between transportation noise and annoyance’ (2002) 8. Highways Agency’s ‘Design Manual for Roads and Bridges’, Vol.11, Section 3, Part 7 ‘Noise and Vibration’ (2008)
Max Forni is a Member of the Institute of Acoustics and is a Chartered Engineer with more than twenty years’ experience in assessment of noise and vibration issues associated with infrastructure and planning schemes, from environmental assessment and design stage to completion and operation.
Max Forni works for Mott MacDonald, an international firm of multi-disciplinary consulting engineers. He currently leads a team of noise and vibration specialists from Mott MacDonald’s Southampton Offices.
Published: 10th Dec 2010 in AWE International