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Noise propagation

Noise changes both in level and frequency as it travels or propagates from a source to a receiver. There are a number of factors that affect the degree of change including distance, ground absorption, meteorological effects, and the extent of natural or man-made shielding between the source and receiver.

Traffic noise acts like a cylindrical ‘line’ source consisting of a number of point sources. This affects the geometric spreading of the noise and means that noise levels tend to reduce by approximately 3 dB for each doubling of distance between the source and receiver.

The amount of ground absorption between a noise source and a receiver affects the amount of reflection. Hard surfaces, such as car parks or water bodies, are relatively reflective and will not normally result in any reduction in noise level. By contrast, soft surfaces such as loose dirt, grassland or bush, can provide an additional reduction of up to 1.5 dB. As a result noise can reduce by up to 4.5 dB for each doubling of distance between a source and receiver when both ground absorption and distance attenuation effects are considered.

There are a range of meteorological conditions that can have both positive and negative impacts on the propagation of noise. Such effects are associated with a range of meteorological parameters including wind, temperature, humidity and rain. Given the lack of control over such effects, most noise models use neutral meteorological conditions as the basis for prediction. NZS 6801:2008 provides further detail on this subject.

The topography and natural terrain between a source and receiver can act as a shield. This reduces noise levels by restricting the direct transmission of noise along a propagation path and by absorbing some of the noise. Man-made features such as buildings, barriers, walls, embankments and bunds can also provide shielding from noise in the same way as natural features. Depending on the alignment of a road and the site geometry, the first row of houses or buildings next to a road may offer shielding from noise to second and subsequent rows. The following pages on noise barriers and bunds specifically discusses the shielding offered by these features.

Road alignment

The alignment of a road is the controlling factor to the location of noise sources. For example, road alignment may affect the degree to which: the effects on protected premises and facilities can be avoided, remedied or mitigated; the way noise is propagated between the source and receivers; and the nature and level of noise generated. Factors to be considered when determining the route and the alignment of a new or altered road include the availability of land, the nature and purpose of the new or altered road, and the safety and geometric design standards.

When the route for a new road is being considered, noise mitigation measures should be explored at the time the road alignment is selected. This may involve optimising the distance between the road and any protected premises and facilities in order to exploit distance attenuation effects or using the topography of the land to shield protected premises and facilities from the noise source (see the noise propagation section below).

The underlying topography affects not only the need for features such as cuttings and retaining walls but also the gradient of a road. This affects the extent to which drivers of vehicles need to vary traffic speed, change gear and use their brakes. Each of these factors affect the engine load, which in turn affects the amount of noise generated. By reducing the gradient of a road the amount of noise associated with vehicle acceleration and deceleration as well as exhaust and engine breaking can be reduced.

The impact of gradient on truck noise is particularly noticeable. For example, trucks tend to use their engine brakes from around -2% gradient, whereas cars will tend to use their engine brakes on steeper gradients of more than -8%. A 5% reduction in road gradient can reduce noise levels by around 1.5 dB.

For altered roads, especially in urban areas, those designing roads should continue using the horizontal alignment to maximise any shielding effects offered by existing natural or man-made features.

The process of determining the geometric design requirements for a new or altered road alignment can also be used to reduce any likely noise effects. This includes using the shielding effects of embankments, cuttings, retaining walls and solid safety barriers. In addition, it includes design speed considerations (see the traffic management section above).

Traffic management

Vehicle speed is a key factor affecting the level of noise generated from a given road. In general terms, as vehicle speed increases, so does the level of noise. Although it is theoretically possible to manage noise levels by changing posted speed limits, in New Zealand such changes must be made in accordance with Land Transport Rule: Setting of Speed Limits 2003 and its amendments. The rule is primarily focused on the safety and efficiency of road networks. It does not consider the relationship between traffic speed and noise.

In urban areas, traffic calming measures can be more effective than simply altering speed limits. This is particularly so in residential areas. Typical traffic calming measures may include road narrowing, traffic islands, pavement changes and speed humps. Accessibility for large and emergency vehicles needs to be maintained.

Travel demand management is aimed at reducing the total number of vehicles on a transport network, fundamentally by reducing the need to travel, use of alternative modes such as public transport and encouraging multiple occupancy of vehicles. Reducing the volume of traffic on a particular road will reduce the noise emissions. However, significant volume reductions are required to make a noticeable change to noise emissions.

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