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 upon 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.