Natural sounds are a part of the environmental noise surrounding humans. In rural areas the sounds from insects and birds would dominate the ambient sound character, with noises such as wind flowing through vegetation increasing as wind speed increase. Work by Fégeant (2002) stressed the importance of wind speed and turbulence causing variations in the level of vegetation-generated noise. In addition, factors such as the season (e.g., dry or no leaves versus green leaves), the type of vegetation (e.g., grass, conifers, deciduous), the vegetation density and the total vegetation surface all determine both the sound level as well as spectral characteristics.
Ambient sound levels are significantly affected by the area where the sound measurement location (or a listener) is situated. When the sound measurement location is situated within an urban area, close to industrial plants or areas with a constant sound source (ocean, rivers, etc.), seasons and higher wind speeds may have an insignificant impact on ambient sound levels.
Sound levels in undeveloped rural areas (away from occupied dwellings), however, are impacted by changes in season for a number of complex reasons. The two main reasons are:
- Faunal communication is more significant during the warmer spring and summer months as various species communicate in an effort to find mates. Faunal communication is normally less during the colder months, with ambient sound levels measured during the winter period frequently being very low.
- The occurrence of temperature inversions, and
- Seasonal changes in weather patterns, mainly due to increased wind speeds and potential gustiness of the wind.
Effect of Temperature Inversions
On a typical sunny afternoon, the air is the hottest near the ground surface and temperature decreases at higher altitudes. This temperature gradient causes sound waves to refract upward, away from the ground and results in lower noise levels being heard at a measurement location. In the evening, this temperature gradient will reverse, but, during certain meteorological conditions, the normal vertical temperature gradient could be inverted so that the air is colder near the surface, with a warmer layer blanketing the lower layer. This is illustrated in the image below.
When such an inversion layer is present, some of the sound waves will be refracted by the temperature gradient, with the refracted sound waves returned to the ground. This effect has been noticed near airports and roads, where noises can be heard over greater distances at night than at other times of day (Parnell, 2015; Saurenman, 2005), and reported by Van der Berg (2003) for WEF noises. Like wind gradients, temperature gradients can influence sound propagation over long distances, and complicate sound level measurements as well as propagation modelling. Redirecting the wave propagation direction due to a change in the density of the air which influences the speed of sound.
Effect of Wind
Wind alters sound propagation by the mechanism of refraction, that is, wind bends sound waves. Wind nearer to the ground moves more slowly than wind at higher altitudes, due to surface characteristics such as hills, trees, and man-made structures that interfere with the wind. This wind gradient, with faster wind at higher elevation and slower wind at lower elevation, causes sound waves to bend downward when they are traveling to a location downwind of the source and to bend upward when traveling toward a location upwind of the source. Waves bending downward means that a listener standing downwind of the source will hear louder noise levels than the listener standing upwind of the source. This phenomenon can significantly impact sound propagation over long distances and when wind speeds are high. Over short distances wind direction has a small impact on sound propagation as long as wind velocities are reasonably slow, i.e., less than 5 m/s.
Wind speed frequently plays a role in increasing sound levels in natural locations. With no wind, there is little vegetation movement that could generate noises and faunal noises (normally birds and insects) dominate, however, as wind speeds increase, the rustling of leaves increases which subsequently can increase sound levels. This directly depends on the type of vegetation in a certain area. The impact of increased wind speed on sound levels depends on the vegetation type (deciduous versus conifers), the density of vegetation in an area, seasonal changes (in winter deciduous trees are bare) as well as the height of this vegetation. This excludes unanticipated consequences, as suitable vegetation may create suitable habitats and food sources attracting birds and insects (and the subsequent increase in faunal communication).
Accurate Ambient Sound Levels Measurements
Accurate sound level measurements (or residual noise level measurements when in the Western Cape) is critical for a comprehensive noise impact assessment for a project. While one can work on the SANS 10103:2008 tables (recommended rating levels for certain areas), or an applicable bylaw, it is ideal to define actual ambient sound levels at a project area. While some acoustic consultants only collect short-term measurements, EARES always recommend and promote the measurement of ambient sound levels over a longer time period, especially in a rural or sub-urban environment (shorter measurements near roads or industrial could be acceptable). This is due to the high variability associated with typical environmental sounds as illustrated in the image below:
Developing a project in a quiet area without adequate noise pollution control measures could result in costly administrative or legal penalties. On the other hand, developing a project in a “noisy” area could result in significant savings if noise pollution control measures unnecessary.
This however does require accurate ambient sound level measurements, coupled with accurate noise propagation modelling; services in which EARES specializes.