I live a kilometre north of a busy motorway. When the wind is coming from the south the noise of the motorway is noticeably greater than when the wind is coming from the north. Assuming a wind speed of a mere 30 kilometres per hour, how can the wind direction affect the level of traffic noise I hear when the speed of sound is more than 1235 kilometres per hour.
• Wind is the single most influential meteorological factor within approximately 150 metres of a noise source such as a highway.
The wind’s effects are mostly confined to noise paths close to the ground. The reason for this is what is known as the phenomenon: the wind speed is lower in the vicinity of the ground because of friction.
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This velocity gradient tends to bend sound waves downward when they are travelling in the same direction as the windand upward when in the opposite direction. This process, called refraction, creates a noise reduction upwind from the source of the sound and a noise increase downwind from the source.
Over distances greater than 150 metres, vertical air temperature gradients are more important. This is because under certain stable atmospheric conditions, temperature increases with height either from the ground up, or from some altitude above the ground. Such an inversion occurs when a layer of warm air is trapped between layers of cold air. This inversion increases the speed of sound with increasing altitude, causing sound waves to be refracted back towards the ground. This would lead to an increase in ambient noise levels for far-away listeners.
Victor Zeuzem, San Mateo, California, US
• The wind does not appreciably speed up the sound and, even if it did, this would not explain why the sound should be louder. What happens is that the sound is refracted, or “bent”, in rather the same way as a ray of light is refracted as it passes from air into water.
This happens because wind speed is not constant with height. At 100 metres altitude, say, the air is moving at 30 kilometres an hour. Closer to the ground, however, trees and buildings get in the way, so the wind speed is lower. At ground level, in between the blades of grass, the wind speed is close to zero.
Sound moving horizontally through air when there is a velocity gradient like this will be bent upwards if it is moving against the wind, and downwards if moving with the wind.
“Sound moving horizontally through air will be bent upwards if it is moving against the wind”
The best way to visualise this is to imagine a row of joggers with their arms hooked together running in a straight line on a beach. If the sand is uniformly firm, they all run together at the same speed and the line of joggers moves straight ahead. Now imagine that the sand is moist but firm at the end of the line of joggers nearest the water (providing fast-going conditions), and dry and soft at the other end away from the water (providing very slow running conditions). In this case the line will curve around because the fast runners have to stay hooked to the slow runners.
So, by the same reasoning, if sound travels 30 kilometres an hour faster at 100 metres altitude than it does at ground level, the sound wave front, which can be thought of as a planar disturbance, bends downwards.
Hugh Hunt, Department of Engineering, University of Cambridge, UK