New Scientist - Last Word
contributions by Dr Hugh Hunt

15 April 2009

Q: Surround Sound

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?
Jim Turton, by email, no address supplied

A: 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.

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

(there were several other contributions - see the full New Scientist article at 15 April 2009)

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