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A PHYSICIST WRITES . . .
(October 2002)
Autumn: “season of mists and mellow fruitfulness” ... and the M4 roar! We live about three-quarters of a mile north of the motorway, and autumn seems to bring an increase in the number of days (or more often, nights) when the traffic is audible. Sometimes, if we are outside, it is like the sound of an enormous waterfall just out of sight.
How does this happen? It can’t be the wind, because the noise seems to be loudest when the air is still. There are in fact several atmospheric effects all working together here.
Usually, any sound becomes quieter as it spreads out from its source. But if it is channelled to stop it spreading, then it does not lose its strength — putting your ear to a pipe, for example, enables you to hear a noise from the other end quite clearly, even over a long distance.
Another effect comes into play when sound waves are travelling at a shallow angle up through air of varying temperature: the direction of travel gradually bends away from the warmer layer of air — whether this is above or below. So in “normal” conditions, with warm air near the ground and cooler air above, the noise from the M4 spreads upwards more than it does sideways (perhaps a balloonist could confirm this?), and we hear none of it at home.
But in autumn (I guess) the sudden cool of the evening makes it more likely that a layer of cold air will settle over the ground, with warmer air above. Some of the traffic noise will then be trapped and channelled out through this layer, flowing like a sheet of water for a long distance without getting quieter. Hence our mildly disturbed nights.
Actually the sheet-of-water analogy is a bit misleading, because the noise from each individual vehicle spreads out over the ground in all directions, not just due north and south. This is how we get the sound-impression outside of a wide waterfall. It also explains why in the small hours I am sometimes able to hear the noise of a high-speed bike on the M4 continuing for anything up to a minute. I just hope the rider is more wide awake than I am.
Peter Soul
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