Sonic anemometer

One thing we’ve noticed with our new house is how variable the background noise is. We now live within earshot of State Highway One and we can hear the distant rumble of trucks and other vehicles on it. The noise isn’t large – and it’s amazing that after just three weeks in the house we no longer can perceive it – we only ‘hear’ it when we specifically listen for it. But when we do listen for it, just how loud it is depends a lot on wind direction (not neglecting of course, time of day).

If the wind is blowing from the north, not only do we warm up a bit but we get an increase in the noise. Conversely, a southerly wind, though unpleasant in terms of temperature, means the road becomes pretty well inaudible.

The reason is that the sound waves travelling from the road to the house are travelling in the air. Sound needs air (or some other material) to travel in – you may for example have seen the demonstration of an alarm clock in a vacuum jar – you can’t hear it because there’s no air to carry the sound from it. Sound waves literally get taken along by the wind. If you happen to be downwind, the sound will be a bit louder; if you are upwind, a bit quieter.

Also, but not noticeable, is that the length of time taken for the sound to get from the road to the house will change. If the wave is travelling with the wind, it will travel that bit faster, equal to the speed of sound in still air plus the speed of the wind. If the wave is travelling against the wind, the speed is that bit lower (the speed of sound in still air MINUS the speed of the air).

This phenomenon is used by a sonic anemometer to measure the wind flow. These rather frightening looking things have a number of transducers and sensors that emit and detect sound – by timing just how long sound takes to go from one to another the speed and direction of the wind can be worked out – pretty accurately too. They are pretty common sensors for taking around on field trips where you need to know what the wind is doing.

 N.B. I think they look frightening because of the shiny ‘prongs’. It looks like either it’s a very painful medical instrument of some description or high voltage arcs are meant to jump across between prongs. Neither is the case.

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