If You Can’t Hear Anything, It Doesn’t Mean There is Not Sound


The Sunday Nation


06 March 2005


George Wandera has two questions: First he has observed that when aeroplanes are flying within audible range, they leave their noise behind. He wants to know, “does this mean that all aircrafts travel faster than sound?”

The answer is no. Only one commercial aeroplane travels faster than sound – the Concord – and it was grounded last year. But why is the noise of a plane left behind? This has to do with difference between the speed of sound and that of light.

Suppose an aeroplane is flying past at 500 metres away. To see the plane, light must travel from it to your eye. At 300,000km per hour, the light takes about two millionths of a second to reach you. This duration is negligibly small.

Sound, on the other hand, is much slower than light – about 330 metres per second in air (1,200 km/h). So, the noise from the aeroplane takes about one and a half seconds to reach to you. If the plane is moving at, say, 300km/h, it will have covered 125 metres in the duration that its noise takes to reach your ears.

So, the noise the aeroplane makes when it is at some position x in the sky reaches your ears when the plane is at point y. The distance between x and y is 125m Thus it appears as if the plane is moving faster than sound.

But what if the plane was actually faster than sound; what would we hear? That is a story for another day, for now Wandera’s second question is also about the noise made by aeroplanes. He learnt in school that sound is a form of energy and that energy cannot be destroyed. But when a plane is very high in the sky, its noise is not heard from the ground. What happens to the sound?

When sound is produced by a source, it radiates in all directions. So the further it gets from the origin, the more space there is to occupy. Thus the intensity (amount of energy per square metre) reduces as the sound moves further and further away – it is inversely proportional to the square of the distance. In other wards, if, say the distance is doubled (factor of two), the intensity drops to a quarter (1/4) since two squared is equal to four.

When the plane is a few kilometres away, the sound energy is dispersed over a very wide area, therefore, intensity is too low to be detected by the human ear. The fact that we can’t hear it does not mean that the noise isn’t there…which brings to mind the story of Schroedinger’s cat:

The cat is placed inside an airtight metal box. Inside the box, there is a glass capsule containing some lethal, poisonous gas. The box is shaken thus breaking the gas and releasing the poison. The cat takes a breath of the gas and a question arises: What kills the cat? Is it the poison or the act of opening the to confirm that it is actually dead?

  Back to 2005 Articles  
World of Figures Home About Figures Consultancy