This week, I finally managed to carry out a little experiment I've been planning for some time. The delay was because I needed access to a military jet-fighter.
You must have noticed how, when a jet-fighter flies past, the noise it makes appears to come from behind the aircraft. When I was a kid, I mistakenly believed that this was because the jet was flying faster than the speed of sound. The real reason is that the light arriving at your eyes from the jet is travelling at approximately 300,000,000 metres per second, whereas the sound arriving at your ears from the jet is travelling at around 330 metres per second—roughly a million times slower. This means that the light arrives at your eyes pretty much instantaneously, whereas the sound arrives at your ears slightly later, depending on the distance of the jet. By the time the sound arrives, the jet has moved several fusilage lengths further along its flightpath, meaning that you are seeing the jet where it is now, but you are hearing the jet where it was a short while ago.
We are able to judge the direction from which a sound is coming because we have two ears. When the sound is coming from the right, say, it arrives at our right ear a split second before it arrives at our left. It is also, thanks to the inverse-square law of acoustic waves, and to the fact that our ears point in opposite directions, louder in our right ear than our left. Our brains use these differences and other subtle cues to calculate the direction of the sound. Amazing, or what?
I finally got to perform my experiment on Monday evening. I was in the garden watering my tomatoes when I spotted a jet-fighter travelling down the valley towards me, low and fast. Its flightpath would take it about 100 metres in front of where I was standing. So I dropped the watering can and hurried to a location on the patio with a better view.
As the jet flew past, the sound appeared to be coming from a few plane-lengths behind the aircraft. Then I took out my other piece of vital experimental apparatus—my right index finger—and inserted it firmly into my right ear. As if by magic, the sound from the jet suddenly appeared to be coming directly from the aircraft. Unable to detect the direction of the sound with data from only one ear, my brain quite sensibly deduced that it must be coming from the plane.
For the remaining few seconds that the jet was in view, I repeatedly removed from and inserted into my ear my index finger, causing the direction of the sound to move repeatedly back and forth.
Give it a go some time. You don't really need a jet-fighter; any noisy, fast-moving aircraft will do.