Sunday, 5 June 2011

Sound at the Speed of Light

Sometimes a small point will intrude into the muddy pool that is my consciousness and send a ripple to the surface, demanding further examination. Occasionally, if I am not otherwise occupied, and indeed, often when I should be, a curious and entertaining facet of our Universe will emerge insisting on philosophical scrutiny.

One such notion appeared to me today. I was listening to BH on Radio4, as I often do on a Sunday, whilst dripping marvellous marmalade absent-mindedly onto my trousers from my toast. A piece came on from someone who had noticed a strange phenomenon pertaining to the chimes of Big Ben.

It appears that at a distance of about 300m from the tower housing the iconic clock and its campanological companion, when listening on the radio to the "bongs" which announce the passage of time, it is possible to apparently hear the chime before it actually happens.

Astonishingly, The Wireless will play you the sound of the bell approximately one second before you hear it with your own ears. How intriguing! How can this be?

Well, it appears that this illustrates the enormous difference between the speed of light and the speed of sound. As a child, sound and light were somehow related inasmuch both conveyed information to us, only of course at seven years old, I didn't really think of seeing and hearing as information. Nevertheless, they seemed to be manifestations of the same kind of thing: What our senses receive. So, it seemed quite strange that later, when I began to have a grasp of physics, to discover that they are in fact two entirely different mechanisms, moving two different types of energy in entirely different ways, one compressive and relying on a medium, and the other, radiation, able to travel through a vacuum (ok, leaving out "the Ether" as a concept, now largely abandoned) as a mish-mash of electricity and magnetism (that I frankly still can't quite reconcile in my poor, overly-spherical head).

But here we have a perfect example of how the two differ.

So, how do we hear the bell on the radio before it actually seems to chime within our actually range of hearing.
Well, it is less mysterious in actuality, but nonetheless delightful in explanation:
The microphones that pick up the chime for the BBC, and which carry the sound to the radio transmitters and thence to the radios of Britain and the World, are mere metres away from the source of the sound. As such, the characteristic "bong" reaches us as the speed of radio waves, plus the time it takes the sound to go a few meters to the microphone. In effect, instantaneously.

Whereas, being physically 300m from the source of the sound means that, at the speed of sound (340 metres per second at sea level) it takes approximately a second or thereabouts to reach our eardrums.
So, on the radio, it arrives pretty much as it happens but over the intervening London air, it arrives about a second later.

Isn't that a delightful and strange thought?