Sunday, November 20, 2005

Weird Science: Speed Of Sound


Sound wave velocity is not a constant. Sound waves depend very much on which medium (no not the ones channeling spirits) they travel through. Through gases they travel slowest, in vacuum not at all since there must exist some medium to transmit sound, through water very quickly and steel and quartz even faster.

So what is a sound wave anyhow? Well, it is roughly described as a vibration in an elastic medium (such as air, water or steel) that is heard by the human ear. So, when a tree falls in the forest and nobody hears it - it, by this definition, did not create sound waves and hence didn’t make any sound.

Don’t sweat it - it will drive you crazy.

So just how fast do these sound waves that we hear actually travel? Okay, through air they travel about 1,130 feet/second if the air is at sea level pressure and around 72 degrees F. Now, transmit the same sound waves through water and they race along at 4,800 feet/second, and in steel this jumps to 16,000 feet/second and finally in quartz all the way up to 18,000 feet/second.

Pretty fast huh? Well, compared to light it is a complete slug since light travels at around 186,300 miles/second or 669,600,000 miles/hour. Sound, on the other hand, travels through air at around 740 miles/hour.

That makes light speed around 904 million times faster than sound.

And that is why, when you see a flash of lightening in the distance, you don’t hear the sound until after, sometimes quite a long time after depending on the distance of the lightening flash.

There are also some interesting properties of the human ear in relation to what sounds can be heard, but that is for another day.
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