The History:

During World War II [1], American pilots were given hollow metal spheres to be used when their planes went down over water. The spheres, being rather heavy for their small size, didn’t exactly act as great flotation devices. So why were these pilots given chunks of metal? Well, they were told that if they dropped the sphere into the ocean, their current position could be worked out by allies in the region, and they would be rescued. However, as foreign intelligence was soon to find out, the spheres were just that – hollow metal spheres – no electronics, no radio equipment, no imbued magical properties. So was the military just playing with it’s pilots, or was there actually something to these spheres?

Well, some of the details are still kinda top secret, but with a little physics and an active imagination, it’s possible to work out how they were used. Arrgh! Scary! I just said the P word. Don’t worry, I’ll try to summarize things without delving to deep into the physics.

Simple Facts: The Ocean

  • The deeper you go, the colder it gets
  • The deeper you go, the higher the pressure

Simple Facts: Soundwaves

  • Sound travels in waves
  • These waves bend towards where the sound travels _slower_
  • Sound travels faster in warm temperatures than cold temperatures
  • Sound travels faster in high pressure than low pressure

Combining the facts:

Normally, the temperature difference of water is quite gradual as you go deeper; however, there is a zone (at around 500m) where the temperature change is quite rapid. Therefore, any soundwaves entering this area, will be bent down. Once you are past this zone, at around 1km deep, the pressure becomes the dominant effect, and sound will bend upwards.
So, if a sound originates in the sweet spot, between these two areas of bending, it gets trapped, and ends up travelling great distances. This is called the sound channel:

Sound Channel

Note, if a sound originates outside of this channel, it gets deflected somewhat, but doesn’t bend back enough to get trapped. Note also, that the lines here show the path of the wave, not the wave itself. View each line as a zoomed out version of the wave pictures above.

Keep your eye on the sphere:

So, the spheres that the pilots were dropping into the ocean were of a specific thickness that would be crushed by the ocean pressure at about 1km deep. This would cause a “ping” that could be detected by underwater microphones thousands of miles away. By triangulating the sound (kinda like how GPS works) the Navy was able to work out where the sphere was dropped and go rescue the pilot!

Implications – LOFAR:

I don’t believe it – I am about to recommend a Tom Clancy novel, The Hunt for Red October. Tom goes into details as to how the US Navy took this technology and used it to detect submarines that entered the sound channel. I’d totally recommend buying it here for those of you who have enjoyed reading this posting.

Implications – Nature:

It turns out that nature beat us to the punch. Whales have been using the sound channel for years to communicate with each other over long distance via Whale Song!



[1]Ralph Chatman (a technical editor for Clancy on Hunt for Red October) suggests that we didn’t know enough about the sound channel during World War II to have used it. Thanks Ralph for pointing this out. This goes somewhat counter to what Robert Muller suggests in his UC Berkley Course on Physics for Future Presidents; however, I am more interested in the physics rather than the history so I’ll leave it for you two to sort out who’s right 🙂