After BoingBoing posted my ice dispersion sounds, many people were thinking about the cause of such sounds and the resemblance to the famous laser gun sounds in Star Wars. First: the dispersion of sound waves, meaning higher frequencies reach the listeners ears earlier than deeper frequencies, is not only an effect experienced in ice sheets, metal is another solid that can perfectly manipulate the speed of frequencies travelling through the material. Because huge thin metal plates are very rare to find (infinte plate called in physics), long wires are the best to experience the dispersion effect. The longer the wire the stronger the down glissando effect. Slinkies are good toys to demonstrate that. Robin Whittle built the possibly longest slinky of the world with 21 metres and suspended it with elastic threads. Hitting the slinky will cause a very strong dispersion effect:
In fact a sound wave in a long wire travels back and forth and produces an echo effect. This sound happened when a fly hit a long wire that Uli Wahl suspended on a hill to make the wind resonate in the metal like an aeolian harp:
The beat like rhythm is how the wave bounces back and forth on the wire. Uli recorded this sound with a very simple tiny telephone resonator, similar to this gadget called echoblaster. Another way to do this is here:
Ben Burtts laser gun sound for Star Wars was produced by hitting an antenna tower guy wire with a wrench. And I think he mixed the movement of the swords with the humming sounds of a poorly grounded TV.
Returning to the ice sounds at last: there was some speculation about how thick ice should be to experience the dispersion effect. I don’t know so much about the physics involved, but as I mentioned, I think the thinner the ice the more likely one can experience the effect. But I have also heard that Lake Baikal must make tremendous sounds with ice as thick as one or two meters during wintertime. I remember that the cracks that I recorded went through the lake from one side to the other and I could feel the ice sheet vibrate. I thought the cracks went just through the place I was sitting and the descending tones were the more far away parts of the crack reaching me with a time lag. By the way, this structure was captured at Lake Baikal, it is supposed to be the focal point for the ice break up in spring:
And this PDF written by Gunnar Lundmark is a nice observation about how the sound of ice can indicate if it is too thin for skating – “A person with ”gold ears”, absolute pitch, can estimate the thickness of the ice with an uncertainty of 5% just by listening.”
And finally some people pointed to Werner Herzogs “Encounters at the End of the World” and the seal sounds featured in the film that should not be confused with the dispersion sounds of ice – although there are some ice sounds in the film the descending tones are the calls of seals. The seals were recorded by Douglas Quin, whose CD “Antarctica” is highly recommended.