The first time you hear ice sheets cracking, it’s probably not what you would expect. You can hear the effect more clearly in sound artist Andreas Bick’s recordings. But what’s going on? Why would ice cracking sound so much like striking a long metal wire, and what does that have to do with Star Wars?
There are a few different kinds of ways in which waves move through a material, the most common being longitudinal waves, such as in air (also called compression or pressure waves), transverse waves, such as in a string (also called shear waves), and surface waves, such as on the surface of water.
But there are a couple of other much more complicated waves, and one of them occurs mostly with sheets of solid materials, like metal… or ice. This wave is called a flexural (or bending, or Lamb) wave, where the particles of the medium expand and contract perpendicular to the wave direction (see it in action in a railroad). One of the unique and startling properties of flexural waves is that high-frequency components move faster than low-frequency components! You can see this in action in this video:
Materials that do this crazy varying propagation speed are called Dispersive, and both metal and ice are dispersive media.
But what about the Star Wars blasters? Turns out the original movie sound effects were created with one of the handiest dispersive materials around—a metal wire. Quoting Wikipedia, which is quoting two nifty books on sound design:
Ben Burtt, a sound designer who worked on the Star Wars films, came up with the sound of blaster fire during a family backpacking trip in the Pocono Mountains in 1976. Burtt hit the guy-wire of an AM radio transmitter tower with a hammer and recorded the sound with a microphone close to the impact.
If you want to try it yourself, you can use a slinky: