# Why does ice make such peculiar sounds?

The pitch of the sounds start high and end up low. Is this some kind of doppler effect?

No. This is chirp induced by dispersion, which is the acoustic version of the same phenomenon for light. This refers to the fact that, generically, sound waves of different frequencies travel at different speeds.

This can be caused by the material properties of the medium, and it can also occur if the sound wave is confined in some kind of waveguide (like a hole or a sheet of ice). For each individual sample a full analysis is necessary to understand which bit of the circumstances caused the dispersion (so it's hard-to-impossible to tell you any specifics about the individual recordings you've given) but the phenomenon is fairly generic so it really doesn't matter much.

If you have short, sharp sounds, the waveform will typically have a very high harmonic content, i.e., if you decompose it as a superposition of monochromatic waves at different frequencies, it will cover a broad band of such frequencies. For the sounds you've given, the higher frequencies travel faster, and you get that descending chirp as the sound transitions to the lower frequencies which take longer to arrive.

The reason this sounds so similar to a sci-fi 'blaster' sound is because this is how blaster sounds were first recorded: Star Wars sound engineer Ben Burtt created (recorded) that sound by recording the guy wire of a tower when he tapped it with a spanner:

Image source

Under tension, the transverse waves have exactly the same dispersion as the ice, so the physics is the same, and the sound is the same.

For more discussion of this, see this previous question on this site.

It's also worth mentioning that the equivalent phenomenon for light ─ taking a short pulse and passing it through a dispersive element so that the higher and lower frequencies arrive at different times ─ is the heart of what makes Chirped-Pulse Amplification (the dominant technology for creating intense pulses of light) work.

This phenomenon is known as Acoustic Dispersion.

Acoustic dispersion is the phenomenon of a sound wave separating into its component frequencies as it passes through a material.

This happens because the phase velocity of a sound wave is a function of frequency in a dispersive media $$^{\dagger}$$. Much similar to how electromagnetic waves disperse when they pass through prism.

Sound waves consist of waves of many different frequencies. When a sound wave travels through air, its component frequencies usually travel together at the same rate, so they all reach the human ear more or less simultaneously. But due to ice being a dispersive media the sound waves propagating through it disperse. Hence high pitch frequencies get separated from low pitched one. And this gives that sci-fi effect to the sound that you hear.

This has been explained over here:

• What makes frozen lakes sing?- NPR's Skunk Bear

• Skipping Stones on Ice Makes Crazy Sci-fi Sounds

Useful:

• $$^{\dagger}$$Why do some types of waves disperse?