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So acoustic dispersion is the phenomenon responsible for the classic Star Wars laser sound. An answer in this thread recently also linked to some great recordings of ice sheets which exhibit the same effect. I understand that the high frequencies reach the listener before the low frequencies, but I don't understand why this happens in some materials and not others?

Can anyone explain the science behind this in layman's terms?

So I've been mulling over the Wikipedia entry and it seems like the key is to understand phase velocity. I've been looking at some interesting java applets which make some sense, but I'm having trouble connecting this to anything real - like what's actually happening in the material, or to think of it another way what's the process? How does it differ between different substances?

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