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?


1 Answer 1


When there's shear waves (like in an ice layer or a steel rail, the high frequencies travel faster than the low frequencies due to higher shear wave speed. A sound impact source results in a downward chirp sound at some distance away for the wave travelling through the material. Try it if you are near a frozen pond with a thrown rock.

In the atmosphere, there's no shear wave but dispersion still occurs from damping and heat absorption in the nitrogen, oxygen, and water vapor molecules. This affects higher frequencies more due to the shorter wavelength and greater heat transfer between the hot higher pressure peak of the wave and the cooler lower pressure part of the wave. This damping of higher frequencies slows them down a little bit (dispersion) and makes the more refracted when the sound speed is not constant (from wind and temperature variations). also high frequency sound sources tend to be more directional making a big difference at long distances in turbulence from refraction. The low (bass) frequencies seem unaffected because they start out more omnidirectional and the have less dispersion from less sound absorption.

You ask a very good physics question! I hope my explanation helps (and is accurate).

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