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Q & A: How sound travels

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Most recent answer: 10/22/2007
How does sound travel through a liquid?
- Jacquelyn (age 12)
Hi Jacquelyn -

Whether it's in a solid, liquid, or gas, sound travels in waves. These waves move by particles colliding with one another. One hits another, and that one hits another, etc. It's like a domino effect. Heat travels in this way too, but with heat the molecule motions are random. Waves consist of collections of molecules moving back and forth together. Some waves travel as variations in pressure, such as sound waves, while others involve sideways motions of the molecules, relative to the direction the wave is going. Examples of sideways wave are the waves on the surface of water in a swimming pool. Both kinds of waves travel in liquids.

In solids, the molecules are very close together, and they have strong chemical bonds between them. To get a bit of solid to compress you have to squeeze really really hard on it. This compressibility is related to the speed of sound in the solid, for the pressure waves described above. The density of the solid is also important -- the more dense the material, the slower the sound will travel. For a crystal, sound waves may even have different speeds in different directions because it may be easier to stretch or compress the bonds along one axis of the crystal than another.

In liquids, the molecules do not follow a rigid pattern in space like those in a crystalline solid, and the bonds between molecules are usually weaker and constantly break and re-form. Raising the pressure in a tiny bit of the liquid will cause the molecules there to move to lower-pressure regions, pushing on the molecules already there, and raising the pressures there. The molecules have some inertia, so they in fact go a bit farther than it would take to even out the pressure, and the pressure in that little region where the pressure was initially higher becomes lower than the average pressure in the liquid, and the molecules are pulled back where they started. The whole process then repeats until all the energy is carried away in the sound waves. This is why you get multiple waves spreading out from the place where a rock is dropped in a pond.

Gases are just like liquids when it comes to pressure waves. Gases are less dense than the liquid versions of the substance, and they also are more compressible -- it doesn't take as much pressure to squeeze a gas into a smaller volume as it would to make a similar change in a liquid. Sound travels faster in less dense materials and slower in materials more easily compressed. Usually the compressibility change has a much bigger effect than the change in density, and the net effect is that sound travels slower in a gas than in a liquid of the same substance.

~Bob and Tom

(published on 10/22/2007)

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