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Is it possible for sound wave to do net work on air particles? As in can a sound wave make the air move in one direction so that it can for example move a sail boat ?

I think since molecules gyrate about a mean position even though they are in the direction of wave propagation no net work is done but I want to confirm this idea.

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2 Answers 2

up vote 2 down vote accepted

actually i wanted to ask whether sound wave does work on the medium, air, itself. like raise its temperature or something ?

Yes. The main reason sound decreases in amplitude with distance is not due to absorption; it's because sound sources emit roughly spherical radiation and are subject to the inverse square law. So an ideal plane wave in an ideal atmosphere would not attenuate with distance, because it's not subject to inverse square law.

But sound is also absorbed by the air, so even a plane wave will slowly decrease in amplitude with distance. This means the sound energy is being turned into heat energy, and increasing the temperature of the air slightly. The amount of energy absorbed varies with humidity and affects high frequencies first:

Absorption of Sound in Air versus Humidity and Temperature

Damping of Air of High Frequencies

Damping of Air of High Frequencies (Dissipation)

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Well, I guess it depends on what you mean by a sound wave. There are wave pulses that can be created in air that can absolutely do work on something else. There are Smoke Ring Launchers, which can create waves of air that could definitely, I don't know, knock over a playing card or something. But as for sound waves, like you said, you're dealing with air molecules being jostled around from an equilibrium position. As soon as the pressure waves cause the air particles to clumb together in some localized region, there are going to be areas of low pressure in the surrounding area that the air will rush back in to fill. So, the air is not going to have a lot of space through which to act on another body, or a lot of time to do so either for that matter.

Of course, sound waves CAN do work. You can break glass with sound. But it's usually not much.

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thanks for answering, actually i wanted to ask whether sound wave does work on the medium, air, itself. like raise its temperature or something ? –  Pushpendre Apr 20 '14 at 13:43
Well, sound is a mechanical wave (along with waves in water that we are more familiar with), so it can transfer energy, but not matter. So, no, sound does not do net work on the medium it travels through. –  D. W. Apr 21 '14 at 5:20
Think about it like this - focus on one specific thin imaginary vertical wall of air in front of you. When you speak, the sound wave originates in your mouth, and is directed forward, traveling perpendicular to the "wall". Your mouth creates a buildup of pressure in one region, which spreads in to the less dense space in front of it. As the dense region diffuses in to the less dense region, it makes that region more dense, and propagates forward in this way. When it gets to your "wall", the region of density passes through, but the air molecules themselves (rel. to initial pos.) are unmoved. –  D. W. Apr 21 '14 at 5:23

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