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Why Sound Waves Reflect

Author: N.H. Crowhurst

 Speed of sound is determined by density and elasticity of medium.

Sound will travel through anything except a vacuum, but the speed at which it travels is set by the density and elasticity of the material through which it is traveling. When sound, traveling in air, comes to the end of the air, it will start to penetrate whatever it strikes. The wave consists of both pressure and movement of the particles of the material through which it goes. The power in the given area of a sound wave is found by multiplying the pressure by the velocity, or rate of particle movement. Transmission in air uses a combination of large movement with small pressure, compared to transmission in, say, a brick wall.

Since the brick wall is much heavier, or denser, than air, the sound wave will not move the brick as much as it does the air. The pressure transmitted to the bricks will be the same as that built up in the air where it strikes them, but because the movement in the brick is very small, something different happens in the air where it touches the wall - it does not follow the same pressure and movement combination as air elsewhere, because the air hardly moves at all. This means that the pressure of the sound wave is almost doubled at this point. This "surplus" pressure starts another sound wave, directed away from the wall.

If the original wave strikes the wall at an angle, instead of "head on," the increase in pressure will follow the wave along the wall, as different parts of the original wavefront reach the wall. This results in the wave leaving the wall, just the same way that light gets reflected from a mirror.

 Balls indicate action of reflecting sound wave

Last Update: 2010-11-03