Absorption

Author: N.H. Crowhurst

When a sound wave changes medium, some of it is reflected and some of it is absorbed

Sound is never completely reflected. Some of it is also absorbed. If a sound wave hits the wall of a room, most of it will be reflected back into the room. Some of it, however, will go on into the wall. Some of it may even go out into the next room. This is the way sounds in one room can be heard in the next - through the wall.

Each time a wave encounters a change in medium, this happens. Some of it goes into the next medium; some of it gets reflected. The proportions depend on the differences between the two media (in the example, the air of the room and the substance of the wall), the wavelength of the sound waves, and the angle at which they strike.

If a sound wave is traveling parallel to a wall, the wavelength along the wall will be the same as in the wave traveling in air. Very little absorption will occur because the wavelength of the same frequency in the wall is much longer (since sound travels faster in the material of the wall).

If the wave hits the wall "head on," the reflection and absorption will divide according to the density and elasticity of the wall material compared to air. But if the wave hits at a particular angle, the wavelength along the wall due to the striking wave may be the same as the natural wavelength for this wave in the wall.

Because this tends to make the wall take up more of the sound wave, there is a critical angle at which a sound wave will strike a surface, at which it will absorb much more than either a head-on strike or traveling parallel.

Possible ways for a sound wave to strike a wall