## Reflection of sound

### Reflection of sound

1. Because sound propagates in the form of waves, it shows both the phenomenon of reflection and refraction. When sound wave travelling in a medium strikes the surface separating the two media, a part of incident wave is reflected back into initial medium obeying ordinary law of reflection while the rest is partly absorbed and partly reflected or transmitted into second medium.

2. When a sound wave gets reflected from a rigid boundary, the particles at the boundary are unable to vibrate. Thus, a reflected wave is generated which interferes with the oncoming wave to produce zero displacement at the rigid boundary. At these points of zero displacement, the pressure variation is max. This implies that the phase of wave is reversed but the nature of sound wave does not change i.e., on reflection the compression is reflected back as compression and refraction as rarefaction. If the incident wave is represented by the equation:
y = a sin( ωt – kx ), then the equation of reflected wave takes the form
y= a’ sin (ωt+kx+π) = -a’ sin (ωt +kx)
where a’ is the amplitude of reflected wave.

3. A sound wave is also reflected if it encounters a rarer medium or free boundary or low pressure region. A practical example is when a sound wave travels in a narrow open tube. When the wave reaches an open end, it gets reflected. The force on the particles there due to the outside air is quite small and hence, the particles vibrate with the increasing amplitude. As a result, the pressure there remains at the average value. This implies that there is no change in the phase of wave but the nature of sound wave is changed i.e., on reflection the compression is reflected back as refraction and vi- versa.
If the incident wave is :
y=a sin ( ωt – kx )
, then the equation of reflected wave takes the form
y= a’ sin (ωt +kx)
where a’ is the amplitude of reflected wave.