Reflection of Sound:
The bouncing of sound waves from the interface between two media is termed as the reflection of sound.
(i) Consider a wave travelling in a solid medium striking on the interface between the solid and the air. The compression exerts a force F on the surface of the rarer medium.
(ii) As a rarer medium has smaller resistance for any deformation, the surface of separation is pushed backwards.
(iii) As the particles of the rarer medium are free to move, a rarefaction is produced at the interface. Thus, a compression is reflected as a rarefaction and a rare faction travels from right to left.
(i) A longitudinal wave travels in a medium in the form rarefactions. Suppose a of compressions and left to right reaches a rigid wall.
(ii) The compression exerts a force F on the rigid wall. In turn, the wall exerts an equal and opposite reaction R=- F on the air molecules. This results in a compression near the rigid wall.
(iii) Thus, a compression travelling towards the rigid wall is reflected back as a compression. That is the direction of compression is reversed.
(i) When the sound waves are reflected from the curved surfaces, the intensity of the reflected waves is changed.
(ii) When reflected from a convex surface, the reflected waves are diverged out and the intensity is decreased.
(iii) When sound is reflected from a concave surface, the reflected waves are converged and focused at a point. So the intensity of reflected waves is concentrated at a point.
(iv) Parabolic surfaces are used when it is required to focus the sound at a particular point. Hence, many halls are designed with parabolic reflecting surfaces.
(v) In elliptical surfaces, sound from one focus will always be reflected to the other focurs, no matter where it strikes the wall.
