What is mean by rellection of sound? Explain.
(a) Reflection at the boundary of a rarer medium
(b) Reflection at the boundary of a denser medium
(c) Reflection at sound in curved surfaces

Explanation

When sound waves travel in a given medium and strike the surface of another medium , it can be bounced back into the first medium is called as reflection.
a)Reflection in rarer medium:
(i) Consider a wave travelling in a solid medium striking on the interface between the solid and the air.
(ii) The compression exerts a force f on the surface of the rarer medium.

(iii) As a rarer medium has smaller resistance for any deformation, the surface of separation is pushed backwards (Figure).
(iv) As the particles of the rarer medmm are free to move, a rarefaction is produced at the interface.
(v) Thus, a compression is reflected as a rarefaction and a rarefaction travels from right to left.
b) Reflection in denser medium :
(i) A longitudinal wave travels in a medium in the form of compressions and rare factions.
(ii) Suppose a compression travelling in air from left to right reaches a rigid wall.
(iii) The compression exerts a force F on the rigid wall.
(iv) The wall exerts an equal and opposite reaction R = -F on the air molecules. This results in a compression near the rigid wall.
(v) Thus, a compression travelling towards the rigid wall is reflected back as a compression. That is, the direction of compression is reversed.

c) Reflection in curved surfaces :
(i) When sound waves are reflected from curved surfaces, the intensity of reflected waves is changed.
(ii) Sound reflected from a convex surface:
Reflected waves get diverged.
Intensity is decreased.
(iii) Sound reflected from concave surface:
Reflected waves get converged.
Intensity is concentrated at points.
(iv) Parabolic surfaces -required to focus the sound at a particular point. Hence, many halls are designed with parabolic reflecting surfaces.
(v) Elliptical surfaces -sound from one focus will always be reflected to the other focus.
(vi) No matter where it strikes the wall.