When sound waves are incident on a boundary between two media, a part of incident waves returns back into the initial medium (reflection) while the remaining is partly absorbed and partly transmitted into the second medium (refraction) In case of reflection and refraction of sound

(1) The frequency of the wave remains unchanged that means

                w_i = w_r = w_t = w = constant

(2) The incident ray, reflected ray, normal and refracted ray all lie in the same plane.

(3) For reflection angle of incidence (i) = Angle of reflection (r)

(4) For refraction

(5) In reflection from a denser medium or rigid support, phase changes by 180° and direction reverses if incident wave is y = A₁ sin

(6) In reflection from a rarer medium or free end, phase does not change and direction reverses if incident wave is y = A_I sin

(7) Echo is an example of reflection.

If there is a sound reflector at a distance d from the source then time interval between original sound and its echo at the site of source will be