Define wave velocity in relation to a wave

A wave passes from one medium to another medium. Mention one property of the wave out of speed, frequency or wavelength (i) which changes, (ii) which does not change.

What do you mean by reflection of sound? State one condition for the reflection of a sound wave. Name a device in which reflection of a sound wave is used.

What is meant by an echo? State two conditions necessary for an echo to be heard distinctly.