A student is investigating how we are able to hear an echo in everyday life.
(a) Define an echo.
(b) State any two conditions that must be satisfied for a person to hear a clear echo in air.
(c) Explain, with reasons, why echoes are usually not heard inside a small classroom even though the walls reflect sound.
(d) The persistence of hearing of the human ear is about 0.1s. Show qualitatively how this value leads to the requirement that a reflecting surface be several tens of metres away from the sound source for a distinct echo to be heard. Base your answer on proportional reasoning without performing a numerical calculation.