A student is performing an experiment in an open field to study echoes. He claps once and uses a stopwatch to measure the time gap between the original sound and its echo. The air temperature during the experiment is 20∘C.
(a) Define an echo.
(b) Taking the average speed of sound in air at 20∘C to be 340ms−1, show that the reflecting surface must be at least a certain minimum distance from the student for a distinct echo to be heard. Calculate this minimum distance.
(c) Explain qualitatively how this minimum distance will change on a hot summer afternoon when the air temperature rises significantly above 20∘C. Justify your answer without doing any numerical calculation.