Which of the following best explains why an echo is heard when a sound is produced near a large, flat wall?
Sound waves reflect off the wall and return to the listener after a short delay.
Sound is a mechanical wave. When it strikes a large, flat, hard surface such as a wall, most of it bounces back into the same medium. This bouncing back of sound from a surface is called reflection. The reflected wave travels back to the listener; if it arrives after a small time delay (at least about 0.1 s), the listener's ear hears it as a separate sound, and that repeated sound is called an echo.
Sound does not pass through a solid wall and reappear on the other side to form an echo; passing through would be transmission, not reflection. Sound is not stored by the wall and re-emitted slowly either, because a wall does not absorb sound and then release it as sound again. So the only correct mechanism is reflection followed by a short time delay.
Hence, the answer is Sound waves reflect off the wall and return to the listener after a short delay.
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