What is an echo?

An echo is defined as a reflected sound wave that returns to the listener after bouncing off a surface. This phenomenon occurs when sound waves travel through a medium, such as air, and strike a hard surface, such as a wall or a mountain, before reflecting back toward the source. The delay between the original sound and the returning sound allows a person to perceive the echo.

For an echo to be clearly heard, there must be a sufficient distance for the sound to travel to the surface and back; typically, this requires that the surface be at least 17 meters away for the echo to be distinguishable from the original sound. This principle is commonly observed in large, open spaces or environments with hard, reflective surfaces.

Other choices do not accurately describe what an echo is. For instance, a continuous sound with no reflections does not involve any bouncing back, so it cannot be defined as an echo. Similarly, a sound that is generated internally pertains to sounds like body noises and does not involve reflection. Lastly, a wave traveling in a vacuum describes a state where sound cannot propagate because it requires a medium; hence, this option does not apply to sound waves or the concept of an echo.