When a sound wave travels from one medium to another, an echo is produced by:

An echo is produced when sound waves reflect off a barrier and travel back to the source of the sound. This phenomenon is closely tied to the concept of acoustic impedance, which is the measure of how much resistance a medium offers to the passage of sound. When sound waves transition from one medium (such as air) into another (such as water or a solid), the difference in acoustic impedance between these media determines how much of the sound is reflected back versus how much is transmitted into the new medium.

When the acoustic impedances of the two media differ significantly, a larger portion of the sound wave reflects back, creating a clear echo. Therefore, it's the variation in acoustic impedance that plays a crucial role in the intensity of the echo that is heard.

The other options do not directly contribute to the phenomenon of echoes. Frequency pertains to the pitch of the sound but does not influence the reflection process itself. Optical density is related to the passage of light through a medium and is not applicable to sound waves. The gain setting typically refers to amplification in audio systems rather than the physical principles governing sound wave propagation and reflection.