The main difference between echo and reverberation is that

Analyze the question: It asks for the main difference between an echo and reverberation, both of which come from the reflection of sound.

Concept:

• Echo: A reflected sound heard as a separate, distinct repetition of the original sound. The reflecting surface must be at least about 17 m away, so the time gap between the original and reflected sound is at least 0.1 s (the ear's persistence of hearing).
• Reverberation: Reflected sound that reaches the ear in a very short time (less than 0.1 s) after the original sound. It blends with the original sound and prolongs it instead of being heard as a separate sound. It is common in halls and small rooms.

Check the options:

• A. Wrong. Both echo and reverberation are reflected sound. Neither is refracted sound.
• B. The time interval between the incident sound and the reflected sound is shorter for reverberation. This matches the concept above: reverberation has a gap of less than 0.1 s, while an echo has a gap of 0.1 s or more.
• C. Wrong. Amplitude is not what separates them, and an echo is usually weaker than the original sound.
• D. Wrong. Neither is defined by its source. Reverberation is not specific to speakers, and echoes are not specific to cliffs.

Answer: The correct option is B.

Tip: Remember "Reverb = Rapid return." If the reflection comes back in under 0.1 s, it merges with the original sound (reverberation). If it comes back later, you hear it separately (echo). Also remember the minimum distance for an echo: d = (v × t)/2 = (340 × 0.1)/2 = 17 m.