A sound wave traveling downward with a speed of about 4,000 m/s suddenly slows to 1,500 m/s not far below the Earth’s surface.What has the sound wave most likely encountered?

Sound speed depends on the material it travels through. A sudden drop from about 4,000 m/s to 1,500 m/s most likely means the wave entered a different kind of rock with different properties. An underground lake or open cavity would disrupt or strongly reflect the wave rather than simply indicate a change to another rock’s speed.

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