Sound needs a medium โ cannot travel in vacuum. Speed in air โ 343 m/s at 25ยฐC. Speed in water โ 1500 m/s. Speed in steel โ 5000 m/s. Echo: reflection of sound from a hard surface. Minimum distance for echo = 17.15 m (so reflected sound arrives after 0.1s). SONAR uses echo to find depth of ocean or detect submarines.
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