The reflecting surface must be sufficiently smooth and shaped such that the sound is reflect back and not dispersed.
Sound travels at approx 340 metres per second and the average human brain easily can distinguish sounds that are 1/2 second apart. That means the sound must travel at least 170 metres so that the reflecting surface must be half that distance away. While a greater distance will create a more distinctive echo, the strength of the returning sound fall off rapidly (at twice the square of the distance to the reflecting surface). In normal circumstances 170 metres (= approx 500 feet) is a good distance. A cave or convex reflecting surface will allow a greater distance.
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Conditions necessary for a congruent triangle? arethey must know the funky chicken dance
You are too close to the surface from which the sound waves are reflected. As a result you brain cannot distinguish between the original sound waves and the reflexted sound waves (the echo).
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The conditions necessary for rusting are water (or water vapour) and air, specifically Oxygen. If one condition is absent and the other is present then rusting would not occur but if both are present then rusting will occur
The total distance traveled by the sound wave is 2 m. At a rate of 330 m/s, it will take 0.00606 seconds for the bat to hear the echo.