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The minimum distance between the source of sound and the obstacle for an echo to take place is (speed of sound is 340 m/s): (a) 17 m (b) 172 m (c) 17 cm (d) 34.4 m In a Stethoscope, sound of heartbeats travel to the stethoscope tube :
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1) the minimum time interval for humans to distinguish between two sounds is about 0.1 seconds. Therefore, the minimum distance for an echo to occur is 2 * 0.1 seconds * 340 m/s = 34.4 meters.