passage through the air at speed greater than the local velocity of sound. The speed of sound (Mach 1) varies with atmospheric pressure and temperature: in air at a temperature of 15 C (59 F) and sea-level pressure, sound travels at about 1,225 km (760 miles) per hour. At speeds beyond about five times the velocity of sound (Mach 5), the term hypersonic flight is employed. An object traveling through the Earth's atmosphere at supersonic speed generates a sonic boom-i.e., a shock wave heard on the ground as a sound like a loud explosion
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