Assertion: boom's foot | /r/HasProperty | 9,100 m
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Source sentences:
text: For example, the speed of sound at 30,000 feet (9,100 m) is about 670 miles per hour (1,080 km/h), but an aircraft must travel at least 750 miles per hour (1,210 km/h) (Mach 1.12, where Mach 1 equals the speed of sound) for a boom to be heard on the ground.