<P> Depending on the aircraft's altitude, sonic booms reach the ground two to 60 seconds after flyover . However, not all booms are heard at ground level . The speed of sound at any altitude is a function of air temperature . A decrease or increase in temperature results in a corresponding decrease or increase in sound speed . Under standard atmospheric conditions, air temperature decreases with increased altitude . For example, when sea - level temperature is 59 degrees Fahrenheit (15 ° C), the temperature at 30,000 feet (9,100 m) drops to minus 49 degrees Fahrenheit (− 45 ° C). This temperature gradient helps bend the sound waves upward . Therefore, for a boom to reach the ground, the aircraft speed relative to the ground must be greater than the speed of sound at the ground . 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 . </P> <P> The composition of the atmosphere is also a factor . Temperature variations, humidity, atmospheric pollution, and winds can all have an effect on how a sonic boom is perceived on the ground . Even the ground itself can influence the sound of a sonic boom . Hard surfaces such as concrete, pavement, and large buildings can cause reflections which may amplify the sound of a sonic boom . Similarly grassy fields and lots of foliage can help attenuate the strength of the overpressure of a sonic boom . </P> <P> Currently there are no industry accepted standards for the acceptability of a sonic boom . Until such metrics can be established, either through further study or supersonic overflight testing, it is doubtful that legislation will be enacted to remove the current prohibition on supersonic overflight in place in several countries, including the United States . </P> <Table> <Tr> <Td> </Td> <Td> This section needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (January 2016) (Learn how and when to remove this template message) </Td> </Tr> </Table>

Why do you hear a boom when a plane breaks the sound barrier