Just like to point out that a person on the ground cannot hear the boom unless the aircraft is flying faster then the speed of sound at ground level. So... assuming the air is dry and 20C that mean if an aircraft is flying Mach 1 at 30,000ft a person on the ground wont hear the shockwave (boom) unless the aircraft is flying faster then 343.2 metres per second (768mph) at ground level. If the speed is slower the shockwave will actually curve back upwards.
You can hear the boom at any altitude, for example shooting out in the desert of Barstow CA. In the middle of the day you couldn't see the fighters most of the time BUT you could definitely hear the boom from the ground. Sounded like a bomb detonation in the distance at first.
Sorry Blaze, but the shockwave will not reach the ground unless the aircraft is going faster then the speed of sound at ground level. When you hear the boom out in Barstow it's because those aircraft are flying faster then the speed of sound at ground level.
From the US Airforce...
Sonic Boom Refraction
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 58 degrees Fahrenheit, the temperature at 30,000 feet drops to minus 49 degrees Fahrenheit. 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 is about 670 miles per hour, but an aircraft must travel at least 750 miles per hour (Mach 1.12, where Mach 1 equals the speed of sound) for a boom to be heard on the ground.
Also a sonic boom will not harm a person again from the US Airforce...
The strongest sonic boom ever recorded was 144 pounds per square foot and it did not cause injury to the researchers who were exposed to it. The boom was produced by a F-4 flying just above the speed of sound at an altitude of 100 feet.
In recent tests, the maximum boom measured during more realistic flight conditions was 21 pounds per square foot. There is a probability that some damage -- shattered glass, for example, will result from a sonic boom. Buildings in good repair should suffer no damage by pressures of less than 16 pounds per square foot. And, typically, community exposure to sonic boom is below two pounds per square foot. Ground motion resulting from sonic boom is rare and is well below structural damage thresholds accepted by the U.S. Bureau of Mines and other agencies.