Sonic boom the ground. For example, the speed of sound at is about , but an aircraft must travel at least (Mach 1.12, where Mach 1 equals the speed of sound) for a boom to be heard on the ground. 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
Swept wing air in front of the aircraft begins to compress. This creates a force known as wave drag. This wave drag increases steeply until the whole aircraft is supersonic and then reduces. However, shock waves can form on some parts of an aircraft at a speed where the aircraft is moving at less than M=1.0 because the "local" speed of sound is determined by air temperature. A rapid drop in air density caused by aircraft contours will reduce the local air temperature and will correspondingly cause a drop in the local speed of sound that can cause shock waves to form.