Sonic boom is sufficiently long. A new metric has emerged, known as "perceived" loudness, measured in PLdB. This takes into account the frequency content, rise time, etc. A well-known example is the snapping of one's fingers in which the "perceived" sound is nothing more than an annoyance. The energy range of sonic boom is concentrated in the 0.1–100 hertz frequency range that is considerably below that of subsonic aircraft, gunfire and most industrial noise. Duration of sonic boom is brief; less than a second, 100 milliseconds (0.1 second) for most fighter-sized aircraft and 500 milliseconds for the space shuttle or Concorde jetliner.
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.