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.
Large aircraft and cost reasons. Barring a few military types, no practical large aircraft has ever had more than four engines. As aircraft get bigger, it therefore becomes necessary to design bigger engines. The airspeed of a fan blade must be kept below the speed of sound in order to avoid damaging and noisy shock waves. This maximum speed of the tip sets a limit on the rate of rotation. For a given rate of rotation, the tip of a larger fan will travel faster. So to keep down the top speed of a large engine, the fan must spin more slowly.