Supersonic aircraft 5 is often referred to as hypersonic. In this region the problems of drag and heating are even more acute. It is difficult to make materials which can stand the forces and temperatures generated by air resistance at these speeds, and hypersonic flight for any significant length of time has not yet been achieved. A sonic boom is the sound associated with the shock waves created whenever an object traveling through the air travels faster than the speed of sound. Sonic booms generate significant amounts of sound energy, sounding similar to an explosion or a thunderclap to the human ear.
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.