Definition: speed of sound

Sound waves travel at approximately 750 mph at sea level, but slow down at higher altitudes. The speed of sound is called "Mach 1." Supersonic refers to speeds from Mach 1 to Mach 5 (1 to 5 times the speed of sound), and hypersonic ranges from Mach 5 to Mach 10 (5x to 10x).