The speed of sound varies depending on altitude, temperature and the medium through which it travels. For example, at sea level in a standard atmosphere, at a temperature of 59-degrees Fahrenheit (15 Celsius), sound travels 761 miles per hour (1,225 km/p/h). At a temperature of 32-degrees Fahrenheit (0 Celsius) the speed of sound drops to 742 mph (1,194 km/p/h). In altitudes above sea level the speed of sound is again different and will vary depending on prevailing factors.
The reason for this variance is that sound waves travel by exciting molecules. When a sound wave hits a molecule it will vibrate, thereby transferring the vibration to adjacent molecules, which pass it on in a like manner. If molecules are packed very tight, the sound wave can travel very fast, increasing the speed of sound. When molecules are not as densely packed the speed of sound slows.
Temperature and altitude affect atmospheric density, changing the speed of sound. Sound will also travel faster through water than through air, because water is a denser medium. Likewise, sound travels faster through steel verses lower-density materials like wood, or atmospheric conditions. For this reason you might see an old movie depicting someone putting an ear to a railroad track to listen for an approaching train, as sound will reach the listener faster through steel rails than by air.
When a jet travels faster than the speed of sound, it is said to break the sound barrier. This creates a shock wave or sonic boom and an instantaneous “shroud” around the jet. The shroud is actually a white vapor cloud, which when caught in fast-speed photography, makes the aircraft appear as if it is emerging from a white wormhole.
Mach 1 refers to breaking the initial sound barrier, or going from subsonic to supersonic speeds. Military jets routinely travel supersonically. When a jet travels twice as fast as the speed of sound, it is traveling at Mach 2. Three times faster than sound refers to Mach 3, and so on.