Sound speed varies with altitude, temperature, and medium. Sound waves travel through vibrating molecules, and denser mediums increase speed. Breaking the sound barrier creates a shock wave and a white cloud of vapor. Mach 1 is the initial sound barrier, and military jets travel supersonic.
The speed of sound varies according to 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 degrees Celsius), sound travels at 761 miles per hour (1.225 km/p/h). At a temperature of 32 degrees Fahrenheit (0 degrees Celsius) the speed of sound drops to 742 mph (1.194 km/p/h). At altitudes above sea level the speed of sound is still different and will vary depending on the prevailing factors.
The reason for this variation is that sound waves travel through exciting molecules. When a sound wave hits a molecule, it will vibrate, thus transferring the vibration to adjacent molecules, which similarly transmit it. If the molecules are tightly packed, the sound wave can travel very fast, increasing the speed of sound. When the molecules are not as densely packed, the speed of sound slows down.
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. Similarly, sound travels faster through steel than low-density materials such as wood or atmospheric conditions. Because of this you may see an old movie showing someone putting an ear on a railroad track to hear an approaching train, as the sound will reach the listener faster through the 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 “cloak” around the jet. The veil is actually a white cloud of vapor, which when captured in high-speed photography, makes the aircraft appear as if it were 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 supersonic. When a jet travels twice as fast as sound, it travels at Mach 2. Three times faster than sound refers to Mach 3, and so on.
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