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## The Speed of Sound

by | Aug 31, 2018

F-18 Breaking Sound Barrier

Sound is generated by a vibrating object acting on molecules around it and travels as waves.  It is kinetic energy that travels from molecule to molecule. Thus, in order for sound to travel it must be transported through a medium capable of conducting waves, such as a gas, liquid or solid.  So, there is no sound in a vacuum such as space (all those explosions in sci-fi movies are actually silent!).

What is the speed of sound?  That depends on what medium it is traveling through. The closer the molecules are to each other and the tighter their bonds, the faster sound travels. Thus, temperature affects the speed of sounds, as does pressure.

• Air: At sea level at 59 degrees with zero humidity sound travels just a tad over 761 mph.  At 32 degrees at sea level the speed of sound is 742 mph. At 30,000 feet speed of sound is about 678 mph.
• Water: Sound travels at 3,315 mph through water at 46°F. The speed of sound increases in water with temperature and pressure. It is also faster in salt water.
• Solids: Sound speed can vary greatly in solids. Here’s a helpful chart (note that 447 meters per second is 1,000 mph):

Mach: “Mach” is how fast with reference to the speed of sound something (usually a plane) is traveling.  Thus, at Mach 1, a plane would be traveling exactly equal to the speed of sound. At Mach 2 the plane would be traveling at twice the speed of sound. Of course, the speed of sound depends on how high above sea level the plane is and what the temperature is.

Sonic Boom: A sonic boom can be created when a plane goes faster than the speed of sound.  This is because as an object passes through the air, it creates a series of pressure waves in front of it and behind it, similar to the bow and stern waves created by a boat. These waves travel at the speed of sound, and as the speed of the aircraft increases the waves are forced together, or compressed, because they cannot “get out of the way” of each other, eventually merging into a single shock wave at the speed of sound.  As an aircraft flies at supersonic speeds it is continually generating shock waves, dropping sonic boom along its flight path, similar to someone dropping objects from a moving vehicle.

Here’s a pretty cool compilation of jets creating sonic booms:

Thunder: A sonic boom created by a plane sounds similar to thunder because thunder is also a sonic boom. What happens is lightning super heats the air around it which causes a sudden increase in pressure and causes surrounding air to expand violently at a rate faster than the speed of sound which creates a shock wave which expands outward from the lightning bolt. The shock wave extends out about 30 feet then changes into the ordinary sound wave we know as thunder. According to the National Lightning Safety Institute, thunder is seldom heard beyond 10 miles under ideal conditions.

Here’s a compilation of lightning with thunder:

# 1 Comment

1. If a fighter jet goes supersonic in a dive, the pilot should stay supersonic (altitude permitting) during the pull out to minimize the shock wave damage on the ground. Slowing down to subsonic would send a concentrated wave to a point on the ground .

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