Waves can take two basic forms: transverse, or up-and-down motion, and longitudinal, or material compression. Transverse waves are like ocean waves or the vibrations in a piano wire: you can easily see their movement. Compression waves, by comparison, are invisible alternating layers of compressed and rarefied molecules. Sound and shock waves travel this way.
Compression waves can travel only through some kind of material medium, such as air, water or steel. A vacuum cannot carry compression waves, as it has no substance to conduct the energy. Their dependence on a medium means these are mechanical waves, and the medium determines their speed of movement. The speed of sound through air, for example, is 346 meters per second. A dense material such as steel conducts sound at 6,100 meters per second.
If you could see a compression wave moving through the air, you would see an area of molecules compressed in the direction from which the wave was traveling. The molecules become more and more rarefied after the maximum compression point, until you see an area of lowest pressure having the fewest air molecules. The air becomes progressively denser after that point, until you reach a maximum compression again. The distance between maximum compression or rarefaction points is one wavelength. As the frequency of a wave goes up, its wavelength becomes shorter.
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Two or more waves, crossing the same point in a medium, interfere with each other. You can see this if you drop two stones in a still pond; ripples spread out and overlap with each other. The same happens with compression waves. If a compression point meets a rarefied point, the two cancel each other out. If two compression points meet, they reinforce each other, creating a point having twice the pressure.
A jet moving through the air faster than the speed of sound produces a sonic boom. As the jet moves forward, air molecules pile up in front of it, like snow in front of plow. The compressed and rarefied layers of air do not move straight out from a source, as you get with sound. The shock wave forms a cone-shaped pattern with the tip just ahead of the plane, and compression waves moving out behind it in ever-larger circles.