Sonic Boom for Dummies

Today’s NASA Picture of the Day is of an F18 Hornet undergoing a sonic boom. The circular cloud you see in the picture is actually heavily compressed vapor that is almost liquid that is created when an object travels at the speed of sound. This was main problem in creating sonic airplanes, how does one make an airplane that will go through a wall of liquid without being damaged nor loosing any of its speed.

Scroll below the image for an explanation of how the cloud is created.

Sound is actually a longitudinal wave, meaning that every time a sound is made, a pocket of compressed air is created. This pocket is called a wavefront, and travels through air to your ear. If a sound is continuous, multiple wave fronts are created, thus forming a continuous wave. The distance between them is the wavelength, and it depends on two factors: the frequency of the sound and the velocity (speed and direction of movement) of the object.

When the sound emitting object is stationary, it sends out a new wave front at a constant rate. For example, a C note has the frequency of 260Hz, which means that a new wavefront is being sent out every 0.0038 seconds! This might seem like a really short time, but sounds travels quite fast, so by the time a new wavefront has been emitted, the old one traveled 1.2 meters.

Now imagine that the piano was moving while someone was playing the C note. The old wavefront is 1.2 meters away from where it was emitted, but the piano has moved some distance too, so the new wavefront is closer to the old one, and the wavelength is shorter. But what if the wavefronts and the piano were moving at the same speed, in other words both traveling at the speed of sound? This would make the new wavefront appear exactly where the old one was, compressing the air molecules more and more every time. That is exactly what happens when an airplane hits the speed of sound, the air becomes compressed so much from the continuous sound emission that it turns to liquid, thus forming a hard barrier (called the sound barrier), that you see in the picture, to get through.