Friday, December 13, 2013

Sonic Booms

Why does a sonic boom ... boom? That is, why is it so loud? Above a plane flies at twice the speed of sound (it's Wonder Woman's plane, which is why you can't see it, just its sound waves), and we can see it sound waves dissipate over time/distance. (The sound waves get less dark to show the sound energy is dissipating.)

Compare this to a helicopter hovering in one spot:

A person standing at the red dot will hear the helicopter with a constant "loudness," or magnitude. Compare this to the person standing at the red dot when the plane flies by, in the picture at top, at mach 2. The guy at the red dot hears nothing until ... BOOM! ... and the plane has already passed him by. Then the magnitude decreases.

Below, we have a plane flying at half the speed of sound. Note how the guy at the red dot hears the plane with increasing, then decreasing, magnitude.

The plane is again loudest once it has passed the guy.

Notice that the sound waves are all distinct, as with the helicopter picture. Compare this to the plane flying at mach 2 above, or the still image of a plane flying at mach 1.3 below.

See how all the sound waves seem to converge at the red dot? Here is the simplest explanation of a sonic boom that I know, which avoids things like compressed pressure waves, etc.: in a sonic boom, you are hearing the sound of a plane from different places in the sky at the same time. In the above picture, there are 6 circles that go through the red dot, corresponding to the idea that that guy is hearing the sound of the plane from 6 spots in the sky at the same time. Of course, in real life, you don't hear sound from distinct, discrete places, but the concept is the same.

By the way, the "cone" that is formed by these circles is called a Mach cone. It is an example of a mathematical envelope, which we discuss in a previous post.

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  1. Whoa! What a brilliant way to explain this phenomenon! Instant enlightenment!
    More like this, please!

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