I’ve never really understood how something travelling faster than teh speed of sound causes a sonic boom.
Secondary, like when man first broker the sound barrier, did the scientists *know* a sonic boom would occur, or was it a surprise and they all were like “WTF was that, did we just break something”?
Thirdly, is a sonic boom guaranteed when something breaks the sound barrier, or do they sometimes not happen?
In: 58
A plane makes noise. If the plane is moving slower than that noise, no sonic boom. If the plane meets or exceeds that speed, it will make a sonic boom. Here’s how. Let’s pretend that the frequency of noise the plane is emitting is a 1Hz sound (it is not 1Hz, but it makes it simpler to just say, wait one second and see what it looks like. If I said 2Hz, I would just say wait 0.5 seconds. if it were 100 Hz, I would say wait 0.01 seconds… you get the idea. 1Hz is just a simpler number to work with). Let’s watch what happens to our plane.
See [this diagram I made](https://imgur.com/a/Et0Zg5g). The top three images represent a plane traveling at subsonic speeds. The circle around the plane represents the sound emanating from it. If you were to set up a bunch of pressure gauges around the plane while it was not moving, you would see the pressure rise and fall at 1Hz because that is the sound it is making. So let this circle represent the peak of that pressure increase. That peak travels out in a perfect sphere at the speed of sound in all directions.
After 1 second of travel, notice that the radius of the sound sphere has increased by the speed of sound. The plane has traveled forward during that time, so it is no longer situated at the center of that sound sphere. However, the sound is traveling faster than the plane. We can see this because it is still within the radius of the sound sphere. Remember also, the plane is still making that same 1Hz noise, that means exactly 1 second after the first peak pressure wave was emitted, another peak is reached, so a new sphere is drawn. (Try and remember that the pressure is smoothly transforming from high pressure to low pressure during the entire travel of the plane, but trying to draw infinite circles would not be easy, nor would it be helpful, so I’m only drawing the peak pressures).
Wait 1 more second and you see that the original sphere is now twice as far in front of the plane as it was in the previous picture. The plane is traveling slower than the sound so it will fall further and further behind. Each sphere of sound, having started inside the previous one and expanding at the exact same rate, will always stay inside the previous sphere.
Now look at the bottom row, representing a supersonic plane. It emits a peak pressure wave. 1 second later, the pressure wave has expanded by the speed of sound, but the plane is now slightly in front of that pressure wave because it is traveling faster than it. It emits a new peak pressure wave. This wave is partially outside of the first wave because the plane was in front of the peak when it was emitted. If you wait 1 more second, you’ll notice now the plane is completely outside the first sphere. It is traveling faster than sound, so it will continue to increase its lead.
Take a look where the pressure waves intersect. Don’t forget, the whole area inside the circles is smoothly changing pressure. Sometime, the high pressure peak of one wave will hit the low pressure trough of the other wave. The pressures will equalize and there will be absolutely no sound there. In other places, marked by these intersections, the peak pressure of one wave will line up exactly with the peak pressure of another wave. The two pressures will add on top of each other making an even higher pressure wave, meaning an even louder sound. That louder sound of stacked pressure waves is called a sonic boom. My diagram might be a bit tough to see how to extrapolate further since I only did 3 pressure waves, but there are [plenty of other diagrams](https://i.stack.imgur.com/rDMBl.jpg) out there that may make it easier to see. I just wanted one of my own to comment on and draw arrows and label stuff appropriately for eli5 purposes.
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