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?
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When you move through air you displace it, and a “wave” is created which sounds like “boom”.
[Here it is for water](https://upload.wikimedia.org/wikipedia/commons/thumb/5/5c/Fjordn_surface_wave_boat.jpg/1200px-Fjordn_surface_wave_boat.jpg); if you were standing on shore you would only “hear” this when the waves reached you. The water would have relatively small waves (it would be relatively “quiet”) and then suddenly that big wake would reach you and it would sound like thunder rumbling.
And [here it is for air](https://history.nasa.gov/SP-60/i-5-1.jpg), and that is a photograph NOT a drawing. Speed of waves (sound) in air is different than in water, but you can see that the same principle applies.
The [sonic boom](https://res.cloudinary.com/dk-find-out/image/upload/q_80,w_1920,f_auto/A-iStock_000017498245Large_pxojdi.jpg) happens when the object is AT the speed of sound, the waves “collect” instead of being able to disperse fast enough. This creates pressures and turbulence on the airplane, which is why airplanes try to go THROUGH the speed of sound asap, either go faster or go slower, and not maintain that speed.
As far as whether they expected it, [here’s the description of the first flight](https://www.space.com/16709-breaking-the-sound-barrier.html) that achieved it, and as you can see it was called the “sound barrier” in 1935 (long before they attempted it), implying that it was a barrier that could not be passed. For the flight in 1947, they went close a number of times and experienced huge turbulence, plus the plane was built to withstand 18 g’s, so as sturdy as they could make it, so I would say yeah they expected it.
You can think of it this way: a plane makes noise, and that noise goes in all directions…
When the plane goes *almost* as fast as the noise (read: nearing the speed of sound), the noise in the front can not escape any more, but rather more and more noise is added… this is at its max when the plane is exactly at the speed of sound. This collected noise is basically the “sonic boom”.
Once the plane is faster than the speed of sound, the problem is gone: there is no longer noise collecting, and the boom is over.
On top of what people are saying, [Here](http://www.lon-capa.org/~mmp/applist/doppler/d.htm) is a simulation that you can use for a visual explanation. Click to place an object, click and drag to place a moving object.
One correction to several people in this thread. The sonic boom is NOT made ONLY as the aircraft passes thru the speed of sound. Any supersonic object is CONSTANTLY making a sonic boom.
Just like a boat is constantly making a wave, so too is a supersonic aircraft constantly making a sonic boom. The wave/boom just tends to only hit you once.
To answer OP’s third question. Yes, every supersonic object will create a boom. But depending on the size and speed and noise generation, may not be that loud. The crack of a whip is a sonic boom, as the tip of the whip goes supersonic. The zip of a bullet flying by is also a sonic boom.
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.
On top of everything else here, the idea of sound being a “barrier” has nothing to do with it being thought of as a “physical” barrier to be broken, but rather a technological and engineering challenge.
Planes are reliant on fluid flow patterns to actually fly (and control their own flight), but fluid flow starts to get weird when you get into the transonic regime (i.e. 80% the speed of sound or above), all of which presents a problem for controlling the plane as well as getting air into the engines (hence why a lot of the early attempts at breaking the sound barrier used rocket engines; it simplifies the problem because you don’t need to get air into the engines).
However, the issue of strange physics near the sound barrier was already known, and had been documented pretty extensively in WW2 when planes were becoming powerful enough to get near the sound barrier in dives, particularly the American P-38 Lightning. So breaking the sound barrier wasn’t done blindly or on accident.
Imagine a boat moving slowly in completely still water. It doesn’t produce a wake, but it will produce small waves/ripples around the boat. If it drives past a dock, i the dock will get hit by a bunch of ripples as the boat passes. That is like a subsonic airplane.
Now if you move the boat fast, it will be moving faster than the ripples/waves can form in front of the boat. The boat now produces a wake and whenever it drives past the dock, one massive wake hits it. That is like a supersonic aircraft’s sonic boom.
Read that as Sonic Boomers at first, thought there was a new generational lingo in the making…
Here’s the ELI5: normally sound is in front and behind something making a sound.
If it goes to fast and out runs its sound, all the sound is behind it now.
The sound builds up behind it. Too much sound skooshed together behind it goes boom.
I find it easiest to understand from the point of the observer. If something noisy is coming your way, the noise starts out faint, and gradually increases in strength as it closes in on you. It gets loudest when the object is closest to you, and then the noise fades out after it has passed you.
However when something goes faster than the speed of sound it gets closer to you faster than the sound is able to reach you in order to do the gradual build up. Instead right after the object has passed the sound goes from 0-100 in an instant, and then fades out as normal.
That instant fade in is what we call a sonic boom
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