Sonic Booms

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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

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.

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.