# Thread: Sonic boom

1. ## Sonic boom

Is there a sonic boom when the flying object/plane slows down, below to the speed of sound and if not, why not?

2. I'm not an expert but the way I understand it the sonic boom is there all the time that the plane is flying faster than sound. When it slows to less than the speed of sound the sonic boom is no longer there.

It has to do with air compression along the leading edges of the airplane.

3. Syzygys, you appear to be of the opinion that sonic booms result from a plane crossing the sound barrier. That's not what happens. A sonic boom results when the plane goes faster than the speed of sound. For example, if a plane flies at hypersonic speeds across some continent, people under the plane's path from one edge of the continent to the other will hear the plane's sonic boom.

4. Originally Posted by D H
Syzygys, you appear to be of the opinion that sonic booms result from a plane crossing the sound barrier. That's not what happens. A sonic boom results when the plane goes faster than the speed of sound.
Lol I think that's the same thing.

Explanation (Wiki):
When an object passes through the air, it creates a series of pressure waves in front of it and behind it, similar to the bow and stern waves created by a boat. These waves travel at the speed of sound, and as the speed of the object increases, the waves are forced together, or compressed, because they cannot "get out of the way" of each other, eventually merging into a single shock wave at the speed of sound. This critical speed is known as Mach 1 and is approximately 1,225 kilometers per hour (761 mph) at sea level.

5. Originally Posted by nonsense
Syzygys, you appear to be of the opinion that sonic booms result from a plane crossing the sound barrier. That's not what happens. A sonic boom results when the plane goes faster than the speed of sound.

Lol I think that's the same thing.
Not clearly. The sonic boom begins to be produced when the plane reaches Mach 1, and is produced continuously from then on until the plane slows to below Mach 1.

Anything flying faster than Mach 1 is producing a sonic boom, anything flying slower is not.

6. Crossing the sound barrier and going faster than the speed of sound is the same thing ice.

7. SONIC BOOM

8. There a picture is worth a thousand words.

9. Originally Posted by nonsense
There a picture is worth a thousand words.
Play the video connected with it - you can watch the cone of the boom follow the jet.

http://www.youtube.com/watch?v=-d9A2oq1N38 There's a good video right at the beginning, again at about 1:45, and the best right at the end.

10. Originally Posted by cosmictraveler
SONIC BOOM

Not actually a sonic boom, just condensation forming in the reduced-pressure regions.
It can happen in sufficiently humid air at significantly below mach 1.

11. Originally Posted by Syzygys
Is there a sonic boom when the flying object/plane slows down, below to the speed of sound and if not, why not?

My late grandfather, was one of the first in britain to pilot a jet.

Jets can break the sound barrier, and this is what causes the sonic boom.

12. Originally Posted by Reiku
...Jets can break the sound barrier, and this is what causes the sonic boom.
That is mainly words with little understanding in them. What is more descriptive of the physics follows:

When and object moves thru the air (or any fluid) at speeds less that the speed of sound in that fluid the fluid does not compress, but just moves out of the way. This is why the analysis of streamline flow around a car (for example to determine the coefficient of drag, in a computer before ever building the car) always treats air as an incompressible fluid. -Anyone who has pumped up a bicycle tire and first learns this thinks that is strange.

The speed with which a fluid can get out of the way of an object moving thru it depends upon the speed of the individual molecules (or atoms) in the fluid. This is related to the temperature of the fluid. If the temperature is increased fourfold, then the average speed of the molecules is only doubled. Thus the speed of sound increases approximately as the square root of the temperature. The air at altitude is cooler than at sea level, so a jet could go faster at low altitude and still not exceed the speed of sound than at high altitude, but of course pushing the dense lower air out of the way, even without compressing it, takes a lot more energy.

As you get near the local speed of sound, the inertia of the air being pushed out of the way does cause some slight compression of it and that takes energy also. (Of course, just giving speed to the air is also a transfer of energy to the air.) It is for these reasons that the commercial jets fly significantly slower than the speed of sound.

When any object does move thru a fluid faster than the speed of sound, it is continuously compressing air significantly. This is like the sudden release of gas in an explosion, but continuous. Like an explosion, humans hear the expanding pressure wave made by the object exceeding the speed of sound as a "boom".

13. Originally Posted by Oli
Not actually a sonic boom, just condensation forming in the reduced-pressure regions.
It can happen in sufficiently humid air at significantly below mach 1.
I just wanted to show what the sonic boom would look like if it could be seen. I'd think that picture is a good example of the boom if it could actually be seen, don't you? Or do you have a better picture you want to add?

14. No better picture, but the boom would start at the nose of the aircraft....

Edit: one "objection" I have is that there's a similar picture on the net of a B-2 Spirit stealth bomber with shock wave and the woo woos post it repeatedly, screaming "It IS supersonic! We knew it!!"

15. Originally Posted by Billy T
That is mainly words with little understanding in them. What is more descriptive of the physics follows:

When and object moves thru the air (or any fluid) at speeds less that the speed of sound in that fluid the fluid does not compress, but just moves out of the way. This is why the analysis of streamline flow around a car (for example to determine the coefficient of drag, in a computer before ever building the car) always treats air as an incompressible fluid. -Anyone who has pumped up a bicycle tire and first learns this thinks that is strange.

The speed with which a fluid can get out of the way of an object moving thru it depends upon the speed of the individual molecules (or atoms) in the fluid. This is related to the temperature of the fluid. If the temperature is increased fourfold, then the average speed of the molecules is only doubled. Thus the speed of sound increases approximately as the square root of the temperature. The air at altitude is cooler than at sea level, so a jet could go faster at low altitude and still not exceed the speed of sound than at high altitude, but of course pushing the dense lower air out of the way, even without compressing it, takes a lot more energy.

As you get near the local speed of sound, the inertia of the air being pushed out of the way does cause some slight compression of it and that takes energy also. (Of course, just giving speed to the air is also a transfer of energy to the air.) It is for these reasons that the commercial jets fly significantly slower than the speed of sound.

When any object does move thru a fluid faster than the speed of sound, it is continuously compressing air significantly. This is like the sudden release of gas in an explosion, but continuous. Like an explosion, humans hear the expanding pressure wave made by the object exceeding the speed of sound as a "boom".
Very informative, thank you.

16. Originally Posted by Oli
No better picture, but the boom would start at the nose of the aircraft....

Edit: one "objection" I have is that there's a similar picture on the net of a B-2 Spirit stealth bomber with shock wave and the woo woos post it repeatedly, screaming "It IS supersonic! We knew it!!"
This one?

17. That's the one.
Wow: the B-2 really IS supersonic and you've proved it...

18. But its top speed is only 450 MPH! Must be photoshoped!

19. Secret government technology!
They're lying to us!

20. Originally Posted by Oli
Secret government technology!
They're lying to us!
Or headed down thru colder high altitude air - See post 15 for the physics.

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