Curiosity - Relative speeds in relation to the sound barrier?

  • Thread starter some bloke
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  • #1
some bloke
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So, I was reading through this thread:
https://www.physicsforums.com/threa...lgun-make-a-lot-of-noise.985309/#post-6309691
and I managed to make one of my usual questions which need more specific knowledge on the subject than I can find on the web.

I (think I) understand that a sonic boom is not so much an event as a continuous phenomenon, where the soundwaves of an object all pile together and create a far louder noise than the object made in the first place.

If you put something in a wind tunnel (in this case, a theoretical wind tunnel of sufficient length for our experiments which produces laminar flow of air), what is the speed of sound in relation to the moving air?
EG if you have the air travel at mach 0.25 and flew a rocket against the wind at mach 0.85, would there be a sonic boom;

a: for an observer moving with the air, seeing the air as stationary and the rocket moving at mach 1.1?
b: for an observer outside the wind tunnel, seeing both move at sub-sonic speeds but in different directions?
c: for an observer on the rocket, seeing the air move past at mach 1.1?

Cheers!
 

Answers and Replies

  • #2
anorlunda
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I am curious as to how the speed of sound changes as the speed of the surrounding material changes

All motion is relative. The surface of the Earth moves at 1000mph due to its spin. More motion around the sun, more motion as the sun moves relative to the galaxy. Will that help you to answer your own question?
 
  • #3
A.T.
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if you have the air travel at mach 0.25 and flew a rocket against the wind at mach 0.85, would there be a sonic boom;

a: for an observer moving with the air, seeing the air as stationary and the rocket moving at mach 1.1?
b: for an observer outside the wind tunnel, seeing both move at sub-sonic speeds but in different directions?
c: for an observer on the rocket, seeing the air move past at mach 1.1?
The shock-wave exists in every reference frame. But to hear the boom you must pass through the shock wave.
 
  • #4
cjl
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The sonic boom exists in all frames, but as A.T. said, you'll only hear it if it passes over you. The observer on the rocket will likely not hear it.
 
  • #5
some bloke
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Ok, so there will be a sonic boom?

So the speed you need to travel is relative to the medium in which you travel?

So if something was traveling just over the speed of sound, and there was a sonic boom built up behind it, and then the wind picked up behind it, could it essentially blow the shockwave back over the vessel (assuming the increased wind speed brought the travel to sub-sonic speeds)?

Another example, if a bullet travels at just-supersonic speeds, if fired with the wind in a hurricane, such that the difference between the bullet and the air was just sub-sonic, would it prevent the sonic boom?
 
  • #6
A.T.
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So the speed you need to travel is relative to the medium in which you travel?
Yes.
 
  • #7
jack action
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Anything that moves into a fluid creates pressure waves. Pressure waves are the same thing as sound waves, only much, much more stronger. Just like sound waves, they travel at the speed of sound, which mostly depends on the local temperature in an ideal gas.

If the object that created the sound wave by pushing on the fluid goes faster than the speed of sound, then the 'second' pressure wave will reach the 'first' pressure wave and they will add up, thus building up the pressure. When this is the case, something has to give up at some point and that is when there is a release of energy (kinetic energy is converted into thermal energy). This is called a shock wave and you can hear a sonic boom when it passes you by.

That being said, pressure waves travel into the fluid, so if the fluid moves, the velocity of the fluid is added to the one of the pressure wave.

The shock wave (where the pressure waves pile up) has a defined position in space. To hear the sonic boom, you just need to be at that location. It doesn't matter if you move or not. The shock wave location depends only on the position of the source with respect to the fluid:

shock-waves-jpg.jpg

You cannot hear the shock wave you've created, as it will always be in front of you.
 
Last edited:
  • #8
cjl
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That's not really how a shock wave is formed. Shock waves occur when the fluid has to actually change velocity in a supersonic flow, so you'll only get them immediately around the airplane where the flow has to turn or slow down (and that turning or slowing is propagating into the fluid faster than a small amplitude disturbance would, so the flow is unable to react to the object until the shock itself). Both of what your diagram labels as "shock waves" would be more accurately described as sonic booms or the mach cone. Shock waves can also have different angles from what your construction would imply - a shock wave at mach 5 can still be perpendicular to the incoming flow, for example.

What your diagram does show though is how the sonic boom will travel once you're far from the airplane. Once the flow no longer needs to turn or slow down, the sonic boom does travel at the speed of sound, and therefore follows the mach cone.
 
  • #9
A.T.
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You cannot hear the shock wave you've created,...
Unless you make a turn.
 

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