Curiosity - Relative speeds in relation to the sound barrier?

• some bloke
In summary, a sonic boom is 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?
some bloke
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!

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?

some bloke said:
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.

cjl
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.

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?

some bloke said:
So the speed you need to travel is relative to the medium in which you travel?
Yes.

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:

You cannot hear the shock wave you've created, as it will always be in front of you.

Last edited:
vanhees71
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.

jack action said:
You cannot hear the shock wave you've created,...
Unless you make a turn.

1. What is the sound barrier?

The sound barrier, also known as the "sonic barrier," is the point at which an object is traveling at or faster than the speed of sound. This speed is approximately 767 miles per hour (1,235 kilometers per hour) at sea level and can vary depending on factors such as altitude and temperature.

2. How does the speed of sound compare to other speeds?

The speed of sound is relatively slow compared to other speeds, such as the speed of light. While the speed of sound is approximately 767 miles per hour, the speed of light is about 670,616,629 miles per hour. This means that light travels almost 874,030 times faster than sound.

3. Can anything travel faster than the speed of sound?

Yes, there are objects that can travel faster than the speed of sound. These objects are typically designed to break the sound barrier, such as supersonic aircraft or bullets fired from a gun. However, it is currently not possible for any object to travel faster than the speed of light.

4. How does the speed of an object affect the sound it produces?

The speed of an object can greatly affect the sound it produces. As an object approaches the speed of sound, the sound waves it produces begin to compress and overlap, creating a shock wave. This results in a loud, booming sound, commonly known as a sonic boom. As the object continues to increase in speed, the shock wave becomes more intense and can cause damage to nearby objects.

5. How does the speed of sound affect aircraft?

The speed of sound has a significant impact on aircraft design and performance. As an aircraft approaches the speed of sound, air resistance increases dramatically, causing drag and potentially damaging the aircraft. For this reason, aircraft that are designed to break the sound barrier must have specific aerodynamic features and materials to withstand the effects of supersonic flight.

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