The Sonic Crack!

[Cedric 132]

Hi there, I'm trying to understand the mechanics of the sonic crack as the bullet leaves the rifle barrel.

 

When a bullet from my .17hmr leaves the barrel it emits a sonic crack as it breaks the sound barrier. What I don't understand is, is that all it does - emit a crack at that one point - or does it continue as it travels. If the former, am I correct in thinking any rabbits at, say, 100 yds. would only hear the crack from that distance (obviously not the one I've shot !! ).

 

Also, do you think the quarry can pick up the directionality of the sound, sometimes adjoining hedges might reflect / deflect the sound.

 

Thanks for any enlightenment !

[Hawkeye. 26]

I think the 'crack' heard is the exit of air from the barrel

[Alsone 789]

It emits the crack as it passes through the sound barrier, a mini sonic boom caused by a build up of sound waves in front of the object. So you only get the crack once. Once it's faster than the speed of sound, it outruns the sound waves so you don't get a build up again.

 

The point at which you get the crack will depend on how fast the bullet accelerates and thus how far from the barrel it is when the sound barrier is reached.

 

As the bullet decelerates, you don't get a build up because it's slowing.

 

No mention of bullets but some explanation with aircraft:

 

http://www.scientificamerican.com/article/what-happens-when-an-airc/

Edited March 18, 2014 by Alsone

[Born Hunter 17,909]

For as long as the bullet is super sonic there is the sonic crack.

[Born Hunter 17,909]

It emits the crack as it passes through the sound barrier, a mini sonic boom. So you only get the crack once.

 

The point at which you get the crack will depend on how fast the bullet accelerates and thus how far from the barrel it is when the sound barrier is reached.

 

You're wrong. The superposition of sound waves from the bullet is continuous for as long as the bullet is supersonic. You will only hear one crack but the phenomenon itself is continuous throughout the bullets supersonic trajectory.

[Alsone 789]

 

It emits the crack as it passes through the sound barrier, a mini sonic boom. So you only get the crack once.

 

The point at which you get the crack will depend on how fast the bullet accelerates and thus how far from the barrel it is when the sound barrier is reached.

 

You're wrong. The superposition of sound waves from the bullet is continuous for as long as the bullet is supersonic. You will only hear one crack but the phenomenon itself is continuous throughout the bullets supersonic trajectory.

 

 

Ian I don't claim to be an expert. I knew it only happened once and roughly why but I readily admit I had to google the exact mechanics as I'm no physicist.

 

However, if you read the article I linked above, then it states the crack is singular because once the object breaks the sound barrier, it out accelerates the sound so you no longer get the build up of waves that cause the crack / boom. The object itself continues to emit whatever noises it ordinarily makes be that the sound of the engine or the sound of the object pushing through the air, but you don't get another boom or crack as the sound can no longer accumulate in front of the object.

[Deker 3,478]

It emits the crack as it passes through the sound barrier, a mini sonic boom caused by a build up of sound waves in front of the object. So you only get the crack once. Once it's faster than the speed of sound, it outruns the sound waves so you don't get a build up again.

 

The point at which you get the crack will depend on how fast the bullet accelerates and thus how far from the barrel it is when the sound barrier is reached.

 

As the bullet decelerates, you don't get a build up because it's slowing.

 

No mention of bullets but some explanation with aircraft:

 

http://www.scientificamerican.com/article/what-happens-when-an-airc/

The bullet goes through the speed of sound in the barrel, from the moment it leaves the barrel it is decelerating, it isn't any distance from the barrel when it breaks the speed of sound, it has already done that in the barrel.

 

The fact is the air pressure wave becomes audible a few feet after the bullet exits the barrel.

[Born Hunter 17,909]

 

 

It emits the crack as it passes through the sound barrier, a mini sonic boom. So you only get the crack once.

 

The point at which you get the crack will depend on how fast the bullet accelerates and thus how far from the barrel it is when the sound barrier is reached.

 

You're wrong. The superposition of sound waves from the bullet is continuous for as long as the bullet is supersonic. You will only hear one crack but the phenomenon itself is continuous throughout the bullets supersonic trajectory.

 

 

Ian I don't claim to be an expert. I knew it only happened once and roughly why but I readily admit I had to google the exact mechanics as I'm no physicist.

 

However, if you read the article I linked above, then it states the crack is singular because once the object breaks the sound barrier, it out accelerates the sound so you no longer get the build up of waves that cause the crack / boom. The object itself continues to emit whatever noises it ordinarily makes be that the sound of the engine or the sound of the object pushing through the air, but you don't get another boom or crack as the sound can no longer accumulate in front of the object.

 

 

If that's what it says then it is also wrong. Simply, if a bullet travels over your head at supersonic velocity you will hear a crack originating from the bullet as it passes over your head not from the barrel of the gun that fired it. The article is wrong!

[Born Hunter 17,909]

Look, here is a very simple animation that illustrates the supersonic crack you hear from projectiles traveling with velocities greater than the local speed of sound.

 

http://en.wikipedia.org/wiki/Sonic_boom

 

You see that point where the waves overlap?...... it travels out like a cone......... that's the 'crack'. It's a shockwave caused by a constructive build up of sound waves from the projectile.

 

[Alsone 789]

Look, here is a very simple animation that illustrates the supersonic crack you hear from projectiles traveling with velocities greater than the local speed of sound.

 

http://en.wikipedia.org/wiki/Sonic_boom

 

You see that point where the waves overlap?...... it travels out like a cone......... that's the 'crack'. It's a shockwave caused by a constructive build up of sound waves from the projectile.

 

 

I think we're on the same page here.

 

There's 1 crack that originates when the bullet breaks the sound barrier but obviously you don't here it until the sound reaches you which is what the illustration is showing.

Edited March 18, 2014 by Alsone

[Born Hunter 17,909]

 

Look, here is a very simple animation that illustrates the supersonic crack you hear from projectiles traveling with velocities greater than the local speed of sound.

 

http://en.wikipedia.org/wiki/Sonic_boom

 

You see that point where the waves overlap?...... it travels out like a cone......... that's the 'crack'. It's a shockwave caused by a constructive build up of sound waves from the projectile.

 

 

I think we're on the same page here.

 

There's 1 crack that originates when the bullet breaks the sound barrier but obviously you don't here it until the sound reaches you which is what the illustration is showing.

 

 

No, the crack is continuously being created for as long as the bullet in supersonic as shown by the animation. In that animation the projectile is traveling with a constant velocity greater than the speed of sound, it doesn't break the sound barrier at any point, it's velocity is constant. So by your reasoning there should be no sonic crack? The crack you hear is from the shockwave that is formed from the constructive build up of sound waves originating from the bullet as it travels past you, not from the point which it breaks the speed of sound, which is within the barrel of the gun itself.

[Cliff Ray 185]

Alsone I am afraid you are wrong there is definitely a sonic boom/crack created for the entire time the object is supersonic. BTW I am a physicist

[walshie 2,804]

I'm not a physicist, but it makes sense that the bullet would make a sonic boom for the entire time it is supersonic. You might hear it at a different time to someone standing somewhere else, but I'd hazard a guess a field is too small to notice the difference.

[Cliff Ray 185]

If that wasn't the case there would never have been the problems with Concorde's routes. They could simply have accelerated over somewhere unimportant and then quietly flown over whatever towns they liked.

[TheRealChuckNorris 42]

And it's also the reason why you can hear the rounds crack over your head if you're down in the butts at a longer distance target range.