Supersonic crack question


October 6, 2010, 09:49 PM
Hey Everyone,
I have an interesting question some of you might be able to answer.

I understand that a suppressor/silencer removes the actual BANG from the expanding gases when a round is fired, but the bullet passing through the air (if fired over the speed of sound) still makes a loud supersonic crack.

Now, lets say a round is fired from a supressed rifle and it traveles JUST over the speed of sound, would that supersonic crack be any different from the same round fired at 3000 FPS? Is a sonic boom always a sonic boom, or does the speed matter and the faster round have a louder sonic crack?

Thank you in advance for answering.


If you enjoyed reading about "Supersonic crack question" here in archive, you'll LOVE our community. Come join today for the full version!
Jim Watson
October 6, 2010, 09:58 PM
It would probably take a sound level meter to tell a difference.

I have worked in the pits at long range rifle matches and the crack of a bullet passing overhead at maybe 1400 fps after 1000 yards of travel is still pretty sharp; I wear ear plugs to pull targets.

October 6, 2010, 09:58 PM
The size of the projectile will make the it louder...the speed of it will make a "sharper" crack, more speed means more sound waves compressed together.

I think that would also mean it would be louder...but its late, and I'm too tired to think very hard on it.

Thats just my un-educated guess...

October 6, 2010, 11:06 PM
It is my understanding that the sound is made when the object (the bullet in this case) first exceeds the speed of sound, and crossing that threshold is what makes the sound, and that it doesn't matter how much faster it goes after that. In other words, it's crossing that line that makes the sound, it doesn't really matter what happens afterwards.

October 7, 2010, 08:56 AM
Well, if its at the threshold you get "transonic" noise, its almost like a pop. Then when the round breaks the sound barrier you get "supersonic" crack which is loud and echos. When a bullet produces a supersonic crack its hard to tell them apart from other bullet sizes and speed with the human ear.

October 7, 2010, 09:30 AM
In my opinion the larger bullets are louder than the smaller ones. It also makes a big difference where you are at. Shooting the rifle or sitting behind it means the shock wave does not pass your ear, you only hear it as it bounces off of objects. If you are downrange, then it gets much noisier.


October 11, 2010, 09:12 PM
The ballistic crack is being produced as long as the projectile is traveling at a velocity greater than the speed of sound. Think about it. If the crack were given off only at the transition to supersonic, a suppressor would quiet it since the bullet is traveling at a supersonic velocity before it leaves the suppressor. Aircraft/bullet...the physics is the same. Wiki is your friend on this one.

Shadow 7D
October 11, 2010, 10:55 PM
The crack is still audible, and the rifle is still damn loud, but the sound is really different, and the actual gun sounds (the expanding gasses) disperse quicker, beside being a higher pitch and quieter.

So you end up with a the sound of a gun fired, that is harder to identify and locate. Or that was at least my experiences doing the scrub work for the 3rd ranger bat after I was injured waiting to ship out.

October 11, 2010, 11:40 PM
No doubt about it, the bigger the projectile, the bigger the boom.

October 12, 2010, 01:26 AM
The supersonic threshold is where the boom begins and will follow indefinately.
The space shuttle booms across 100s of miles when it re-enters. A stationary object (you) hears a boom but the boom is continueous as long as the object is supersonic.
If a plane flew around the world at supersonic speed, the boom would be heard everywhere the plane went around the whole world as the plane passed.
Mach 1 , 2 , 3 or 50 would all boom the same but would be more intense as best I can understand from reading a lot of science stuff.

And yes the bigger the object the more kewl the boom. 22s only make a crack.

rocky branch
October 12, 2010, 03:45 PM
Also depends on just how close the passing projectile is to one's head.

October 12, 2010, 05:37 PM
the sound increase flattens out above 2100-1300 fps. 30 cal peaks at about 138 dB, .22 cal at about 135 dB.

October 12, 2010, 07:05 PM
Well, if you really need your drugs or your arse to go that fast. . .

But seriously, it doesn't matter too much. Once it's above the sound barrier, it's not going to get any louder. Generally, though, the easier it makes the transition, the lower the boom. A small caliber / high velocity bullet will be less noticeable than a larger bullet that generates more air resistance.

October 12, 2010, 07:39 PM
A little sidestep question for those knowledgable...

How does the shape of the projectile effect it? Or does it effect it at all?

(My edjamacated guess is it does effect it, but so little that it'd hardly be measurable with civi. equipment.)

What I'm really wondering here... (been toying with the idea of sub-sonic .308 rounds). Comparing a 220gr. Hornady round nose .308 bullet, to a 220gr. Sierra MatchKing .308...The way the air moves around each bullet is extremely different, does this translate (even just a couple of fps) into a difference of speed in which the bullet breaks the sound barrier?

Does the round nose "stack up" the air and thus effect it's speed of sound? Or does the MatchKing pierce through the air to effect it's speed of sound?

Like I said, I think it does change (just like altitude and barometric pressure change it ever-so-slightly), but I doubt it'd be a measurable difference.



October 12, 2010, 08:06 PM
JWF, the speed of sound is a function of density. the denser the medium, the faster the speed. Hence the speed of sound is faster in water than air, and faster in steel (about 18,000 fps, IIRC) than in water.

the bullet shape doesn't have much to do with the speed of sound.

Now, the bullet shape does have a lot to do with supersonic drag....

Shadow 7D
October 12, 2010, 09:00 PM
It does, the smoother the air flow, the less the boom, look up 'quite' supersonic aircraft, there are some designs that are theoretically able to minimize to or drastically reduce the boom,

I would hypothesize that a spitzer type bullet, or a blunted spire with a boat tail, or hollow tail, ie, very good ballistic coefficient, would produce less boom than a double semi wadcutter.

Zak Smith
October 13, 2010, 01:37 AM
The speed of the projectile does not have to exceed the speed of sound in the medium for some parts of the airflow to be supersonic.

October 13, 2010, 02:04 AM
The way the air moves around each bullet is extremely different, does this translate (even just a couple of fps) into a difference of speed in which the bullet breaks the sound barrier?

It makes a huge difference.
A spitzer with a boat tail has very smooth air flow. This means the air speeds up very little as it flows over the surface of the bullet.
While the opposite end of the spectrum would be something like a shotgun slug with a horrible aerodynamic shape. A shape which creates such massive airflow disturbances that this turbulence has air at speeds much faster than the projectile itself especially after they round sharp edges and protrusions.
This turbulence can create eddies crashing into one another which itself creates additional noise. This means even such a projectile well below the speed of sound can create a supersonic crack because some of the airflow still exceeds the speed of sound.
While a very aerodynamic projectile can get closer to the speed of sound without producing a crack.

If you were to even more irregular projectiles lacking aerodynamic shape like square projectiles the effect would be increased even more.
Of course the poor aerodynamics also mean the projectile is slowing at a much faster rate, so both it and the turbulence it is creating is going to drop down below the speed of sound at a faster pace and shorter range.

A larger projectile is also disturbing more air, so naturally will create more turbulence and noise.

To an extent however tissue also flows around a projectile in a similar manner (which is why spitzer bullets must be designed to become destabilized and yaw or tumble within a short time after impacting tissue to do much damage) to airflow. So without compensating expansion and/or destabilization the more aerodynamic rounds also damage less tissue for a given amount of energy and penetrate more because tissue flows around them with much less resistance.

If you enjoyed reading about "Supersonic crack question" here in archive, you'll LOVE our community. Come join today for the full version!