Here’s something that never occurred to me before: So, if a person fired a bullet in my general direction, and it passed me at greater than Mach 1, it would make a sonic boom that I would hear (the way we hear a whip crack)? (Let’s suppose that, in order for me to be able to distinguish the boom from the sound of the gun being fired, the shooter was either quite far away from me or was using some type of silencer.)
That’s right. But you wouldn’t hear a the crack of supersonic bullet until it was already past you. That’s behind the infantrymen’s saying “You never hear the one that gets you.”
The first airplane to go supersonic, the Bell X-1, was shaped like a .50 caliber round because that shape was known to be stable when passing through Mach 1.
The bullet isn’t making a noise like an aricraft does.
The noise of aircraft engines is not what makes a sonic boom happen. If that were true, then the space shuttle would not make a sonic boom during its descent.
But it does make a sonic boom; see my post upthread for a video link.
So does a supersonic bullet.
So does anything traveling faster than the speed of sound.
“Far away” is easy. Give him a 50-cal sniper rifle, and put him a mile away. Muzzle velocity is somewhere between Mach 2 and Mach 3, but of course drops fast as the bullet travels. Call it an average of Mach 2, about 1400 MPH, during its flight from the shooter to you. The bullet will reach you in 2.5 seconds; if it passes right next to you, you’ll hear the shock wave just milliseconds after that (depending on how close it passes).
The sound of the gunshot (the muzzle blast), traveling at the speed of sound (!), will reach you in another 2.5 seconds, easily distinguishable from the bullet’s sonic boom.
Loud as it is, the sound made by a jet engine has far less energy than the shock wave it produces at supersonic speeds. Bullets create sonic “booms” (actually more of a crack like sound" that are unassociated with the boom of being fired. When one goes by you, you hear a crack, followed by the report of the weapon being fired.
The “noise piling up” theory is not an accurate description of the physical phenomenon.
Tris
eta: Curses, foiled again.
You can get a comparable effect with light when a particle travels faster than the speed of light in a medium like air or water:
Others have answered this but thought you might enjoy a photo of the sound barrier.
Yes, it is invisible, usually. In the right conditions though water may condense at that boundary. Here is a pic (might need to scroll down a little…I know it seems an article about something else) of a plane going through that boundary and causing a disk of water vapor to form denoting the pressure wave itself. Pretty neat.
In fact, guns with silencers are also used with low-velocity ammunition, precisely for this reason. The silencer would be pointless, if the bullet we still supersonic.
EDIT: Whack-a-mole, it’s a bit misleading to caption that picture as the plane passing through a barrier: If it were a video, you wouldn’t see the plane passing through the “wall” of cloud that it’s forming, but rather that the “wall of cloud” would appear to move with the plane (it wouldn’t all be the same cloud, just that the air in the immediate vicinity of the plane forms visible water droplets). As the plane accelerated, the “wall of cloud” would still stay with the plane; it’d just form a sharper cone.
It isn’t a sound barrier at all in that photo, no matter how often it’s called that in e-mails, it’s a photo of the Bernoulli Effect on a plane that is actually subsonic. The increasing cross-sectional area of the airplane as you go aft accelerates the air, lowering its pressure. If the humidity is high enough, the low-pressure air goes to saturation and water condenses out of it. Toward the aft end of the plane, the cross-sectional area reduces, air slows down and recovers pressure, and the water is reabsorbed.
Actually, to be specific, it’s called the Prandtl–Glauert singularity.
Here’s a pretty good example of how it “stays” with the aircraft.
To forestall the inevitable “but I thought…” questions, let’s note right here that the Ultimate Speed Limit, c, is the speed of light in a vacuum. Light passing through air, water, glass, rock crystal, mica, etc., travels at a lower speed than c, sometimes substantially. Hence it’s possible for something to travel faster than the speed iof light within the medium in question.
According to Airman Doors, USAF’s link this occurs at transonic speeds. Seems close enough to me.
It happens at transsonic speeds, yes, but that doesn’t make it the “sound barrier.” The sound barrier is simply a colloquial term used to describe the difficulty in achieving and sustaining controllled supersonic flight in an aircraft.
That seems kind of pedantic. The sound barrier is encountered at transonic speeds.
Maybe I should not use the colloquial term “sound barrier” but I figured everyone understood what I meant in this context.
Be male, blond, in the military, and very agile?
As was suggested by other posters, it’s misleading to refer to a “photograph of the sound barrier.” Given that we seem to agree on what the sound barrier is, it’s impossible to photograph “the difficulty in achieving and sustaining controllled supersonic flight in an aircraft.”
A Prandl-Glauert singularity is not a sound barrier.
A condensation cloud is not a sound barrier.
A shock wave is not a sound barrier.
A photograph of any of these things is not a photograph of the sound barrier.
It’s possible I’m being pedantic, but the propagation of muddled terminology is often a big reason for the confusion of lay people. In their quest for understanding, they’ve come here for the Straight Dope, so let’s give it to 'em straight.