Well, I watching a documentary on stock cars the other day, and I got to thinking about speed, and then my mind wandered over to the sound barrier. I realized I know nothing about it. What speed do you have to pass? I know that there is a sonic boom, but what kind of sound is it and how long does it last? Does anything special happen when you break it? Help, its troubling me!
The speed required to break the sound barrier gets lower and lower as you increase in altitude.
It is a quick boom, much like the name implies.
Sometimes, the water vapor inthe air surrounding a jet going supersonic will condense into fog, but that’s the only real even that happens aside from the sound.
I bet this would get more of a response inGeneral Questions…(hint, hint)
Just hopping in here before this thread gets moved:
The speed of sound in dry air at sea level is 340 meters per second, or six hundred and something miles per hour. The sound “barrier” is aptly named; it is very difficult to get a plane to pass that speed, but once past the speed of sound, flight is almost as easy as it is at subsonic speeds. This is because sound is a compressional wave, that is, wavefronts consist of regions of higher or lower pressure than average. When a loud object, such as an airplane, is travelling at the speed of sound, all of these wavefronts pile up on top of each other, producing a region just in front of the plane where the air pressure is very high. Once the plane is past the sound barrier, the wavefronts no longer overlap, except along a cone traling behind the plane. When this cone intersects an observer, that observer hears a sonic boom. This cone exists for as long as the plane is travelling at supersonic speeds, but it passes any given observer very quickly. The phenomenon is similar to the wake produced by a boat moving faster than the speed of water waves. This is a lot easier to explain with diagrams; anyone out there less lazy than I care to find some links?
Excellent explanation, Chronos! Now try the same for the terminal velocity of an unladen European swallow in a head-first dive 
Here’s what the USAF has to say about sonic booms: http://www.gulflink.osd.mil/aljubayl/aj_ref/n10en040/sonic.htm
It mentions SOS is “about 750 miles per hour at sea level”, but I’d always heard it was 760 miles per hour.
coldfire, — is the swallow carrying a coconut?
ducks and runs
The speed of sound in air is around 750 mph. But it fluctuates a bit depending on pressure/temperature of the air. It’s got a lot to do with the compressibility of the “fluid” (air is more compressible than water…the speed of sound is faster in water).
Larrigan, you ignorant slut! I said unladen
Coldy, obviously the terminal velocity of the swallow in a head first dive would be 0.
when it hit the ground…
OK, WisePaddy. Now what’s the terminal velocity of an Irishman who just got a kick in the arse by a large Dutch boot?
The likelihood of that happenning is fairly slim, even if you can get your clogs off the ground… 
This question has been asked and answered several times in GQ. (No, not the one about the Dutchman and the Irishman!) The previous respondents are correct except that the speed of sound does not depend on pressure and varies more than a little with temperature:
– Elementary Classical Physics, Vol. 2, Weidner & Sells
And, yes, Joe Kittinger did jump out of a balloon and almost broke the sound barrier in free fall. (He hit about Mach 0.95.) There was no sonic boom because the air was too thin, which is why he was falling so fast. He had no sensation of falling.
But if two swallows had a coconut on a string…?
Sheesh, you guys can make [ul]anything[/ul] mundane and pointless! And I stand corrected on the value in mph.
By the way, are the swallow’s wings spread or flush?
A similar effect you can see:
A boat displaces water as it advances and creates a bow wave. As it goes faster it eventually is sitting between two waves at bow and stern and it has reached it maximum displacement speed. At that point it takes an inordinate amount of power to get the hull to climb over the bow wave but if and when it is done, the hull is no longer in displacing mode but planing and it takes much less power to go faster. If you sail a light sailboat you can get it to plane and you get quite the feel of it. If the wind is not quite strong it make be difficult to start the plane but once started it is easy to keep it.
Stepping through the sound barrier is similar in a way. It takes a lot of power because you have in effect pushed the sound waves you are making ahead of you all together. Once you pass that point it’s easy sailing again.
I let this go by once but I have to speak up – there is a large increase in drag as you near sonic velocities. (Drag is the force that impedes your forward progress when flying. The less drag, the faster you can go.) That’s not exactly what is meant by the term “sound barrier” but it’s close enough. However, contrary to previous posts, the drag does not drop off again after passing through the speed of sound. The rate of increase drops dramatically, but the drag doesn’t level off or reduce. It continues to increase, only more slowly.
The analogy with boats and wakes fails because an aircraft remains totally immersed in the atmosphere – there’s no equivalent to stepping, hydroplaning, or anything similar, for aircraft.
What does happen at transonic velocities (Mach 0.9 to 1.1) is difficulty controlling the aircraft. It is much easier to maintain a stable configuration at speeds higher or lower than the speed of sound than it is at transonic velocities. Part of the reason for this is the formation of local shock waves as different parts of the aircraft go supersonic at different times. (The air over an airplane’s wings travels at different speeds at different points on the wing – that’s what generates lift.) In terms of control, the boat analogy is valid.
The difficulty in breaking the sound barrier wasn’t primarily in going fast enough – all that requires is a bigger engine. The hard part was maintaining controlled flight.
Well, I guess I was mistaken. I did think you needed an extra boost to surpass the sound barrier. The way it was explained to me is that, at that critical point you are pushing against all your sound waves combined and condensed into one and after you are over mach1, you have left them behind and no longer need that boost. I guess it could be that I am entirely mistaken on this point or that this is true but the effect is minor or negligible compared to the general common drag.
What you say (that different parts of the airplane cross the barrier at different moments and this makes control difficult) seems to confirm there are some critical forces appearing at that critical moment. If the forces just increased linearly, there would be no difference at that critical point.
I remember asking this to a Concorde pilot but, even though this is a yes/no question, his answer was so involved I gave up. I began to think he did not understand my question. I do remember I was holding in my hand the disk of one of the wheel brakes and even though it was quite big it weighed close to nothing. It was hard to believe this was the real thing.
Thank you so much fro all of the responses! My mind can now rest!! Oops, your probably right, I should have posted this in GQ…well, there’s a Newbie mistake for ya
There are links to some interesting photographs in the thread Photgraph sound barrier?