I know when you pass through the sound barrier, a sonic boom results… but what about when you slow down back through the sound barrier? Any resulting “Boom”?
Max
I think the BOOM is for the people on the ground and not in the plane. As a plane approaches the speed of sound the ride gets bumpier as it starts pushing through the bunched-up wavefront of sound. It is this violence that ripped apart and killed a few people making the first attempts on the sound barrier. Once the plane is faster than sound the wavefront is behind the plane. It’s this bunched up wavefront people on the ground hear but I don’t know that the people on the airplane hear anything although they now experience a smoother ride (less turbulence).
Then again, when the plane drops below the sound barrier I suppose the wavefront being dragged behind the plane can catch up and maybe they will experience a BOOM. Not sure though.
Any Concorde/Fighter Pilots on this board?
Not 100% on this but …
The sonic boom is correctly identified as a ground phenomenon. My impression, however, is that this is not a single event which occurs when the speed of sound is passed, but is the sound that occurs when the shock wave created by supersonic motion passes over the hearer. That is, the sonic boom trails along with the aircraft as long as it is supersonic.
If this is correct then there is no corresponding boom when passing back below the sound “barrier”, but the shock wave ceases to exist and its accompanying boom is no longer heard.
FWIW, the first attempt on the sound barrier was successful. Ask Chuck Yeager. The shock wave doesn’t go away at supersonic speeds. In fact it gets stronger. The problem is not violence induced by the whole body shock wave, it’s that different parts of the aircraft go supersonic at different times and shock waves pop up everywhere, causing a loss of control (if not designed for). It’s this transonic region that is dangerous. Once the entire aircraft is supersonic the control problems are much simpler. They’re a little different than subsonic control problems, but a lot simpler than transonic control. So typically a fighter aircraft will cruise subsonic and go supersonic with the afterburners, passing quickly through the transonic regime.
I could be wrong, but I seem to recall that the sonic boom is evident to ground observers when the space shuttle lands, indicating that it does happen both ways.
"Sonic boom is an impulsive noise similar to thunder. It is caused by an object moving faster than sound – about 750 miles per hour at sea level. An aircraft traveling through the atmosphere continuously produces air-pressure waves similar to the water waves caused by a ship’s bow. When the aircraft exceeds the speed of sound, these pressure waves combine and form shock waves which travel forward from the generation or “release” point.
As an aircraft flies at supersonic speeds it is continually generating shock waves, dropping sonic boom along its flight path, similar to someone dropping objects from a moving vehicle. From the perspective of the aircraft, the boom appears to be swept backwards as it travels away from the aircraft. If the plane makes a sharp turn or pulls up, the boom will hit the ground in front of the aircraft."
From Sonic Boom
Should’ve added this as well, from the same site as above.
"The sound heard on the ground as a “sonic boom” is the sudden onset and release of pressure after the buildup by the shock wave or “peak overpressure.” The change in pressure caused by sonic boom is only a few pounds per square foot – about the same pressure change we experience on an elevator as it descends two or three floors – in a much shorter time period. It is the magnitude of this peak overpressure that describes a sonic boom.
There are two types of booms: N-waves and U-waves. The N-wave is generated from steady flight conditions, and its pressure wave is shaped like the letter “N.” N-waves have a front shock to a positive peak overpressure which is followed by a linear decrease in the pressure until the rear shock returns to ambient pressure. The U-wave, or focused boom, is generated from maneuvering flights, and its pressure wave is shaped like the letter “U.” U-waves have positive shocks at the front and rear of the boom in which the peak overpressures are increased compared to the N-wave."
Yes, you definitely hear a sonic boom during space shuttle landings.
Not only do you hear a sonic boom from the space shuttle, you hear two, about 1/10 second apart. The first sonic boom is created by the wings and fuselage. The second sonic boom is created by the tail. If you’re ever in Titusville during a shuttle landing, I’ll bet you hear it before you see it.
Sound is generated in waves. imagine a point surrounded by concentric circles. The point is producing the sound, the circles are the waves of sound.
Now start that point moving. The point will move, but the circles will expand around the stationary point they were originated, so that as the point moves, the circles are no longer concentric, but bunched up in front, spread out behind (hence the doppler effect.)
As it approaches the speed of sound, the front edge of those circles (the area of the circle in the direction the point is moving) get closer together and closer to the point.
When it hits the speed of sound, all of them (circles and point) overlap. When it’s passed the speed of sound, the circles now are no longer concentric at all, but intersecting. In two dimensions, they intersect at two points, one above the plane, one below. If you expand into three dimensions they are expanding spheres that intersect in circles. At those points of interaction, the spheres interact constructively, they sound that the point makes increases.
So as that circle of interaction moves over the ground, the people it passes hear a REALLY loud noise.
You are 100% on this, but I’m not sure what you mean by ‘ground phenomenon’. You certainly hear above ground too.
Sorry, just shorthand for “people on the ground hear this, not people in the aircraft”. I didn’t mean to exclude people between the ground and the aircraft who are falling to their deaths, or people underground but close enough to the surface to still hear the boom, or people in the bath lying back with their ears underwater, or people above the aircraft, parachuting down from a balloon at 100,000 feet, etc.
Did I miss anyone?
Extremely true, I’m surprised that I forgot to mention it. I used to watch it land at Edwards AFB.
One of my favorite things was seeing a boom coming at you. Sometimes, if the plane was low enough, and you were in just the right kind of vegetation, you could see the plants move as the wave moved toward you. It was absolutely one of the coolest things I’ve ever witnessed, a shock wave travelling right at me.
Here’s another interesting thing…
I liked watching the jets at altitude. They would casually cruise along until reaching the supersonic corridor. Then, you would see them pick up speed. Strangely, they would suddenly seem to disappear once going fast enough. It was not due to being too far away, they would just seem to vanish.
Well, in my lizard-like brain, I decided that this weird effect was due to the light being refracted around it as the air pressure increased with speed.
Does anyone have a clue on this?