Sonic boom Questions

  1. Here's what I sorta understand about sonic booms:

    The boom is caused (and I say all this tentatively) by "shock" waves (is this tantamount to air resistance caused by the plane running into the air?). And it occurs at a certain point (which can vary with temperature and air pressure).

    Ok, here's where I get confused:

    Once the plane breaks the sound "barrier" (doesnt seem like much of a barrier?), where does it go or exist in etc?

    Is the phenomenon of sonic booms simply the plane "outrunning" the sound it makes?
  2. jcsd
  3. Sonic boom is the shockwave that develops in front of the plane when it goes faster than sound. At faster than sound speeds, the pressure disturbances created by the plane cannot travel upstream and instead coalesce into a shock wave - essentially an extremely thin (in the range of micrometers) region of high pressure and temperature gradient.
  4. FredGarvin

    FredGarvin 5,084
    Science Advisor

    You weren't one of the people trying to originally break that barrier. The transsonic region ended up being the toughest nut to crack. Aerodynamically, there are some very wierd things that happen in this region and once you go sonic, some things happen opposite to what subsonic theory dictates. Do not lose your frame of reference. This was a huge technological advancement in the day. Just because it is done routinely does not mean that the feat itself was not remarkable.
  5. The ' sonic boom' is behind, behind of the plane. The shockwave is the one in the 'front'. Cause and consequence.
    Last edited: Nov 24, 2005
  6. True - in fact, I'm still trying to understand the very basics of the idea.

    Talking about disturbances in the air pressure around the plane, at normal (sonic?) speeds, what I am picturing is a plane slicing through the air, forming a V-like wake from the nose to the rear and this wake becoming a tighter and tighter V as the plane approaches the sound barrier. Is this correct?

    If pressure is built up in the front (in the form of shockwaves) and released towards the rear (in the form of soundwaves), is it "slipping" from, say, the cockpit window toward the back of the plane in the tenths of a second the air gets slammed into the windshield?

    Sorry, I am having trouble understanding this phenomenon...
  7. russ_watters

    Staff: Mentor

    No. The v-like wake only appears above the speed of sound. The shock wave is made of the actaul pressure disturbances that the plane makes by cutting through the air - like the sound of the wind you hear driving your car. Below the speed of sound, these waves are literally able to "get away" from the plane, as sid was saying, and they move away in all directions. The cone is formed by the waves moving away from the plane as fast as they can - none go forward because the plane is going to fast, so all trail in a v behind the plane.
    What mathphys said isn't really correct: a shock wave is a shock wave is a shock wave and when the plane disrupts the air, the two biggest disruptions come at the nose and tail, so those are the two disruptions that form the biggest shock waves - thus, double-boom you usually hear. In actuality, there are other shock waves coming from every place on the plane where there is a major disruption (leading edges of wings, appendages, etc.). There is no separate build-up and release.

    The best explanation I've seen is using water drops: if you drip water into a pool, concentric circles form as the wave expands from where the drop hit. If you do a steady drip, drip, drip, these waves will continually move away from the impact point. If you start to move the impact point slowly, the waves will still be able to move forward away from the impact point and you'll end up with circles that are not concentric, but still moving away from the impact point in all directions. If you move faster than the waves can move, all the circles you make will be behind the impact point, and their diameters will be larger the further from the impact point, forming a cone. HERE is a picture of the phenomena using a plane, but the circles are there...

    Essentially, all that wind-rushing sound that would normally radiate in all directions is compressed into a single wave - making it very powerful.
  8. DaveC426913

    DaveC426913 16,534
    Gold Member

    The sonic boom IS the shockwave. True, it expands in a cone behind the plan as the distance increases, but at distance zero the shockwave is actually at the nose (as well as any other protruding parts)

    If you were magically able to stand one inch from the plane, you would hear the sonic boom from the nose of the plane, not behindthe plane.
  9. Astronuc

    Staff: Mentor

    The sound barrier is a speed limit of pressure pulses in a gas, or air the case of aircraft.

    Here are some interesting examples -

    Interesting java applet describing sound in subsonic, transonic and supersonic flight. -
  10. I would only hear the sonic boom IF i'm inside the Mach cone(or its envolvent). For me the shock wave is the phenomena that occurs because of the supersonic motion. The surface of the cone (or the corresponding shape) is what is called 'the front' (the surface of 'discontinuity'). The boom is what i hear. And as a spectator, i would always prefer to hear it as far away from the nose as possible :biggrin: .

    What is behind(the gas) is sometimes called the blast wave, to distinguish it from the propagating front, which is called 'the shock (front/wave)'.The terminology i'm using is from physics, i don't know how it is in aeronautics.
    Last edited: Nov 24, 2005
  11. Whoa!, nice applets, thanks astronuc.
  12. Here's a picture of a jet breaking the sound barrier..

    Attached Files:

    Last edited: Nov 25, 2005
  14. FredGarvin

    FredGarvin 5,084
    Science Advisor

    This gives you an idea of what the ssonic boom for the Concorde was like. You can fast forward to about 5:00 into the video. It's worth the DL time. You can clearly hear the double boom.
  15. russ_watters

    Staff: Mentor

    I know - I got the impression that jhe1984 got that mistaken understanding from interpreting what you said:
    ...which isn't correct. As I said, a shock wave is a shock wave is a shock wave and the double-booms you hear are both pretty much the same thing. Both are shock waves and both are sonic booms. In addition:
    Well, you'd hear the sonic boom at the instant the edge of each cone hits you.
    That doesn't make any sense and certainly isn't how it's described in aerodynamics - it sounds like you are applying the dynamics of an explosion to a sonic boom. They aren't the same thing.
    Last edited: Nov 26, 2005
  16. Clausius2

    Clausius2 1,467
    Science Advisor
    Gold Member

    I am gonna try to point out some misconceptions WE usually have about this stuff. Surprisingly people watch these videos thinking the plane is breaking the sound barrier when they see the clouds of vapor condensating, as if the barrier of condensating vapor would remain behind the plane forever and ever. In fact it is the contrary. The shock wave begins to form there where Mach# reaches a transonic value, usually at some point of the fairing (maybe Russ knows where) but not neccesarily on the nose, where the flow velocity is smaller (there must be an stagnation point at the nose). Then the shock progresses upstream to the nose, so the next instant of the picture of jhe1984 would be the condensating front displaced a little bit upstream facing to the nose. Once reached the steady state, the shock will remain --->detached<--- a short distance from the nose, producing a bow shock which takes the form of a cone (Mach Cone) if seen from large distances compared with the characteristic lenght of the plane.
    I am not a pilot, but in my opinion, a pilot may see very little through the cockpit window due to the condensated clouds attached behind the shock bow. This condensated clouds form by means of the extraordinary increasing of pressure behind the shock. The white clouds are NOT the shock front, but they are displaced a short distance behind of the order of the lenght needed by a molecule of vapor to become condensated when entering in the overpressurized surroundings behind the shock. The proper process of condensation takes a short distance from the shock front.
    The sonic boom heard by on ground people is due to the degenerated Mach Wave (at large distances from the plane the Mach Cone degenerates into a Weak --also called Mach---Wave). Such Weak Wave produces a jump of pressure, and our ears work by means of pressure differentials, so our brain and all our auditive system interprets such jump of pressure (weak compared with the jump just on the plane nose though) as an horrible sound. Theoretically, this Weak Wave will spread until the infinity, because the flow behind a Mach Wave is Subsonic. But far away enough it becomes dissipated due to viscous effects of air.
    Hope this clarify a bit. I hope to read soon something about the Prandtl-Galauert singularity, which has a lot to do with this stuff, but I don't know exactly what.
  17. russ_watters

    Staff: Mentor

    Fuselages are tough because they are often irregularly shaped, but for the wings, it's simply the thickest part of the wing - that's where the air has to be moving fastest. On a cylindrical fuselage, like what the Concorde has, it would be at the point where the nose cone (what you're calling the fairing? same diff...) ends and the cylindrical shape begins.
  18. Clausius2

    Clausius2 1,467
    Science Advisor
    Gold Member

    This explanation is very well suited for this case. I mean, the water drops stuff is such a good explanation of the Mach Cone that the mathematics involved in both cases (water drops and compressible flow) are formally similar each other. In the case of waves in water, we refer to this cone as the Froude Cone, being the Supersonic flow in Compressible Flow similar to the SuperCritical Flow (Froude#>1) in Open Channel Flow. I don't know if you have read something about Open Channel Flow but you have hit (unawarely?) the head of the nail.

    To those interested in Shock Dynamics I recommend to read something about Supercritical-Subcritical flows and Hydraulic Jumps (the incompressible version of a shock wave in a free surface flow). This theory has strong applications in Hydraulics when dealing with open channels and how controlling the depth of the flow. Also there is a project in my department (it is currently a reviewed paper in J of Fluid Mech written by three spaniards) funded by US Navy to research on avoiding the Hydraulic Jump formed behind war ships (just onto the Froud Cone) and enhancing a more silent functioning of their ships.
  19. Yes, the edge is the envolvent, that was my point when i said 'or'. Yes, I know that an explotion is not the same thing that a sonic boom. My point there was that a shockwave is not a sonic boom,in a general sense. You may say ahh but the shockwave and the sonic boom are the same according to the terminology i'm using, ok, but in general a shock wave is much more than a sonic boom, there are shockwaves that are not even mechanical waves. That's why i said that i was using terminology from physics.
    From the point of view of the unperturbed gas, the shock front is the envolvent (edge) of the Mach cone, and that is what is called the 'shock' (with the terms 'wave' or 'front' usually added) , the perturbation that is drastically changing the properties of the unperturbed gas in front of it. Maybe, because in my area we are interested in the properties of the gas and its changes, we made this distinctions, and there, they make a lot of sense.
    For us, the shock front/wave is the edge of the cone is the edge of the cone, is the edge of the cone; in front of it we have the unperturbed gas (pre-shock conditions) ,behind it we have the shocked gas (post-shock conditions), IF there is a boom that is what we hear (a 'secondary' perturbation or perturbations traveling across the shocked gas that we are able to detect/hear (yeah the may be in the edge of the cone)).
    For us, it would be ambiguos to say that the shock front/wave is the same thing that the shocked gas, or to say that the shocked gas or the shock front are the same thing that a sonic boom.
    Of course this may be different in aerodynamics, and i'm guessing that is just a matter of convention.
    Last edited: Nov 30, 2005
  20. Clausius2

    Clausius2 1,467
    Science Advisor
    Gold Member

    I think you are discussing semantics, but the physics is the physics is the physics. I am not going to talk about "We", I am going to talk about "Books", and it does not make sense making a distinction between shocks and sonics booms, as it does not make sense differentiate between ships and sheeps. They are trivially different because they have not nothing to do with each other. The sonic boom is a "boom" because it has to do with our auditive system. We listen a boom which is a consecquence of a sharp pressure jump, and this jump is a consequence of the shock itself. The boom is not a shock, it is a boom, man.
    Regarding conventions, one can distinguish a shock wave as that zone of space of thickness GREATER than 0 but very small, in which the gas suffers a sharp variation in the fluid magnitudes. The shock front contains both preshocked and postshocked gas.
  21. I know that they're different things, strictly speaking, that was my point. It is just that some people was saying that they were the same, they were the same, they were the same, and i was trying to explain why they are not.
    I don't know, maybe in 'their Books' there is no distinction and this is a convention in their areas, but in the ' Books' and 'Papers' of 'our' area, of course that there is a distinction, and as you mention a trivial one.
    All i can say that in all books i've seen so far (and they are many), the convention i mentioned is used, which is more precise and adequate to describe the involved physics. Also, it is the standard terminology used in research papers in my field.

    The important thing is to understand the phenomena, no matter how you name it. Nevertheless, i always prefer to use the most precise terminology, to avoid further recriminations :rolleyes: .

    In conclusion, what i'm saying is: a boom and a shock wave are different things from the physical point of view, no matter the terminology.
    Last edited: Dec 3, 2005
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