1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

No sonic boom? Why not?

  1. Sep 19, 2011 #1
    I read that the speed of sound in air at 20 degrees celsius is about 1126 feet per second.
    When I shoot my rifle, there is a noticeable crack which I believe to be the 'sonic boom'. I also have 'sub-sonic' ammo that does not cause the CRACK, because the bullet is traveling slower than the speed of sound.

    My question is, if I shoot an Air Rifle (bb gun) that shoots bb's at an advertised rate of 1400 feet per second, why is there no CRACK (sonic boom) ?

    Is there a MASS requirement for me to hear the boom?
     
  2. jcsd
  3. Sep 19, 2011 #2

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    I don't think that you hear the sonic boom of a bullet that you have fired yourself.

    wQZOj2K1Gfm9oex22NHmpmkQo1_500.jpg

    When the round shockwave reaches your ears, you hear the "bang" from the shot. The conical shockwave behind the bullet that causes the sonic boom never passes your ears. They are inside the geometrically extended cone from the start (or behind the cone if you will). So the expanding cone surface (sonic boom shockwave) never passes them.
     
    Last edited: Sep 20, 2011
  4. Sep 19, 2011 #3

    fluidistic

    User Avatar
    Gold Member

    Oh nice reply!
    Should be "because they are NOT inside the cone"?
     
  5. Sep 19, 2011 #4

    boneh3ad

    User Avatar
    Science Advisor
    Gold Member

    No, he meant inside the cone. The sonic boom is a result of the pent up sound pressure at the shockwave coming off of the object (in this case the bullet). You start in an area where that shockwave will never cross your ears since it starts in front of you, effectively placing you inside the cone.

    Of course, there is the shockwave generated at the barrel by the explosion, and that is where the difference is between the aforementioned guns. You are really just hearing the explosion and associated pressure wave, not the shock and sonic boom associated with the bullet. The bullet itself wouldn't have much of a sonic boom simply because it isn't putting much noise into the flow.
     
  6. Sep 20, 2011 #5

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    No I meant inside the cone in more abstract geometrical terms (infinite cone). I modified the post to make it more clear.
     
    Last edited: Sep 20, 2011
  7. Sep 20, 2011 #6

    xts

    User Avatar

    For those of you who got some military training (don't try it with your riffles!): if someone shoots in your direction from the distance, then, as the bullet passes you by (so the boom cone should cross my ears):
    1. subsonic bullets (e.g. pistol ones) are almost quite silent - that is pretty understandable;
    2. supersonic bullets (e.g. from sniper's carabine) may be heard not as a single 'boom', but rather as a whistling sound, lasting for 1/2s or so - I don't understand the mechanism behind this whistle.
     
  8. Sep 20, 2011 #7

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    Well, they are not supersonic forever. So at large distance they might already gone subsonic. Also: The conic shock wave is affected by obstacles like vegetation and reflected by the ground. This might "blur" it.
     
  9. Sep 20, 2011 #8

    xts

    User Avatar

    True, but you hear 'whistle' rather than 'boom' when bullets are definitely still supersonic, e.g. if you are crawling on no-man's land, and your colleagues continue carabine fire over your head from a distance of 100m or so.

    The bullet of 800m/s muzzle velocity, having effective range of over 1km, cannot slow down to subsonic on first 100m.

    The blur by reflections from the ground seems to be convincing explanation...
     
    Last edited: Sep 20, 2011
  10. Sep 20, 2011 #9

    boneh3ad

    User Avatar
    Science Advisor
    Gold Member

    Bullets are tiny and fast-moving, meaning that the sound they generate will be high-pitched due to the fact that the perturbations they create in the air are small and high-frequency.
     
    Last edited: Sep 20, 2011
  11. Sep 20, 2011 #10
    Excellent answer. Sonic boom just means that the sound generated by an object has piled up into a shock wavefront. But the sound does not magically appear because an object is going fast; it still must create the sound. An object that is very quiet at sub-sonic speeds (e.g. a bullet slicing through the air) will have a quieter sonic boom. Similarly, an object that creates high-pitched sounds normally will create a high-pitched sonic boom. The classic sonic boom heard when a supersonic jet passes overhead is the low-pitch roaring sound of the engines piled up so that it is much louder and sudden.
     
  12. Sep 20, 2011 #11

    DaveC426913

    User Avatar
    Gold Member

    No, it seems to me, the whole thing about a sonic boom is that it is a single wavefront of pressure, not a sustained sound. Regardless of the length of an object (be it a face-on manhole cover or a jet plane) it creates a single pressure wave dependent on the area and on the velocity. i.e. sonic booms do not have a pitch, so small object do not make "high-pitched" sonic booms and large objects do not make "low-pitched" sonic booms.

    (I grant that objects can create a multitude of shock waves, such as leading and trailing edges, but that does not change the principle that any given shich wave is a single pressure wave.)
     
  13. Sep 20, 2011 #12

    cjl

    User Avatar

    Not true. The sonic boom appears exclusively because the object is traveling faster than the speed of sound. The object can be emitting no sound of its own, and it will still create shockwaves simply because it is traveling through the air faster than sound.
     
  14. Sep 20, 2011 #13
    Perhaps we are getting into semantics, but I consider a shockwave a sound, therefore this statement is contradictory.
     
  15. Sep 20, 2011 #14

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    I agree in respect to the initial wavefront. See the conical shock wave in the picture. This is not a "whistling sound, lasting for 1/2s or so". This is something you will hear as a crackle or loud click.

    The whistling sound as described by xts is then heard in the cone and blurs with the initial crackle. But without actual recording we are just guessing around, about human perception.
     
  16. Sep 20, 2011 #15

    cjl

    User Avatar

    A shockwave can be considered a sound of sorts, but your prior statement implied that the sonic boom was the result of other sounds emitted by the object traveling supersonically. For example, you claimed that jet aircraft creating a sonic boom did so because the sound of the engine "piled up". This is false - the reason jet aircraft make loud, rumbling sonic booms has more to do with their size and the resultant intensity of the shock wave produced, which is why (for example) the sonic boom created by the space shuttle during reenty sounds very similar to the one generated by the concorde, even though one of them has jet engines and the other is gliding when the boom is created.
     
  17. Sep 20, 2011 #16

    boneh3ad

    User Avatar
    Science Advisor
    Gold Member

    Any object moving supersonically will create shockwaves, but the shape and intensity most certainly is dependent on more than just the area. In fact, the area has very little to do with it and much more on the overall shape. For example, a sharper object creates a sharper Mach cone. A blunt object creates a bow shock. There are many things that play into it. However, the physical reason for a shockwave is what you aren't grasping.

    An object moving through the air perturbs the air around it, which manifests itself as pressure fluctuations. These pressure fluctuations are sound. The wavelength (hence frequency) of these fluctuations depend on a great many things, but in particular an objects size, speed, shape and any noise-generating devices on the object. When the objects move faster than the speed of sound, these sound perturbations can't keep up and the ones moving forward start falling behind. As more sound waves are created, they coalesce with the previous waves and eventually form a shockwave. This shockwave most definitely will have a characteristic frequency to it based on what frequency the soundwaves generated by the object were before they coalesced. With a plane, you hear and feel an low boom. With a supersonic bullet, you hear and feel a much smaller and higher-pitched crack. Any sustained whistling afterwards is as a result of the sound created normally through the passing of the bullet through the air.

    True, and the frequency of that sound will be dependent on what frequency the perturbations generated by the object are. Smaller objects will generally have higher frequency sound (from things such as shed vortices). With a plane, the sound of the engines and other things just add to the effect (though it certainly won't do anything like double the effect or anything).

    A shockwave is sound. However, you can generate sound purely aerodynamically as opposed to with engines.
     
  18. Sep 20, 2011 #17

    DaveC426913

    User Avatar
    Gold Member

    I am grasping it quite fine.

    An object that is otherwise making no noise will still create a shock wave and thus a sonic boom.
    An object that has a simple geometry, and a correspondingly simple shock envelope will still create a shock wave and thus a sonic boom.

    As I pointed out, there are overtones, because real-wrold objects are not simple and silent, however, in principle the shockwave is a single pressure wave, and has no frequency.

    When chrisbaird said this:
    he was wrong.
     
  19. Sep 20, 2011 #18

    boneh3ad

    User Avatar
    Science Advisor
    Gold Member

    Clearly not based on the rest of your post.

    Objects moving through the air never make no noise. Just because you can't hear the noise doesn't mean it isn't being made. Even the tiniest of pressure fluctuations, which are present as a result of quite literally any object moving through a viscous fluid, are going to result in some degree of noise. This sound is what coalesces into the shock.

    I don't believe anyone ever said it wouldn't. However, your originally said the shock was dependent on area (only sin a sense) and Mach number (it is), which was incomplete and partially incorrect. I was correcting this.

    Not true. A shock is a pressure wave, as you put it, but it is a result of the coalescence of many sound waves, each one of which contains a frequency (it is sound after all). The very fact that it can be described in terms of a wave means it has a frequency. If it didn't have a frequency, you couldn't hear it. I could go on and on.

    No he wasn't. The key point is that any object moving through a viscous medium produces sound. It doesn't just come from the engines or other similar sources.
     
  20. Sep 20, 2011 #19

    xts

    User Avatar

    Two more points about 'whistling sound':

    - I am not sure if it is 0.5s or 0.1s, but definitely it lasts long enough to be noticed as lasting, rather than single 'click'

    - maybe the 'whistle' is somehow related to bullet rotation? Carabine bullets usually make one rotation every 25-35cm, which, at 800m/s gives 2-3 kHz. That might be a pitch of bullet 'whistle'. But, on the other hand the bullet is pretty symmetrical...
     
  21. Sep 20, 2011 #20

    DaveC426913

    User Avatar
    Gold Member

    This is rude and uncalled for. Knock it off.

    The point is that the noise is incidental. A single pressure wave - one compression - is enough to be a shock wave. That is the essence of a shock wave, regardless of the details you've mentioned.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: No sonic boom? Why not?
  1. Super Sonic Flight (Replies: 5)

  2. Why ? (Replies: 3)

Loading...