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Noise travels fast and not so fast?

Discussion in 'General Firearm Discussion' started by sp 101, Mar 22, 2015.

  1. sp 101

    sp 101 here New Member

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    Scientific question for those up for inquiry :

    WHY when Someone shoots a rifle at a 1 mile distance and I hear a cu-lick then a boom. Both sounds come from the same gun, from the same location but they get to my ear seconds apart.

    I have been at the range listening to black powder ignition followed by bullet impact – but that is two different locations as well as two different sounds.

    Why are the sounds arriving at my location at two different times?
  2. boogerhook

    boogerhook Seattle Well-Known Member

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    My non-scientific response: The first is the explosion of the primer and powder in the cartridge, the second is the sonic boom made by the bullet flying faster than sound. Sub-sonic sound waves travel ahead of the object (being pushed) super-sonic sound waves travel in the wake of the object, that is a little later.

    No idea whether its true, just pulled it out of my ... hat.
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  3. solv3nt

    solv3nt Portland Well-Known Member

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    Scientific answer, sound travels around 750mph, or somewhere around 1100fps, this varies with altitude. The first sound that you will year, is the initial explosion of the powder burn, the second sound that you will hear, is the sonic crack of the bullet breaking the sound barrier.

    In this case, the speed of sound makes two debuts, the first being the gunshot. The gunshot is loud, and that sound wave will travel at the speed of sound.

    The second is the sound barrier. Just as chuck Yeager broke the sound barrier in the x1, the bullet is supersonic, and will have a new sound wave traveling behind it. This wave will also sound like a boom, but like all sound waves, will also be traveling at the speed of sound.
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  4. Heywood

    Heywood Prineville Oregon Well-Known Member

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    If you are away from the shooter with a modern weapon, you are hearing the sonic crack of the bullet as it goes by as it is most likely still moving faster than the speed of sound. Then, the sound of the cartridge touching off, arrives after it. This is known as "crack-bang" and for a while people thought they could use it to determine direction and distance of fire which is possible for very limited distances and then only if you know which cartridge is being used.
    If you are standing near the shooter, or next to them. You are probably hearing some sort of echo. Which is much more likely with a slow moving bullet.
    I just re read your post. Sounds like you think you are at a considerable distance from the shooter? If the difference between sounds is as long as you say, you are much closer to the shooter than you assume, because, when the bullet velocity diminishes beyond the transonic range (which is far less than a mile for Most standard hunting cartridges and about half a mile for military cartridges) there will no longer be a sonic crack.
    As an example, the difference in the crack/bang of a 5.56 increases out to about 400 meters, then it deminishes quickly beyond that.
    Of course this all changes when you throw in canyon like terrain.
    Last edited: Mar 24, 2015
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  5. tac

    tac UK, Oregon and Ontario. Well-Known Member

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    Over here in THIS man's Army, we call it crack-thump.

    Case #1 - As you lie there, crapping yourself, you first hear a wickedly sharp crack as the round goes over your head, followed by the distant thump of the gun that fired it.

    This actually GOOD. It means that you are still alive.


    Case #2 - Everything goes black for no apparent reason.

    This is VERY BAD.

    This means that you are dead.

    In case #1 it's the bullet passing over your head at a supersonic velocity - hence the whiplash-like crack. You CAN learn to tell not only what is being fired at you, but figure out roughly how far away it is, too, with practice. We got a LOT of practice.

    In case #2 it's the bullet nailing you through the forehead, in which case you'll never hear anything at all.

    Last edited: Mar 31, 2015
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  6. SmokeEater

    SmokeEater Key Peninsula Member

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    I was walking the 200 yard butts at the Seattle Police range with my brother. He's ten years older than me and has doctorates in Physics, math, languages, and a couple of others. As the bullets passed over our heads, he was muttering something. I asked him what he was doing. He said, "Just figuring out the bullet velocity and the probable caliber by the sound of the bullet"

    I shut up.:eek:
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  7. rick benjamin

    rick benjamin USA, Or, Damascus Secure the drama Silver Supporter 2016 Volunteer

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    I'd say you were a wise man, but then I noticed you run into burning buildings...
    P.S. metoo
  8. MarkAd

    MarkAd Port Orchard Well-Known Member

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    Gunfire and buildings on fire, can a man have more fun??:eek:
  9. SmokeEater

    SmokeEater Key Peninsula Member

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  10. Flopsweat

    Flopsweat Slightly right of center Well-Known Member

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    I saw some of the other stories that you have posted there. My life was saved by someone trained in CPR after I collapsed in an ER (not Seattle, but still...). I surely would have stayed dead if I had keeled over anywhere else.
  11. Medic!

    Medic! What just happened? Has eagle eyes. But cant remember what he saw. Bronze Supporter

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    It's a well known fact that light is faster than sound.

    That's why so many people look bright until the speak! :s0108:
    Flopsweat likes this.
  12. chemist

    chemist Beaverton OR Well-Known Member

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    Aha- gotcha! All your tech talk is belong to us! Sound speed does NOT vary with altitude, only with temperature. That infamous graph of "sound speed vs. altitude" in the CRC is really just a plot of standard temperature vs. altitude.
    Now attenuation varies quite strongly with altitude, but velocity does not. Your muzzle report might die out quicker in the mountains, but it travels at the same speed given the same air temp as the blast from your beachfront shot.

    For extra credit, tell me how humidity affects sound speed.... Anybody?
    Humid air is less dense than dry air at the same temp, so the sound speed increases on moist days, ceteris paribus.