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How loud is the supersonic crack?

Discussion in 'General Gun Discussions' started by Owen, Sep 3, 2008.

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  1. Owen

    Owen Moderator Emeritus

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    If a supersonic bullet passes, say, 6 feet away, how many dBa is it?

    is it velocity, projectile diameter, projectile length, or number of shockwaves dependent?
     
  2. ny32182

    ny32182 Member

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    Exact dB levels I don't know; nor have I ever stood 6 feet away while a bullet passed me down range, but I can tell you this: Based on watching full velocity rifles .223/.30cal being fired with a good suppressor on, I will say that the supersonic crack is indeed distinctly audible when isolated, but doesn't even hold a candle to the sound of the muzzle blast. When shooting an unsuppressed rifle, you can't hear it. It is completely drowned out by the muzzle blast.
     
  3. Jim K

    Jim K Member

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    It is a shock wave, the same thing as a "sonic boom" caused by a jet aircraft. It is not so much dependent on bullet shape as on velocity.

    How loud? I don't know but I have "worked the pits" and it is certainly noticeable as a loud "crack", like a .22 rifle being fired a few feet away.

    Jim
     
  4. rcmodel

    rcmodel Member in memoriam

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    I have been that close both in basic training, where .30 cal MG's were fired over you while low-crawling through an obstacle course.
    Also, military rifle ranges while working in the target pits.

    A .30 cal bullet crack is plenty loud enough to know for sure it just went buy! Not really that loud though. Kind of more a "Snap' sound.

    rcmodel
     
  5. ctdonath

    ctdonath Member

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    Can't tell you dB, but can tell you that an AR15 with silencer (i.e.: muzzle blast suppressed) sounds like an unsuppressed .22LR, as does roughly a .30-06 bullet passing nearby fired from 300m away.
     
  6. jnyork

    jnyork Member

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    I shot highpower rifle for years, and from being in the pits I can tell you it is pretty loud, you want your ear protection for sure.
     
  7. Owen

    Owen Moderator Emeritus

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    In a specific case, I know a an unsuppressed rifle ~163 dBA, and with a suppressor, it is ~142 dBa.

    What I really want to know is, with a perfect supressor, that eliminated 100% of the muzzle blast, so there is only the crack, how loud would the gun be?
     
  8. Artiz

    Artiz Member

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  9. hso

    hso Moderator Staff Member

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    Owen,

    I think the closest you're going to get is the difference between a suppressed rifle and an unsuppressed rifle.

    I don't even recall seeing a calculation for what you're asking for.
     
  10. Owen

    Owen Moderator Emeritus

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    guess I'll have to figure out how to measure it.
     
  11. Vibe

    Vibe Member

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  12. hso

    hso Moderator Staff Member

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    I think it would be the difference between the peak total and the max subsonic component since that subsonic component would be what was left from the muzzle blast of the rifle used in the graph.

    You'd need to have an octave band analysis done and subtract the subsonic from the supersonic, but you may get roughly the same results as subtracting the max subsonic from the total peak on the graph shown above (regardless of whether it was fired from a suppressed rifle) if you assume the subsonic is from the unsuppressed muzzle blast. i.e. 140 - 115 ~ 25 dB
     
  13. Jim Watson

    Jim Watson Member

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    Looks like Vibe has it. If you want field test information, all I can say is to go to a real rifle range and put a sound meter on the edge of the target pits. Time of flight will separate the muzzle blast from the bullet shockwave.

    It is also interesting to hear overhead fire from a subsonic bullet. A .45-70 at 550 yards just kind of "whizzes" by.
     
  14. Travis McGee

    Travis McGee Member

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    Having worked the pits and otherwise heard supersonic riflle and MG bullets passing overhead a few feet, I'd say it was more of a "snap" or "crack" than the BOOM or BANG of the muzzle blast.

    From a sniper's POV, the good thing about suppressing a rifle is that muzzle blast gives away position, but sonic crack does not. In fact, sonic crack alone often confuses the "recipients" about the location of the shooter, and sends them looking in the wrong places.
     
  15. Langenator

    Langenator Member

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    When I've worked the pits, I was never sure if the cracks I heard were from the passage of the bullets through the air, or the sound of them striking the targets.

    The one time I could distinctly ID the sound of bullets passing was on a Bradley platoon live fire gunnery range, where the 25mm sabot (training) rounds were passing a couple hundred yards away (the targets for the Brads were located a good distance from the bunkers the infantry squads were assaulting, for safety reasons), and the crack was quite distinctive, and came from a very different direction from the sound of the 25mm guns firing.
     
  16. Deanimator

    Deanimator Member

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    Standing in the target pits, I find the supersonic crack of a 6.5-.284, .308, or .30-06 fired from 600 yards to be fully the equivalent of a .22lr handgun fired next to me. I find it intolerable without hearing protection.
     
  17. hso

    hso Moderator Staff Member

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    That won't give an accurate measurement because the standard instruments won't capture the ~.35 millisecond "crack" because they're too short. You'll need a laboratory grade instrument capable of measuring impulse noise with durations as short as a tenth of a millisecond.
     
  18. Owen

    Owen Moderator Emeritus

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    thanks Vibe, that's exactly what I needed.
     
  19. brickeyee

    brickeyee Member

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    Even the measurement of muzzle blast amplitude is off with normal dB meters.
    The bandwidth of the circuits in these devices is simply not adequate to capture the actual peak noise intensity.

    The noise exposure time- intensity tradeoffs also fail for gunfire.
     
  20. James T Thomas

    James T Thomas Member

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    unmistakeable

    For you AK-47 fans; the "crack" is not what I would call loud, that is intensity, but it is distinctly attention getting! And from the AK, unique, I might add. I have heard up to 12.7mm from the wrong end, and I knew immediately the enemy's gun. It has a shock wave with it, and you can sense the damage it will inflict.

    I suppose you know, but the boom, boom, boom comes a moment later, unless you are unfortunate to be close up to one, and the time delay between the crack and the gun report can be used to estimate what the range is to the shooter.

    Some of the replies are referring to muzzle blast. The near passing of a bullet intended for you is a deadly "crick, crick, crick," as it sounded to me.
    Not much acoustical energy; decibels, but it get's your attention.

    I know there is a lot to admire about the AK, but I hate them.
    It may be irrational, but I can't dismiss it.

    No one ever needs to shout "hit the dirt."
     
  21. Loomis

    Loomis member

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    Sonic booms are different depending on the size and speed of the object.

    When you crack a whip, you are actually creating a mini sonic boom as the tip of the whip breaks the sound barrier.

    17HMR have very small "sonic booms".

    Aircraft sonic booms can make your house shake and break your windows if they fly low enough and fast enough.
     
  22. MachIVshooter

    MachIVshooter Member

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    All I hear is the muzzle blast that escapes the supressor behind a supersonic bullet, and the echo from the woods.
     
  23. Artiz

    Artiz Member

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    I can assure that, a couple of months ago there was the Quebec Air Show, this was the greatest air show ever, Blue Angels, Thunder Birds, Red Arrows, Snow Birds, and the Canadian Demo CF-18 who was supposed to fly just under the sound barrier, unfortunately, this nice pilot broke it just a little and released the gas at time, because we saw the "white cone" sourrounding the aircraft, mannn... add to the sound of a flying F-18 a broken sound barrier, you get hands on ears, omg that was loud, really loud.
    So a bullet creating a shockwave is NOTHING.
     
  24. misANTHrope

    misANTHrope Member

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    ^

    FYI, the "white cone" appearing around the aircraft is not indicative that the speed of sound has been broken. That cone of condensation appears at about .95 Mach or so. I wasn't there, so I can't say for sure, but I doubt you actually heard a sonic boom. The sudden onrush of sound after those high-speed passes they do at shows is pretty intense, but nothing like an actual sonic boom. And if a pilot at a show inadvertantly broke the speed of sound, he'd almost certainly end up in some deep doo-doo.

    Carry on. :)
     
  25. Artiz

    Artiz Member

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    He wasn't low, he was really high, I mean, really (ok sky's the limit), so we lost a bit of sound, but I think it was supposed to happend (in these shows they like to tell you something and doing the opposite) because he climbed to the point that we could'nt see him, and then we saw (I saw, other people were looking dead center, lol) him coming from the left, really high too and I'm pretty sure he (maybe not) broke it a little to freak us (can we say broke the sound barrier a little? lol), because we clearly heard a really nig noise after we saw the cone (I said "hands on your ears!" to my friend, he said what? I said Do IT!) and then BANG! We heared it, we feeled it.
     
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