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How loud is the supersonic crack?

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

  1. Owen

    Owen Moderator Emeritus

    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 Well-Known Member

    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 Well-Known Member

    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.

  4. rcmodel

    rcmodel Member in memoriam

    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.

  5. ctdonath

    ctdonath Well-Known Member

    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 Well-Known Member

    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

    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 Well-Known Member

  9. hso

    hso Moderator Staff Member


    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

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

    Vibe Well-Known Member

  12. hso

    hso Moderator Staff Member

    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 Well-Known Member

    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 Well-Known Member

    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 Well-Known Member

    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 Well-Known Member

    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

    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

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

    brickeyee Well-Known Member

    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 Well-Known Member


    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."

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