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How many feet per second is the speed of sound?

Discussion in 'General Gun Discussions' started by grimjaw, Jul 2, 2006.

  1. grimjaw

    grimjaw Well-Known Member

    I'm trying to do some guestimations on rimfire ammunition at longer ranges (150-200 yards). I've heard anecdotal evidence that if a projectile moving a supersonic speed falls to subsonic speed, especially if it the fall is rapid, it has an affect on trajectory. I don't know if that is true, maybe someone will come along and enlighten me.

    I read up a little bit on the speed of sound, and I understand that it changes depending on medium pressure (?) and thus decreases with altitude in air. 761 mph is the speed of sound at sea level, but at 11000+ feet, it's 660 mph. I'm using the 761 mph number to do my math.

    761 miles = 4018080 feet
    1 hour = 3600 seconds

    Speed of sound at sea level = ~1116.13 feet per second

    Are those correct calculations?

  2. KimberTLE.45

    KimberTLE.45 Active Member

    Sounds about right. I know subsonic ammo is right around 1050 FPS so 1100 would seem right.
  3. Stiletto Null

    Stiletto Null Well-Known Member

    Sure. :)

    Speed of sound is dependent on a bunch of other variables like temperature and humidity, but ~340m/s is a good number for napkin math.

    Allowing for .3048 m/ft, that comes out to 1115ft/s.
  4. OEF_VET

    OEF_VET Well-Known Member

    That's pretty close. It's somewhere around 1,100 fps, with minor variations based on altitude changes.
  5. Outlaws

    Outlaws Well-Known Member

    Thats about what google says.
  6. 444

    444 Well-Known Member

    I recently spent some time working up a subsonic load for the AR15.
    I found that last week, with a temperature about 110 degrees, in Las Vegas, I started getting a supersonic crack at around 1140 fps. This isn't a solid number but right now, here, it is very close.
  7. RNB65

    RNB65 Well-Known Member

    There is no exact answer. The speed of sound is not fixed. It varies based on atmospheric conditions (temp, air pressure, humidity).
  8. Stevie-Ray

    Stevie-Ray Well-Known Member

    1100 fps is the generally accepted answer.
  9. DoubleTapDrew

    DoubleTapDrew Well-Known Member

    Wow! I didn't know you could download a .223 to subsonic velocities considering it's usually smoking at about 3000fps. That's cool to know. How does it shoot with those loads?
  10. trapperjohn

    trapperjohn Well-Known Member

    its around 1100 f/s, it will vary a bit depending on atmospheric conditions like temp and humidut, but its not a wild variation.

    the reason you want a supersonic bullet ot stay supersonic is to keep it from passing through the shock wave. a supersonic projectile has a shock wave traveling BEHIND it. if the bullet slows down to subsonic velocities it passes through the shockwave which can alter its course slightly.
  11. the naked prophet

    the naked prophet Well-Known Member

    The speed of sound does NOT vary with pressure. It is independent of pressure. It does vary with temperature. http://en.wikipedia.org/wiki/Speed_of_sound I know this from freshman aerodynamics class five years ago.

    The difference between winter and summer, or night and day, will be more than the difference between florida and colorado.

    In addition to that, the air passing over the bullet may exceed mach 1 even if the bullet is travelling well under that speed. This means a semiwadcutter can have a supersonic crack at much lower speeds than a streamlined bullet. Keep that in mind when you develop subsonic loads.
  12. grimjaw

    grimjaw Well-Known Member

    trapperjohn, thanks for the explanation. That is what I was trying to avoid. The ballistics tables I've seen for .22WMR show that for 40gr projectiles you'll be hitting ~1100fps somewhere between 150-200 yards. I don't have a place that I can set up 200 yard paper targets, so it's hard for me to see just what kind of accuracy hit I'll take from that "shock wave", if any.

    prophet, thanks for the correction and info.

  13. Stiletto Null

    Stiletto Null Well-Known Member

    Nitpick. :p
  14. redneckdan

    redneckdan Well-Known Member

    The problem is the transonic barrier, when the projectile drops from super to subsonic, there is a lot of buffeting on the projectile, not exactly a good thing for accurcay because the buffeting is not uniform, every porjectile is differnt.
  15. Dionysusigma

    Dionysusigma Well-Known Member

    What about through other mediums, i.e. liquid and solid? And combinations of them? Say, tomato soup versus chili? :eek: :p

    For that matter, what about different gases? Is it different for hydrogen and xenon?

    What exactly causes sonic booms and cracks? Is it something along the lines of supercavitation of the medium itself?

  16. Stiletto Null

    Stiletto Null Well-Known Member

    Supercavitation? I thought that term's only applicable to liquids.

    Anyhoo, shock waves are the answer. You can get a sonic boom with dramatic changes in subsonic flow patterns too.
  17. TallPine

    TallPine Well-Known Member

    Somewhere back in my old computer code I have the formula for calculating the speed of sound (Mach). IIRC it is dependent on several variables including altitude (air pressure). Probably it was a multi-step computation, you first have to compute air pressure based on altitude ASL and then use that as an input.

    I could go look it up if someone really really needs to know the exact formula.
  18. Creeping Incrementalism

    Creeping Incrementalism Well-Known Member

    If you want to really get technical, the "only vary with temperature thing" depends on air being an ideal gas, which it isn't. Not that that fact makes a significant difference.
  19. the naked prophet

    the naked prophet Well-Known Member

    For most considerations, air at atmospheric pressure (or even a few atmospheres) is close enough to an ideal gas for most approximations. Considering the accuracy and consistency of our chronographs and loads, +/- 10 fps is way more than the error you get from approximating air as an ideal gas.

    The reason that the speed of sound varies with altitude is that the temperature varies with altitude. This website explains it pretty well, with a simple, correct equation. Although that wikipedia page is technically correct, it isn't the formula that I learned in school, and any joker can change it at any time. If you really want to be sure, get an aerospace book. I've got a few for sale ;)
  20. BigFatKen

    BigFatKen Well-Known Member

    This may be a typo; likely they meant median or "The median is the middle of a distribution: half the scores are above......"

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