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From: bercov@bevsun.bev.lbl.gov (John Bercovitz)
Newsgroups: rec.guns
Subject: The Speed of Sound
Message-ID: <35545@mimsy.umd.edu>
Date: 13 Jun 91 01:59:45 GMT
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Approved: gun-control@cs.umd.edu

The speed of sound is referred to fairly often in this newsgroup and
occasionally there are quantification errors.  If we ever start a FAQ,
the following might be useful.

The speed of sound in air is (kgRT)^0.5.  Notice that the only variable
is T.  So the velocity of sound doesn't depend on its pressure.  Subbing
in values for k,g,and R with degrees Rankine for an input and feet per
second for an output: c = 49*(T)^0.5 to about four significant places.
Rem: To get to degrees Rankine, add 460 to degrees F.
Example:  t = 68F so T = 528R
          c = 49*(528)^0.5 = 1126 fps

Another thought mentioned in this group at times is that a supersonic
bullet causes shock waves when it enters a semi-liquid medium.  It's
certainly true that anything traveling faster than the speed of sound
in a medium has shock waves coming off of it.  But the speed of sound 
in water is around 5000 fps.   A critter with a projectile traversing
his carcass is going to experience traumatic shock but not shock waves.
Perhaps that's the source of the confusion 'mongst the gun rag writers.
Nitpicking: It is possible to get some very small local shock waves
at the front of a flat-nosed bullet if it is only slightly subsonic
in the traversed medium.

If you must create shock waves in a critter, you're not going to be
able to do it with a conventional firearm which, due to propellant
limitations, has a maximum projectile velocity of around 4500 fps.
Instead, you'll have to step up to a gas gun at least.

The speed of sound in solids is higher yet.  Lead is not very stiff
but it is quite dense so the speed of sound in it is around 6400 fps.
At the other end of the spectrum, beryllium is quite stiff but has a
low density.  The speed of sound in it is around 42,000 fps.  Steel
and copper lie in between at 19,000 and 16,000 fps respectively.

         JHBercovitz@lbl.gov    (John Bercovitz)