dlogcher@uunet.UU.NET (Dutch) (06/15/91)
#Article 5993 of rec.guns: #From: bercov@bevsun.bev.lbl.gov (John Bercovitz) Newsgroups: rec.guns Subject: The Speed of Sound Date: 13 Jun 91 01:59:45 GMT #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 The speed of sound in air is a const = 343 m/s, or 1114.75 ft/s at 20 degrees C. The proper formula is v = (B/p)^1/2 where B=dP/dV/V and p is the denisty of the medium. This yields a v = 1500m/s or 4875 ft/s in water. This was derived using Young's Modulus... I own three rifles, an AR-15, Ruger mini 14, and Ruger 10/22. I keep these for target shooting, varmint plinking, and home defense. I hope I never have to use them for the latter, but I am prepared to do so if the need arrises Not a nice thing to say, but its not such a nice world to live in. -Dutch
bercov@bevsun.bev.lbl.gov (John Bercovitz) (06/15/91)
In article <35663@mimsy.umd.edu> mailrus!ulowell!dlogcher@uunet.UU.NET (Dutch) writes: ##Article 5993 of rec.guns: ##From: bercov@bevsun.bev.lbl.gov (John Bercovitz) #Newsgroups: rec.guns #Subject: The Speed of Sound #Date: 13 Jun 91 01:59:45 GMT ##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 # The speed of sound in air is a const = 343 m/s, or 1114.75 ft/s at 20 #degrees C. Sure you're not quoting something around 15C? I just checked the CRC handbook and the Pratt & Whitney standard atmosphere tables (based on NACA std atm) and get 1116fps @ 59F (15C). # The proper formula is v = (B/p)^1/2 where B=dP/dV/V and p is the #density of the medium. That's what I said, wasn't it? Can I use r (as in rho) for density and c for velocity? That way I can use p for pressure and v for specific volume. Thanks. c = (B/r)^1/2 but: B = kp (ideal gas) so: c = (kp/r)^1/2 but: r = 1/(g*v) so: c = (kgpv)^1/2 but: pv = RT (perfect gas law) so: c = (kgRT)^1/2 QED Peace? (Sorry about the g in there; I went to engineering school before there was much done in metric, more's the pity.) #This yields v = 1500m/s or 4875 ft/s in water. Did I forget to say approximately 5000fps? Sorry about that. I meant to say approximately because although it's a little lower for pure water, I didn't really want to explain about impure water. Since blood is a lot like seawater (c = 4990 fps @20C), the velocity of sound in blood should be somewhere around 5000fps. I think other "impurities" will raise it some more. OK, so it was a gross generalization but I think the conclusion will be the same: bullets aren't supersonic in game. Regardless, this is an object lesson to me on the subject of skating over explanations. And as long as we're into true confessions, the formula really works out to: c = 49.02 (T)^1/2. I was being a little clever saying that you get to degrees Rankine by adding 460; it's really 459.688. But by saying 460, I pumped up the answer a little so that I could say 49 was the constant rather than 49.02 and yet still get four place accuracy as long as we stay at temperatures that are habitable. I just thought it would be easier to remember that way. (Hate for ya to nail me on _that_ too.) Later, dudes! JHBercovitz@lbl.gov (John Bercovitz)