still use an out-of-date figure of 1,080 fps for the speed of sound. This would be correct at about 24° F. For a number closer to general use, 1,120 fps is better. Rounding off the figures slightly, this is based on about 60° F. (The standard previously given of 661.7 knots at 59° F. is 1,116 fps.) The speed of sound increases as
how much water vapor is in the air. There are a number of formulas that are used to determine the speed of sound that involve density — and humidity if air is the medium. If they are worked out for air, there is very little difference, i.e., density has very little influence. Sound travels faster in moist humid air than in dry air, but again the difference is so slight it can be dismissed. Where density has a powerful effect is when the medium changes. For example, sound moves much faster in liquids and solids than in gases. In both liquids and solids, density has the same effect as in gases: that is, velocity varies inversely as the square root of the density. For ex- ample, at normal temperatures, sound moves in copper at about 11,000 fps. And steel is a very good conductor of sound at about 16,000 fps. Sound moves in water at about 5,000 fps near the surface. As you go deeper in the water, the increased pressure from the weight of all the water on top makes the water more dense and sound travels faster. The molecules that vibrate to transmit the sound waves are closer together. Of course, air has the same situation, but air changes very little in, for example,
one thousand feet while water changes a huge amount in the same distance. Many books on the subject say
that nothing affects the speed of sound in the atmosphere other than tem- perature, i.e., the effect is so slight they discount it. Others say that density and humidity have an insignificant effect. Most books agree that pressure has no influence, although increasing pressure increases the density. For example, the speed of sound at
sea level is 661.7 knots or nautical miles per hour (761 statute or common miles per hour) at a standard temperature of 15° C. (59° F.). At 60,000 feet the speed of sound drops to 573.8 knots. This is based on a standard temperature at that altitude of -56.5° C. (-69.7° F.) A general rule of thumb is that the pressure drops about 1 inch of mercury for each 1,000 feet of altitude gained. If you are using millibars, the correction is 1 millibar for each 8 meters of altitude gained. As altitude increases, these rules of thumb become less accurate and are at their best below 4,000 feet above sea level. In firearm ballistics we normally
give velocities in feet per second instead of knots or miles per hour. Many people
the temperature increases and con- versely will decrease as the temperature drops. The change is about 2 fps for each degree of centigrade. Above the speed of sound is the supersonic range and below it is called subsonic. The speed of sound is important for both the bullet design and performance as well as the louder noise, sometimes described as a crack, that the supersonic bullet makes as it travels through the air. The same noise as an aircraft sonic boom, it is a product of a pressure shock wave suf- ficient to create an audible disturbance. The speed of sound slows with
increased altitude, but this is because of the lower temperature instead of lower air density. Many people believe that the thinner air is the reason for the drop, but this is not correct. Frankly, all of this discussion on
the speed of sound is probably more trouble than it is worth. For our knowl- edge of small firearm ballistics, only temperature needs to be considered, and even normal temperature changes will make very little difference. In most cases we can just use 1,120 feet per second. It is not acceptable to say that the
speed of sound depends on density, humidity, or any other factor, unless temperature is included. Temperature is the most important of all the influ- ences and the only one that needs to be considered in small arms ballistics. Re- member that we have been discussing the speed of sound. Density and some of these other factors have a more notice- able effect on the flight characteristics and trajectory of the projectile, but that is not the subject at this time. Finally, remember what Thomas
Jefferson said, “No free man shall ever be debarred the use of arms.” In the two years following England’s ban of guns, crime with handguns rose 40 percent. Why? A study found that the law targeted legitimate users rather than criminals.
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