392 Table 112 (continued) 



VELOCITY OF SOUND IN AIR 



Hardy, Telfair, and Pielemeier have deduced expressions for /, g, and h as functions 

 of T, V , and C»oo, based on the equation of state in the virial form, and the reader is 

 referred to the original article 4 for details. These investigators 4 have made observations 

 of the velocity of sound in dry air and have also calculated its value under standard 

 conditions on the basis of the thermodynamic equations. The results, based on these 

 two methods of determination, were c = 331.44 ± 0.05 meter sec." 1 and a = 331.45 ± 0.05 

 meter sec. -1 , respectively, for the velocity of sound of small amplitude in dry air with 

 0.03 percent carbon dioxide at 0°C. and at one standard normal atmosphere pressure, as 

 the frequency of the sound approaches zero. The mean of results deduced from the 

 observations of other investigators was found to be c = 331.46 ±0.05 meter sec. -1 



We adopt as the best value a = 331.45 meter sec." 1 for dry air under the stipulated 

 conditions. 



For rough calculations of the velocity of sound in air under conditions in the neighbor- 

 hood of one atmosphere pressure one may use equation (5), with c =- 331.45 meter sec." 1 . 

 To take account of the presence of water vapor in air for such calculations, equation (3) 

 may be rewritten (assuming the mixture a perfect gas) 



= V'f 



(14) 

 or 



<=»V*(*)(?) (15 > 



Reliable observations of dry air with 0.03 percent C0 2 have yielded p = 0.001293 

 gm.cm." 8 , where po = 1013.250 mb. (1 atmosphere pressure). 



Elsewhere in this volume (Table 84) there will be found data permitting the accurate 

 computation of p for moist air as a function of temperature, pressure and relative 

 humidity, making use of the compressibility factor of moist air. 



For 70 we may use the value given by Hardy, Telfair, and Pielemeier, namely 

 70 = 1.4028 for dry air at 0° C. and one atmosphere pressure. 7 for dry air changes 

 relatively slowly with temperature and pressure, under ordinary atmospheric conditions. 



In taking account of the effect of water vapor on 7 when making rough calculations of 

 c, use may be made of the following approximate relation for ordinary conditions 6 



7 = 70 -0.1 j (16) 



Description of Table: 



Table 112 A is based on equation (5), taking c = 331.45 m.sec." 1 as previously ex- 

 plained. Strictly speaking, the table is only valid for dry air considered as a perfect 

 gas, and shows the variation of c with temperature on this assumption. It is, there- 

 fore, strictly applicable only in rough calculations. 



8 Wool f, Wm L., Acoustical tables for air and sea water, Journ. Acous. Soc. Amer., vol. 15, 

 p. 83, 1943. 



(continued) 



SMITHSONIAN METEOROLOGICAL TABLES 



