534 BELL SYSTEM TECHNICAL JOURNAL 



tested is built into an opening between two adjacent but structurally 

 isolated rooms. A loud speaker acts as a source of sound in one room 

 and a portion of the sound energy is transmitted into the second room 

 through the test partition. The transmission loss is taken as 



TL = L, - L2 - log.o MA), 



where Li and Li are the intensity levels in the source and test room 

 respectively, expressed in db, a^ is the absorption in the test room and 

 A is the area of the partition. The levels Li and L2 are measured and 

 plotted with a moving coil microphone and an automatic level recorder, 

 and a beat frequency oscillator is used as a source of tone so that the 

 frequency may be varied continuously. Measurements with a con- 

 tinuous variation in frequency enable resonances in the partition to 

 be much more easily and quickly detected than is possible when 

 measurements are made at discrete frequency intervals. Both pure 

 and frequency modulated tones have been used for the measurements. 

 Results of measurements on a few partitions are given. 



The Optical Behavior of the Ground for Short Radio Waves.^ C. B. 

 Feldman. The role of the ground in radio transmission is first con- 

 sidered generally. In short-wave propagation taking place via the 

 Kennelly-Heaviside layer only the ground in the vicinity of the 

 antennas is involved, and its effect may be included in antenna 

 directivity. The utility of so ascribing the ground effect exclusively 

 to the terminals of a radio circuit rests on the applicability of simple 

 wave reflection theory in which the distance between the terminals 

 does not appear. For this purpose reflection equations, similar to 

 Fresnel's equations for a nonconducting dielectric, are employed with 

 a complex index of refraction. 



The paper describes experiments undertaken to determine the limits 

 of applicability of these optical reflection equations and discusses 

 the results. Particular emphasis is placed on the identification of 

 direct and reflected waves. The existence of a surface wave, foreign 

 to simple reflection theory, is recognized with vertical antennas, when 

 the incident wave is not sufficiently plane. At angles of incidence 

 between grazing and the pseudo-Brewster value the requirements of 

 planeness are severe. The relation of optics to Sommerfeld's theory 

 is discussed. The experiments include tests made -with the aid of an 

 airplane. 



For short-wave communication via the Kennelly-Heaviside layer, 

 use of the modified Fresnel equations is shown to be justified. These 



3 Proc. LR.E., June, 1933. 



