ABSTRACTS OF TECH .W I CAT ARTICLES 189 



When the sound source is cut off there will not only be a decay of 

 sound due to the absorption of the walls, fixtures, etc., but the inter- 

 ference pattern will continually shift causing the actual rate of decay 

 of the sound intensity level at a point to fluctuate about the rate of 

 decay caused only by absorption. This fluctuation may be very 

 pronounced and actually changes from point to point in the room, 

 but may be minimized by the use of a warble tone and if necessary 

 by the proper placement of the transmitter. With these precautions, 

 and they are used in all the reverberation time measurements presented 

 in this paper, the effect of interference is greatly reduced. The 

 average rate or rates of decay 5 (db per second) of the sound intensity 

 level can at once be obtained from the slope of the straight line or 

 lines which best fit the series of experimental points. The reverbera- 

 tion times T (time per 60 db) used in this paper have been calculated 

 using the relation 



T = — 

 8' 



The Effect of Exposure and Development on the Quality of Variable 

 Width Photographic Sound Recording.^ Donald Foster. This paper 

 deals with the dependence of the quality of variable width recording 

 on the conditions of exposure and development. When the widths of 

 the images employed in recording or reproducing are comparable with 

 the w^ave-length of the record the exposed portion of the record is not 

 uniformly exposed. The record is attenuated in amplitude as the 

 frequency is increased, and harmonics are introduced whose relative 

 intensities depend on the contrast of development and on the fre- 

 quency. When the exposure of the record occupies the linear range 

 of the H & D curve, and when the product of the gammas of the 

 negative and the positive is equal to unity, the record is practically 

 free from spurious harmonics. The amount of non-linear distortion 

 is calculated for the case when the over-all gamma is equal to two; 

 and it is shown that Cook's analysis of the aperture effect gives a 

 superior limit to the distortion obtainable by overexposure or by 

 over or underdevelopment. The effect of the unavoidable non- 

 uniform illumination of the images is considered. 



The Vectorial Photoelectric Effect in Thin Films of Alkali Metals.^ 

 Herbert E. Ives. It is assumed that the photoelectric eft'ect ex- 

 hibited by thin films of alkali metals on specular platinum surfaces is 

 proportional at any wave-length to the electric intensity just above 



8 Jour. S. M. P. E., November, 1931. 

 '■> Phys. Rev.. September 15, 1931. 



