LOUDNESS 



393 



threshold points are sHghtly above the zero we have chosen. This 

 means that our zero corresponds to the thresholds of observers who 

 are slightly more acute than the average. 



From these curves the loudness level contours can be drawn. The 

 first set of loudness level contours are plotted with levels above 

 reference threshold as ordinates. For example, the zero loudness level 

 contour corresponds to points where the curves of Figs. 2A to 2J 

 intersect the abscissa axis. The number of db above these points is 

 plotted as the ordinate in the loudness level contours shown in Fig. 3. 

 From a consideration of the nature of the hearing mechanism we 

 believe that these curves should be smooth. These curves, therefore, 



O 80 



< 



m 40 



500 1000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 3 — Loudness level contours. 



5000 10000 20000 



represent the best set of smooth curves which we could draw through 

 the observed points. After the smoothing process, the curves in 

 Figs. 2 A to 2J were then adjusted to correspond. The curves shown 

 in these figures are such adjusted curves. 



In Fig. 4 a similar set of loudness level contours is shown using 

 intensity levels as ordinates. There are good reasons ^ for believing 

 that the peculiar shape of these contours for frequencies above 1000 

 c.p.s. is due to diffraction around the head of the observer as he faces 

 the source of sound. It was for this reason that the smoothing process 

 was done with the curves plotted with the level above threshold as 

 the ordinate. 



^ Loc. cit. 



