Study of Locomotive Whistles 



107 



successively turned so that the relative direction of the observing sta- 

 tion was that indicated by the several radii in figure 1. 



Even had it been possible to move the Rayleigh disk and adjust it 

 to the same sensitivity in all ten positions, and had the time required 

 been no consideration, the scheme of turning the locomotive had many 

 advantages. It minimized, by making more nearly constant the dis- 

 turbing effects of winds, temperature changes and differences, the 

 varying topography of the surrounding landscape, and reflections from 

 houses, trees, and other objects. 



The curve in figure 1 shows that the intensity of the sound from 

 the whistle at right angles to the track was double the intensity along 



Fig. 2. Sound wave generated by an electric spark, showing reflection, refraction, 

 diffraction and absorption. 



the track. The general contour of the curve is about what one should 

 expect from a whistle located as this one was, to the side and rear of, 

 and only four inches from, a steam dome several times as large as 

 the whistle itself. However, the shape of the curve was not determined 

 wholly by reflection. Several other factors contributed to its variation 

 from a circle with the whistle at the center. One of these is that in 

 the case of a whistle located toward the rear of a locomotive boiler the 

 portion of the sound waves that start forward in the direction of the 

 track must pass through the hot air currents arising from the boiler 

 and the hot gases and smoke from the smoke stack. There is conse- 

 quently more or less energy absorption, reflection and refraction. In- 

 deed, the cylindrical stream of hot gases from the stack is .essentially 

 a sound dispersing lens. The sound energy in the shadow of such a 



