Kansas Academy of Science. 



DETERMINATION AND COMPARISON OF SOUND 

 INTENSITIES. 



By Bruce V. Hill, University of Kansas, Lawrence. 



WE are now able to compare the power and efficiency of differ- 

 ent luminants with considerable ease. For similar measure- 

 ments dealing with the energy from sounding bodies we have, 

 however, at present no method. Several properties of the vibra- 

 tions and waves concerned in the two cases, as well as the peculiari- 

 ties of the sense-organs involved, are responsible for this. To 

 compare the illumination power of two lamps, the common pro- 

 cedure is either to produce equal illumination from the two 

 sources on adjacent parts of the same screen, or so to adjust the 

 two that shadows, of equal depth are cast. Now, both of these 

 methods depend upon the fact that light travels very nearly in 

 straight lines. Sound waves, having so much greater wave length, 

 do not even approximate rectilinear propagation. 



The eye is very sensitive to changer of intensity, but not to 

 small shadings of color. With the ear the reverse is true. Very 

 small changes of pitch can be perceived, while variation of loudness 

 is required to make itself noticed. A lamp varying even slowly is 

 unpleasant, while, in music, the accent and shading are necessary, 

 and so by habit we are less able to discriminate slight changes of 

 sound intensity. Again, the ear has a power of analysis which the 

 eye does not possess. A white light may really be made up of a 

 number of different combinations of colors. In the sound from a 

 number of instruments, however, the ear can pick out, not only the 

 separate pieces, but the overtones given by them, amounting in 

 all to hundreds. 



All these things make it impossible to compare tone intensities 

 as we do those of light. The effect of a given source of light 

 might be measured by its chemical action, but, so far as we know, 

 there is no corresponding effect due to sound. There are several 

 effects produced by sound, and of these we will speak briefly, and 

 give a short account of the attempts to solve the problems of sound- 

 intensity measurement. The action of the microphone has long 

 been known. If in the circuit with a battery and a telephone re- 

 ceiver is placed a carbon rod, resting loosely upon carbon supports 

 at its ends, a very slight jar varies the resistance of the contacts 

 and causes a sound in the receiver. Three nails in H shape make 



