MINIMUM AUDIBLE INTENSITY OF SOUND. 427 



gation of that experimenter 12 consisted in the measurement of the 

 pressure variations in a sound wave at the limit of audibility by means 

 of a sensitive resonator. This method was finally discarded on ac- 

 count of the large errors of observation inherent in it, and another line 

 of attack adopted. 13 The relative sensitiveness of the ear for tones of 

 different pitch ranging from 50 to 16,000 vibrations was determined 

 by two methods. One of these involved a direct measurement of the 

 relative minimum intensity by means of the current necessary to pro- 

 duce a sound in a telephone diaphragm pressed against the ear and 

 acting like a piston in the outer chamber of the ear. The other method 

 consisted in the calculation of the relative intensity from the current 

 necessary to produce audibility at a distance 30 centimeters from the 

 telephone. In the latter case the diaphragm was placed in an aperture 

 in the extended plane surface, so that it was theoretically possible to 

 calculate the energy at any point in the neighborhood for any ampli- 

 tude of the diaphragm. By the second method the absolute as well as 

 the relative values of the minimum audible sound for different pitches 

 were determined. The results of the two methods give curves for the 

 relative sensitiveness with differing pitch which are in close agreement. 

 The absolute values are, however, much smaller than those obtained 

 by any other experimenter. Rayleigh 14 doubted the validity of the 

 Wien sensitiveness curve, owing to certain experiments of his own, but 

 in general Wien's results were, for many years, accepted as correct. 



In 1921, Kranz 15 employed a thermophone pressed against the ear 

 as a calculable source of sound, and by gradually reducing the current 

 until he could no longer detect the opening or closing of the circuit, he 

 determined the minimum audible energy. His results have been 

 published for only one pitch. 



Lane 16 has conducted an investigation over the upper range of pitch, 

 above 2000, using a Hewlett tone generator as a source. His experi- 

 ments were carried on at night, out-of-doors, and the disturbance due 

 to the noise of insects is mentioned. His results are therefore open to 

 the same criticism as the work of the early experimenters already 

 noted. He finds the minimum audible intensity to remain practically 

 constant between pitches 2000 to 14000. 



More recently, Fletcher and Wegel 17 have completed an extensive 



12 Wied. Ann., 1889, Vol. 36, p. 834. 



13 Pflueger's Arch. f. Gesammte Physiol., 1903, Vol. 97, p. 1. 



14 Phil. Mag., 1907, Vol. 14, p. 596. 



15 Physical Review, Second Series, March, 1921, Vol. 17, p. 384. 



16 Phys. Rev., Second Series, May, 1922, Vol. 19, p. 492. 



17 Phys. Rev., Second Series, June, 1922, Vol. 19, p. 553. 



