26 KENNELLY AND KUROKAWA. 



ment was made with the receiver open to the air of the room. The 

 frequency of apparent resonance was, however, raised to 1090 o^ . 



Applicatioyi of the Method to the Tuning of Telephone Receivers for 

 increasing their Sensibiliti/ : Referring to Figures 7 and 8, it will be 

 seen that for the given impressed frequency of 921 c\3 on the receiver 

 connected to the fiber tube, varying the length of the tube through a 

 range of 9.4 cm., or one quarter of a wavelength, altered the motional 

 impedance from a maximum to a minimum, or vice versa, and also 

 altered the maximum cyclic velocity of the diaphragm in the ratio of 

 more than 8:1. From this it will be evident that when such a tube 

 is inserted between the telephone cover and the listener's ear, by the 

 insertion of an ear tube into the plug P, Figure 2, the loudness of the 

 sound emitted by the receiver, under steady excitation, can be adjusted 

 by altering the tube length. In other words, the tube length can be 

 tuned to maximum sensibility of the receiver. This principle may 

 have useful applications in measurements with the telephone receiver 

 as a detector. Observations made in this way have shown that this 

 method of tuning is applicable over a certain range of impressed fre- 

 quency. Thus, if the impressed frequency is 900 <>3 , and the receiver 

 has an apparent resonant frequency, under its actual acoustic load, of 

 1000 '>3 , an inserted adjustable tube may enable the resonant frequency 

 to be reached at 900 <>3 . 



Possible Applications of the Method to Architectural Acoustics: When 

 the free impedance of a telephone receiver is measured with the axis 

 of the instrument horizontal, and with its cover facing a wall of the 

 room at not too great a distance, it is found that, near the frequency 

 of apparent resonance, the electric impedance of the instrument is 

 affected by its distance from the wall. Holding the frequency steady, 

 the electric impedance of the receiver is found to vary if the instru- 

 ment is moved nearer to, or farther from, the wall which it faces. 

 This means that the acoustic impedance, at the diaphragm, of the 

 air in front of the instrument, is affected by the distance from the 

 wall in relation to the wave length of the emitted sound. The amount 

 of variation of electric impedance, or the sensibility of the measure- 

 ment to small mechanical displacements, depends upon the sharpness 

 of resonance of the diaphragm. It seems likely that this method of 

 measurement is capable of being applied to architectural acoustics; 

 as, for example, in the measurement of the sound-reflection coefficient 

 of draperies. For such purposes, a telephone receiver should be 

 selected with a small total mechanic resistance in its diaphragm, 

 and with a small damping constant A; i.e., with a large sharpness 



