99 



KENNELLY AND KUROKAWA. 



(230 to 980 dynes per kine) than in mechanic reactance (j 630 to _/' 920 

 dynes per kine). This indicates that the changes in acoustic load, 

 effected h\ altering the tube aperture at the distant end, may pre- 

 ponderate in mechanic resistance, or in mechanic reactance, depending 

 upon the impressed frequency, and the system of stationary sound 

 ^va^'es set iip in the tube. 



Another set of measurements was made with the same telephone 

 receiA'cr and fiber tube -plugged at various tube lengths, l)ut with a 

 row of holes, like those of a flute, bored in the tube wall. The diameter 

 •of each hole was 6.5 mm., and their distance apart, between centers, 4 

 cm. Removable plugs were inserted in these holes. The motional 

 impedance of the receiver was measured, at four steady frequencies 

 between 900 ~ and 1000 cv:! , when one plug was remo\'ed at successive 

 holes along the line. The motional impedance was found to be 

 sensiti\'ely affected by the position of the particular plug removed. 

 The results, although interesting, have not been analyzed, and are 

 therefore not presented here in quantitative detail. They indicate, 

 however, that the acoustic impedance of the air column in a tubular 

 musical instrument, such as a flute, undergoes marked variations 

 when the instrument is manipulated, as in playing. These changes of 

 acoustic impedance might be measured, in the manner described, with 

 respect to a telephone-receiver diaphragm as the source of sound. 



Moiional Impedance and Acoustic Impedance of a Telephone Receiver 

 pjressed against the Fait: When a telephone receiA'er is exposed to the 

 air in a large room, the total mechanic impedance on its diaphragm is 

 the total internal impedance (z^ + z, . + Zi) plus the external im- 

 pedance Z2 of the air in front of the diaphragm, including the cushion 

 of air under the cap. The ordinary motional impedance circle of the 

 instrument is measured under these conditions. When, however, the 

 telephone is pressed against the ear, the external acoustic impedance 

 Z2 is that of the air in the ear ca\-ity, as well as in the cap of the instru- 

 ment. The motional impedance of the instrument under these condi- 

 tions, approximates to that actually presented in the average tele- 

 phonic use of the device as a receiAcr of speech. 



Figure 12 gives the motional impedance circle O A B C of a bipolar 

 receiver. No. 143-2, similar to that used in the tests recorded in pre- 

 ceding figures. The frec^uency of apparent resonance was /^ — 969 <^ 

 when the motional impedance was 177Y44?2 ohms. This receiver, 

 when held against the ear of a listener, as in the ordinary conversa- 

 tional use of the instrument, gave rise to the smaller graph O D E F, 

 which departs very clearly from the strictly circular form. The exact 



