ACOUSTIC IMPEDANCE. 



23 



shape of this graph O D E F is not regarded as of great importance; 

 because, if the instrument was placed against the ear with less or more 

 pressure, or if it was centered differently on the ear; or if some other 

 ear was selected, the shape of the graph would be likely to be altered 

 appreciably. To a first approximation, however, the graph O D E F 

 may be regarded as an approach to the dotted circular motional im- 

 pedance O G E H, which may be described as the motional circle of 



Figure 12. Motional Impedance Graphs of Receiver in Room and pressed 

 against Listener's Ear. 



reference, with the instrument applied to this ear. It will be observed 

 that the diameter of this circle of approximation is 78.5 ^ 44° ohms. 

 The diameter O E has, therefore, substantially the same slope as the 

 free-air motional impedance diameter O B ; but has been reduced in 

 length somewhat more than 50 per cent. The frequency of apparent 

 resonance has also been changed from 969 ~ to 1 100 <>3 , by applying the 

 receiver cap to the ear. 



Figure 13 gives the corresponding mechanic impedance graphs for 

 this receiver and ear. The straight line a b c, parallel to the reactance 

 axis, corresponds to the motional impedance circle O A B C of Figure 

 12, taken in the free air of a room. This means that the total mechanic 

 impedance of the receiver free to the room, was a constant mechanic 



