404 BELL SYSTEM TECHNICAL JOURNAL 



At the end of the tube from the loud speaker are three different 

 impedances. One is the reactance offered by a closed tube of uniform 

 bore. The closure is formed by a well-fitting plunger whose position 

 in the tube may be adjusted. The second is the infinite impedance 

 offered by a rigid wall closing the end of the tube from the loud speaker. 

 The third is the impedance to be measured. All three impedance 

 elements are fixed in position. The loud speaker, tube, condenser 

 transmitter and associated amplifiers are, however, mounted together 

 on a carriage which can be rotated so as to bring any one of these 

 impedances into alignment with the tube. For brevity, reference to 

 these three positions will hereinafter be to positions 1, 2, and 3. 



For any one frequency a balance is obtained for each of the three 

 positions. These three electrical readings and the reactance value of 

 the closed tube are sufficient to determine the impedance being 

 measured. 



A photograph of the apparatus is shown in Fig. 2. 



Fig. 2 — View of acoustic impedance equipment, showing loud speaker, tubes, con- 

 denser transmitter amplifiers and small horn in position for measurement. 



Theory 



Thevenin's theorem ^ states that, in an invariable electrical network, 



the current in any branch is equal to the current that would flow in a 



simple series circuit composed of an electromotive force and two 



impedances. The electromotive force is the voltage that would 



obtain at the branch terminals on open circuit. The impedances are 



the impedance at the terminals looking back into the source of power, 



and the impedance of the branch. 



^ K. S. Johnson's "Transmission Circuits for Telephone Communication," Ch 

 VIll. 



