A METHOD OF MEASURING ACOUSTIC IMPEDANCE 



40J 



Later, a direct method was described Ijy Stewart ^ who measured 

 the change in acoustic transmission through a long uniform tube when 

 the unknown impedance was inserted as a branch. 



The apparatus to be described in this paper measures acoustic 

 impedance directly in terms of a known acoustic impedance and three 

 balance readings of an electrical potentiometer. The only assumptions 

 involved in the method are that the elements of the apparatus be 

 invariable during a measurement, and that the value of the comparison 

 acoustic impedance be known accurately. 



Apparatus 

 Fig. 1 shows the general arrangement of the apparatus. An oscil- 

 lator feeds electrical energy into a loud speaker where a portion is 

 converted into acoustic energy which travels along the tube and into an 



OSCILLATOR 



(3) 



(2) 



(1) 



HEAD PHONES 



Fig. 1 — Schematic circuit of acoustic impedance measuring apparatus. 



attached impedance. A canal about 0.06 inches in diameter picks up 

 the sound pressure at the junction of the tube and the attached im- 

 pedance, and passes it along to a small condenser transmitter. A 

 corresponding voltage, generated by the transmitter, is amplified and 

 the current output of the amplifier passed through a variable resistance 

 in series with the primary of a variable mutual inductance. 



The same oscillator also feeds energy through a second amplifier at 

 the output of which the voltage is balanced (by the null method) 

 against the voltage drop across the variable resistance and the second- 

 ary of the mutual inductance. 



^Phys. Rev., 28, 1038 (1926). 



