A METHOD OF MEASURING ACOUSTIC IMPEDANCE 



409 



observed as oscillations of the impedance at about 5-cycle intervals. 

 An examination of the measurements in this oscillatory region (Fig. 9) 

 will make evident the precision of the apparatus. 



Fig. 10 shows the radiation impedance of a hole, 0.7 inch in diameter 

 and surrounded by a flange which appro.ximates an infinite wall for 



lO 80 



5 



500 1000 2000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 7 — Acoustic impedance of 6 foot e.xponential horn, having end diameters of 



0.7 inch and 30 inches. 



500 1000 2000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 8 — Acoustic impedance of 112 foot open tube, 0.7 inch in diameter, coiled into 

 helix. Measuring frequencies chosen at random. 



