A METHOD OF MEASURING ACOUSTIC IMPEDANCE 



407 



losses through heat conduction, for frequencies near the half wa\e- 

 length anti-resonance, where tlissipative effects are most pronounced. 

 It will be seen from Fig. 5 that there is a close agreement between the 

 theoretical curve and the measured points. It seems reasonable, 

 therefore, to assume that the value chosen for the comparison im- 

 pedance is quite accurate. 



Fig. 6 shows the impedance of a conical horn and F"ig. 7 that of an 

 exponential horn. In both cases, the mouth of the horn projected 

 through a window into open air, so as to minimize reflection effects 



800 



600 



400 



200 - 



200 



-400 



-600 





1000 



2000 3000 



FREQUENCY IN CYCLES PER SECOND 



4000 



Fig. 4 — Acoustic reactance of closed cylindrical tube, 2.4 inches long and 0.7 inch 



in diameter. 



from external objects. Reflection effects from the mouth, where there 

 is a change in impedance, are present, however, and these appear as 

 oscillations of the impedance about a mean which is the characteristic 

 impedance of the horn. By characteristic impedance is meant the 

 impedance that would obtain looking into the throat of the horn were it 

 infinite in length. 



Fig. 8 is the impedance of an "infinite " tube. The tube was actually 

 112 feet long and coiled into a helix. At low frequencies, where the 

 dissipative losses are small, reflection effects from the open end are 



