4 KEXNELLY AXD KUROKAWA. 



Acoustic impedance is the planevector sum of acoustic rcsitifdncr and 

 acousiic reactance. Acoustic resistance absorbs and dissipates the 

 energy of acoustic vibration. Acoustic reactance cycHcally stores and 

 releases, without dissipation, the energy of acoustic vibration. If we 

 denote the acoustic impedance of the fluid on sides 1 and 2 of the disk 

 by Zi and Zj respectively; then the A'ector root-mean-sciiiare ^-elocity 

 of vibration at the disk surface will be: 



F F 



i" = = — rms. kines,-^ or cm. per sec. Z (1) 



Zi + Z2 z 



where z is the total acoustic impedance to velocity of the fluid in the 

 tube, at the disk, 



F acoustic absohms Z /^n 



or z = . (2) 



X or dynes per kme Z 



F being taken as of standard phase. 



If we employ the C G. S. system of units throughout, the acoustic 

 impedances Zi and Zo may be expressed in Aector acoustic absohms. 

 An absohm in acoustic resistance is therefore such a total resistance to 

 vibration at the surface of a disk, subjected to a simple harmonic 

 vibromotive force of 1 rms. dyne, as will cause a Aibrational velocity 

 to be produced of 1 cm. rms. per second, in phase with the vmf. An 

 acoustic impedance is, however, a planevector ciuantity, having both 

 a size and a slope, and is expressible in complex numbers. If the 

 slope of Z is + /3°, the phase of the velocity will be retarded, by (1), 

 through (3° behind the vmf. Acoustic impedance is presented by the 

 air in contact with a vibrating telephone diaphragm, and measure- 

 ments made of the impedance offered by the air to a telephone 

 diaphragm have shown the need for a definition of this quantity 

 and of its unit. 



If the tube, on its two sides 1 and 2, is perfectly symmetrical with 

 respect to the disk AB, the two impedances Zi and Z2 must be equal 

 at least in size if not in slope; but whether equal or not, their sum 

 will be equal to a total vector impedance z acoustic absohms Z . 



The impedance z will manifestly increase as the area of the tube is 

 increased. Neglecting friction at the walls of the tube, the increase 

 will be in direct proportion to the area. If S be the area of the tube 

 in sq. cm., then the acoustic impedance per sq. cm., or acousiic im- 



1 The kine is a name of the C. G. S. unit of linear velocity, or cm. /sec, as 

 proposed at a B. A. meeting. 



