ACOUSTIC IMPEDANCE AND ITS MEASUREMENT. 



By a. E. Kennelly and K. Kurokawa. 



Received, June 15, 1920. 



Presented, May 12, 1920. 



It is the object of this paper to describe some measurements of 

 acoustic impedance and to offer a technique for such measurements in 

 the laboratory. 



Definitions of Acoustic Impedance and Acoustic Impedance Density: 

 If a small thin rigid disk, or circular diaphragm AB, Figure 1, fitting 

 without edge friction, in a smooth cylindrical tube TT, T'T', of S sq. 

 cm. internal area, be actuated, through the central rod, by a simple 

 alternating mechanical force, or vibromotive force (vmf.) F root- 

 mean-square dynes, directed along the axis of the tube; then the 

 effect of the force, neglecting the friction of the air on the walls of the 

 tube, will be to set up an alternating velocity .r cm. per second, in the 



Figure 1. Diagram of Longitudinal Section of an Acoustic Tube with 

 Vibrating Disk. 



air or other fluid contained in the tube. The maximum cyclic ampli- 

 tude of vibration ± Xm cm. about the quiescent position, is supposed 

 to be very small. The fluid filling the tube on each side of the disk 

 may be compressible, as in the ordinary case of an air tube. The 

 maximum cyclic velocity of the fluid Xm in contact with the disk, must 

 be the same as the maximum cyclic velocity of the disk itself. In the 

 case of a rigid disk, this velocity would be the same at all parts of its 

 surface. The magnitude and phase of this velocity for a given vmf. 

 will depend upon the impedance to vibration of the column of fluid on 

 each surface. This impedance to vibration velocity, of the fluid in a 

 tube at a driving section, may be called the acoustic ivipedance of the 

 fluid, at each of the two surfaces of the disk. 



