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BELL SYSTEM TECHNICAL JOURNAL 



the acoustic reactance of the sHt multipHed by the square of the 

 effective area of the diaphragm. These quantities are represented 

 by ri and j m\w in Hg. 2. If the sht, 0, were closed the stiffness im- 



posed by the air chamber on the diaphragm would be equal to yr—, 



where A is the effective area of the diaphragm, V the volume of air in 

 the enclosure and 7 the ratio of specific heats of air. This is the stiff- 

 ness represented by 5 1 in Fig. 2. 



In adjusting the width of the slit to the desired value its resistance 

 was measured experimentally. For this purpose a steady stream of 

 air at low velocity was passed in series through the slit and a capillary 

 tube. The pressure drop through the tube and that through the 

 resistance was then measured with a manometer. The ratio of these 

 values is under this condition equal to the ratio of the resistance of the 

 tube to that of the slit. The resistance of the tube had previously 

 been determined as a function of the pressure difference between its 

 two ends when air was passed through it at a known steady rate. 

 The apparatus is diagrammatically shown in Fig. 5. 



CAPILLARY 

 TUBE 



SLOW, STEAD' 

 VELOCITY 

 OF AIR 





P2 



R2=Ri X pj- 



ACOUSTIC RESISTANCE OF THE ANNULAR 

 SLI T 



R2 



Rl = ACOUSTIC RESISTANCE OF CAPILLARY 

 TUBE 



Pp = PRESSURE DIFFERENCE ON THE TWO SIDES 

 OF THE ANNULAR RESISTANCE SLIT 



P| = PRESSURE DIFFERENCE AT ENDS OF 

 CAPILLARY TUBE 



Fig. 5 — Method used to measure acoustic resistance. 



The response-frequency characteristic of the receiver was determined 

 experimentally. For these measurements it was placed over a cali- 

 brated condenser microphone so as to form a 15 c.c. enclosure between 

 the receiver and the microphone diaphragms. This space was filled 

 with hydrogen to avoid acoustic resonance at the higher frequencies. 

 While current from a vacuum tube oscillator was passed through the 

 receiver coil, the voltage generated by the microphone, as well as the 

 receiver current, was measured. From these values, the calibration 

 curve of the microphone and the volume of the enclosure, the ampli- 

 tude of the receiver diaphragm per unit current is readily determined. 

 Values so obtained, expressed in db, are plotted in Fig. 6. In the 



