518 BELL SYSTEM TECHNICAL JOURNAL 



flat surfaces are all above /<;. To insure that this central stiffened por- 

 tion should vibrate with approximate plunger action, which is more 

 efficient than diaphragm action, it is driven at six points near its 

 periphery. 



Reference to Figs. 15 and 16 and Equation (14) shows that the com- 

 pliance of the air chamber Ct, of the spider legs Cs and shunt tip of the 

 needle arm Cs are determined. Also the mass of the spider m2 and the 

 effective mass of the needle arm Wi, as viewed at the point where it is 

 attached to the spider, are determined. 



The impedance looking into the system from the record is deter- 

 mined by the rate at which it is necessary to radiate energy in order 

 that the reproduction may be loud enough. The power taken from the 

 record is approximately v"^ Zo since Zo is a resistance over most of the 

 band. Experiment has shown this value of Zo to be approximately 4500 

 mechanical ohms. 



But substituting in Equation (13) the value of Wg, and from Equa- 

 tion (14) the value of Cs, we find that the impedance is only 2920 me- 

 chanical ohms. It is, therefore, necessary to use a transformer whose 



4500 

 impedance ratio is ;^(^- From this and a knowledge of filter structures 



the needle-point compliance can be determined. The value obtained is 

 easily realized with commercial types of needle. 



It will be noted that the record is shown in Fig. 16 as a constant cur- 

 rent generator, i. e., a generajtor whose impedance appears high as 

 viewed from the needle point. That this is necessary is obvious when 

 it is remembered that, if the impedance looking back into the record 

 were to equal the impedance of the filter system, the walls of the record 

 would have to yield an amount comparable with one-half the amplitude 

 of the lateral cut. This would cause a breakdown of the record material 

 with consequent damage. 



The design of the system is, therefore, complete except for the resist- 

 ance termination which is supplied by the horn for all frequencies 

 above its low frequency cut-off. The characteristics of the horn will be 

 dealt with later. The resistance within the band looking in at the small 

 end of the horn is G A2 where G equals the mechanical ohms per square 

 centimeter of an infinite cylindrical tube of the same area, and A2 

 equals the area in square centimeters of the small end of the horn. 



Let yl 1 = the effective plunger area of the diaphragm (as previously 

 mentioned this is 13 sq. cm.). The impedance looking back at the 

 diaphragm is 



Zo=Trfc m3 = 2920 mechanical ohms 



