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DIRECT RADIATOR LOUD SPEAKERS 



bination with the existing vacuum tube driving system, is another factor 

 which reduces the response of a dynamic loud speaker at the higher fre- 

 quencies. The impedance characteristics of the vacuum tube power ampli- 

 fiers are generally designed so that the voltage across the loud speaker, for 

 constant voltage applied to the input of the power stage, is independent of 

 the frequency. Therefore, the current in the voice coil decreases with 

 frequency as the impedance increases with frequency. The impedance 

 frequency characteristics of several voice coils are shown in Fig. 7.4. In 



FREQUENCY IN CYCLES PER SECOND 



Fig. 7.4. The impedance frequency characteristics of 1^-inch diameter voice coils of 140, 

 70 and 18 turns and all having 10 ohms d-c resistance. 



the case of a large heavy voice coil the rapid increase of the impedance at 

 the higher frequencies causes a corresponding reduction in the driving 

 force. To maintain the driving force at the higher frequencies requires a 

 relatively low ratio of the inductive reactance to the resistance which for a 

 constant value of the resistance is equivalent to a reduction in the mass of 

 the voice coil. By sectionalizing the voice coil both mechanically and 

 electrically it is possible to reduce the mass and the electrical impedance 

 at the higher frequencies. The constants of the single coil, single cone 

 conventional loud speaker may be chosen to yield uniform response and 

 adequate power handling capacity over the range from 70 to 5000 cycles. 

 This frequency band is commonly used in moderately priced radio receivers 

 in use to-day. Considerable improvement in the reproduction is ob- 

 tained by the use of a wider frequency range. For example, wide range, 

 radio receiver loud speakers reproduce from 50 to 8000 cycles, broadcast 

 station monitoring loud speakers reproduce from 45 to 12,000 cycles, ultra 

 high frequency broadcast and television-sound loud speakers reproduce 

 from 40 to 15,000 cycles. 



