A SOUND PROJECTOR SYSTEM 207 



heated by low voltage alternating current. Two similar tubes in this 

 unit operate as a full wave rectifier and supply rectified alternating 

 current for the plate circuits of the amplifier tubes of both the first 

 and second units. The third unit has a single stage of high power 

 push-pull amplifier tubes and push-pull rectifier tubes and also operates 

 entirely on alternating current. 



These three types are capable of arrangement into combinations to 

 meet the particular need. For small theaters only No. 1 and No. 2 

 are required. In the larger houses the high power unit No. 3 is added, 

 while to meet exceptional conditions two or more of the high power 

 amplifiers may be operated in parallel from the output of No. 2. 



Following the amplifier there is an output control panel. This 

 consists of an auto-transformer having a large number of taps, the 

 taps being multipled to a number of dial switches, to which the sound 

 projectors or loud speakers are connected. By means of this panel, 

 it is possible to match the impedance of the amplifier output to the 

 desired number of horns in order to obtain the most efificient use of 

 the power available and also to adjust the relative volume of the 

 individual horns. 



The ordinary theater installation employs four horns, two mounted 

 at the line of the stage and pointed upward toward the balconies and 

 two mounted at the upper edge or above the screen and pointed 

 downward. This combination has been found to give good distri- 

 bution throughout the house. 



The loud speaker unit used with the horns in theater equipments 

 is essentially that recently described by Messrs. Wente and Thuras.- 

 As brought out in this article, this unit shows extremely high efficiency; 

 about 30 per cent of the electrical power supplied is radiated in the 

 form of sound. This is important since the higher the loud speaker 

 efficiency, the smaller the power capacity of the amplifier needed in 

 the system. The frequency-response characteristic of a typical re- 

 ceiver and horn is given in Fig. 10. An individual horn may be 

 equipped with two, four or nine loud speakers by using the throats 

 shown in Fig. 11. The power capacity for continued safe operation 

 of the horn with one, four and nine throats is approximately 5, 20 

 and 45 watts, respectively (electrical input). The number of horns 

 used is dependent upon the particular installation and is related to 

 the directive characteristic of the horn. If it is necessary to disperse 

 the sound over a large angle, more horns are needed than when it is 

 desired to concentrate over a comparatively small angle. This 

 directive characteristic of the horn is important in talking motion 

 pictures as it is responsible for the illusion of the sound coming directly 



