LOUD SPEAKERS AND MICROPHONES 



273 



upon the value of Bh' where v Is the volume of the coil, but at the higher 

 frequencies the efficiency decreases with increasing mass of the coil. 

 It is advantageous, therefore, to keep v small and to make B as large 

 as is practically possible. Values were selected to give the receiver 

 an efficiency of 55 percent at the lower frequencies. For these con- 

 ditions the relative sound power output was computed by equation (2) 

 on the assumption that the receiver was connected to an amplifier 

 having an output impedance equal to 0.45 times that of the receiver 



-20 



500 



10,000 20,000 



Fig. 7 — Relative computed sound output of high frequency receiver. 



1000 

 FREQUENCY 



2000 5000 



IN CYCLES PER SECOND 



at the lower frequencies. Figure 7 shows the values so obtained. 

 Corresponding values obtained experimentally when the receiver was 

 connected to the horn previously described are shown in Figs. 8 and 9, 

 where the sizes of the rooms in which the values were obtained were, 

 respectively, 5000 and 100,000 cubic feet. Both of these curves differ 

 considerably from the computed curve, particularly as regards loss 

 at high frequencies. The curve of Fig. 8 shows less, and that of Fig. 9 

 more, loss at high frequencies. The computed curve, however, refers 

 to the total sound output, whereas the measured curves give average 

 values of sound intensity in a certain part of the room, values depend- 

 ent upon the acoustic characteristics of the room. 



The number of high frequency receivers that must be used for each 

 transmitting channel is governed largely by the amount of power that 

 the system is to deliver before harmonics of an objectionable intensity 



