A HIGH EFFICIENCY iRECEIVER 



147 



power losses within the receiver it was deemed desirable to measure 

 the efficiency more directly, viz., to measure the actual sound power 

 generated for a given power input. 



O 



FREQUENCY- P.P.5 



Fig. 7 — Efficiency of the receiver. 



The power output may be determined directly by measuring the 

 acoustic pressure in the tube at the sending end. In order to measure 

 this pressure an annular slit was provided on the side of the tube a few- 

 inches from the receiver as shown in Fig. 8. This annular slit had a 



RECEIVER 



RESISTANCE 

 UNIT 



Fig. 8- 



^TPANSMITTER 

 DIAPHRAGM 



-Arrangement of apparatus for measuring efficiency. 



diameter of a quarter of an inch, a width of 0.003 inch and a length of 

 0.040 inch. A slit of these dimensions has an acoustic impedance 

 about fifteen times as great as the tube, so that it has a negligible effect 

 on the sound wave propagated along the tube. This slit led to a small 

 chamber over the face of the diaphragm of a condenser transmitter. 

 The condenser transmitter was connected to an amplifier and an a.c. 

 ammeter. This combination was previously calibrated, so that from 

 the meter readings the pressure over the slit could be determined. 



The input power was determined from the current input and the 

 resistance of the receiver. With this set-up we thus were able to 

 measure both the acoustic power transmitted along the tube, which 



