114 



I'SRL TEST STATIONS 



72*" 



BALANCED 



TRANSMISSION 



LINE 



TO CURRENT 



MEASURING 



SYSTEM 



Figure 47. Circuit schematic for current measurements 

 in unbalanced circuit. 



"■ — 9- 



72<" 

 BALANCED 

 TRANSMISSION 

 LINE 



R = 2" J, 



rwwr 1 



TO CURRENT 



MEASURING 



SYSTEM 



Figure 18. Circuit schematic for current measurements 

 in balanced circuit. 



transformer is removed, the circuit cannot be used 

 for a projector with one side grounded because the 

 current flowing through the stray capacity of the coil 

 and leads will be included in the reading. For the 

 same reason the circuit for the intermediate-frequen- 

 cy system shown in Figure 48 cannot be used. 



Calibration of Unknown Instruments. When a set 

 of calibrated standards is available, the calibration of 

 unknown units is fairly straightforward. A known 

 sound field is established between a standard projec- 

 tor and hydrophone. The latter is replaced by the 

 unknown, the response of which is found in the same 

 field. It is assumed that the unknown will satisfy the 

 acoustic restrictions of proper beaming, absence of 

 standing waves and reverberation, and the use of a 

 testing distance that will not require correction for 

 spherical waves. The sensitivity of the unknown is 

 determined by comparison with the response of the 

 standard, but all coupling losses are to be accounted 

 for. If the unknown is a projector, a standard hydro- 

 phone is used to measure the sound field it generates 

 for a given current or for given power. 



Occasionally a hydrophone is found with only one 

 high lead and the case grounded. A coupling loss 

 measurement may be made on this instrument by im- 

 mersing it in a glass container of water which insu- 

 lates it from ground. However, care must be taken 

 to avoid standing waves between the transducer head 

 and the walls of the container. 



Another case requiring special treatment is the 

 measurement of the current to a high-impedance 

 projector with unbalanced electrical connections. If 

 the water resistance is high enough, the method out- 

 lined previously will give fairly accurate results if the 

 case-to-system ground is removed. If the water resist- 



ance is not sufficiently high, recourse must be had to 

 measuring the impedance of the projector, driving 

 it from a known voltage and calculating the current. 

 If tlie impedance of the projector is high enough, the 

 transmission line can be terminated with a 72-ohm 

 resistor and the projector connected across it. The 

 voltage across the projector can be calculated then 

 from the current through the resistor. 



Greater stress has been placed upon measuring cur- 

 rent than upon calibrating for a given available 

 power. There are several reasons for this. The main 

 reason is that the transformers used in coupling pro- 

 jectors to the line do not act like ideal transformers 

 over this frequency range and hence cannot be repre- 

 sented by a voltage source in series with a resistance. 

 This immediately destroys the concept of available 

 power. Likewise, the power amplifier output does 

 not conform to such a representation and so will not 

 satisfy the conditions involved in the definition of 

 available power. The use of a resistance pad, which 

 would eliminate the impedance variations, would 

 increase tremendously the size of the power amplifier, 

 if the same power output were to be preserved. In 

 addition to this, the problem of providing pads for 

 various projector impedances for this frequency 

 range is tremendous. As a result, all calibrations of 

 projectors are at the moment made on a current basis. 



The actual recording of data for all such acoustic 

 tests follows quite exactly the procedure and printed 

 forms used on the lower frequency systems. 



Frequency Scaling 



The high-frequency system is admirably adapted 

 to perform tests on scale models, with the wave length 

 of the sound shortened on the same scale. Even meas- 



