96 



USRL TEST STATIONS 



SYSTEM I 



TO 



TRANSMITTING 

 TERMINALS 



TO 30A 

 TRANSMISSION 

 MEASURING SET 



OR 

 RECORDING 

 SYSTEM 



PROJECTOR 



Figure 34. Typical circuit arrangement for observations of projector acoustic output versus electric power input. 



by superposition. The rate of rotation is selected on 

 the basis of instrument size, driving torque required, 

 and pattern complexity. For example, small devices 

 may be rotated at the maximum rate, provided the 

 rate of signal variations does not exceed the response 

 rate of the recorder. 



After the preliminary observations and adjust- 

 ments, the pattern is recorded with the turntable 

 moving through a complete revolution. Overall sys- 

 tem and transducer stability may be checked by not- 

 ing the recorded trace at the point of overlap. 



For all critical test conditions, the pattern should 

 be repeated at a second distance. If the pattern 

 changes radically with distance, this usually indicates 

 too small a test distance (see Chapter 5) or the pres- 

 ence of reflections. In the former, the distance should 

 be increased, while in the latter it should be de- 

 creased, the frequency slightly displaced, or other 

 means employed to reduce reflections. 



In general, sharply resonant transducers are in- 



vestigated for pattern characteristics at the frequency 

 of resonance, at frequencies slightly above and below 

 this point, and occasionally at the frequencies of sec- 

 ondary and tertiary resonance. 



Design symmetries or asymmetries which control 

 the beam pattern are investigated by making records 

 about several axes of rotation. The positioning of a 

 transducer for an investigation of design symmetry is 

 illustrated in the sample circuit sheet shown in Fig- 

 ure 31. 



Split transducers, designed for bearing deviation 

 indicator [BDI] operation, may be studied for phase 

 shift and symmetry by means of a special circuit de- 

 signed by the Harvard Underwater Sound Labora- 

 tory. 51 This circuit provides for the halves of the 

 transducer to be in parallel but the connections to 

 one may be reversed so that its output may be in the 

 same phase or opposite to the other. Lag lines giving 

 various phase shifts are readily available and when 

 plugged into the circuit are connected to a three- 



