RECOMMENDATIONS FOR IMPROVEMENT 



135 



Figure 79. Small triangular float which can he anchored 

 anywhere in the lake. Note the diver's helmet which is 

 useful for inspecting underwater equipment. 



frequency for each component may be neglected in 

 the calculations. Although the present corrections lie 

 within ±0.1 db, further improvement along these 

 lines is possible. 



General Improvements— Mechanical 



Since the purpose of the mechanical systems in any 

 acoustical calibration is to handle, hold, or orient 

 the devices, any improvement which will reduce the 

 time involved in rigging and measurement and still 

 provide a minimum of interference in the sound field, 

 is recommended. To minimize the number of systems 

 required, each should be designed to handle as many 

 different devices as possible. 



Electromechanical Equalizer 



Transducer calibrations would be greatly facili- 

 tated if only there were projectors which give a con- 



SIGNAL 



AMPLIFIER 



RECORDER 



AND 

 EQUALIZING 

 SYSTEM 

 I SYNCHRONIZED 



RECORDING 

 PEN 



EQUALIZING 

 CHART 



Figure 80. Schematic diagram of electromechanical 

 equalizer. 



stant sound field over the whole frequency range, or 

 hydrophones which had a uniform response for a 

 constant field at all frequencies, or both. Either in- 

 strument would make possible the direct calibration 

 ol the other. Though no such ideal instruments are 

 available, if is possible to approach the same result by 

 proper control of the amplifier gain, or, what is more 

 convenient, by control of the attenuation preceding 

 a constant amplifier gain. The method is indicated 

 in Figure 80. 



Hydrophones are available which have been cali- 

 brated by reciprocity or other means. Using such an 

 instrument with a fixed attenuation and amplifier 

 gain, a recording is made as the projector sweeps over 

 the range of frequencies. From the calibration curve 

 of the hydrophone, a curve may be plotted which will 

 give the strength of the sound field produced by the 

 projector at each frequency. If a straight line is drawn 

 parallel to the recorder axis and at some desired 

 sound level, the difference between it and the projec- 

 tor curve will be the number of decibels by which 

 the actual field differs from a constant one. If some 

 device connected to the attenuator can be made to 

 follow the curve and thus vary the output level so that 

 it will be proportional to the deviation of the curve 

 from a straight line, the output of the amplifier will 

 be at a constant level. In other words, the record 

 would be that of a flat hydrophone in a constant 

 sound field. If now the coupling loss of an unknown 

 hydrophone is determined and the corrections lot 

 this loss added to the original curve, the resulting 

 control curve will cause the attenuator to correct for 

 both the variations in the projector output and the 

 coupling loss. If this hydrophone is placed in the 

 field of the projector and its output is fed through 

 the attenuator with its gain controlled by the curve, 

 the hydrophone signal will have added, at every fre- 

 quency, the number of db necessary to make its out- 

 put what it would have been if the projector output 

 had been constant. In other words, the record is the 

 response of the unknown hydrophone in a constant 

 sound field. The same technique will give a correc- 

 tion curve for a hydrophone and allow the direct 

 calibration of a projector. Calibration by this meth- 

 od eliminates the time and errors involved in the 

 point by point computations required at present. 

 The data are in such a form that the calibration can 

 be reproduced directly by photographing the rec- 

 orded chart. 



Such a system is under consideration by USRL. 



