EQUIPMENT AND FIELD PROCEDURES 



277 



Figure 5. Oscillograph record showing negligible transients produced by receiver amplifier gain changes in equipment C 

 (input signal maintained at steady value). 



accurate determination of average reverberation 

 levels as a function of time. 



Equipment E 



This equipment used by WHOI is described in 

 reference 9. This reference gives only the details of 

 the recording system; presumably in most details the 

 system does not differ essentially from UCDWR 

 systems A to D. One major difference is the introduc- 

 tion of a logarithmic amplifier which makes it possible 

 to record directly in decibels. In the original version 

 of this equipment ' a seismographic galvanometer 

 \vith a linear response up to 70 c was the recording 

 device. The deflections of this galvanometer were 

 recorded on photosensitive paper. More recently, the 

 galvanometer has been replaced by a recording ar- 

 rangement consisting of a cathode-ray oscillograph 

 and a camera. This system has a linear response up 

 to about 1,000 c; at higher frequencies the overall 

 response, through the logarithmic amplifier, falls off 

 rapidly. 



With this WHOI equipment, the averaging pro- 

 cedure is simplified by superposing on the same film 

 the records from a number of successive pulses. When 

 the reverberation from a number of pulses is re- 

 corded on one film in this manner, the average rever- 

 beration curve can be drawn by eye through the 

 densest portions of the trace. Since the film records 

 the reverberation in decibels, the resulting plot is the 

 desired curve of average reverberation level versus 

 time. A sample record, showing the superposition of 

 reverberation from 12 successive pulses recorded with 

 the seismographic galvanometer, is shown in Figure 

 6. In this illustration, time increases from right to 

 left; the particular features of the galvanometer used 

 by WHOI made it more convenient to present the 

 data in this way. 



Table 1 summarizes some of the important infor- 

 mation concerning the equipment used in various 

 reverberation studies at UCDWR. Most of the items 

 in the table are self-explanatory. The letters A to D 



refer to the electronic setups described in equipment 

 designations; and the figures in parentheses next to 

 the transducer designations in the column labeled 

 "Transducer references" tell where detailed descrip- 

 tions of these transducers can be found in the 

 bibliography. The column labeled "Reference" tells 

 where the results obtained with the indicated equip- 

 ment are discussed. 



Table I. 

 ments. 



Equipment used in reverberation measure- 



13.1.2 Calibration of Proj ector 

 and Receiver 



In order to properly interpret the recorded values 

 of reverberation, and to convert these recorded ampli- 

 tudes to reverberation levels, it is necessary to know 

 the values of the projector output F and the receiver 

 sensitivity F'. These quantities, which occur in the 

 theoretical formulas of Chapter 2, depend not only 

 on the type of transducer, but also on the electronic 

 equipment used. The procedures used in the determi- 

 nation of F and F' in the field are called "calibra- 

 tion procedures." 



The projector is calibrated by measuring the pro- 

 jector output with a standard hydrophone whose 

 response is stable. If an auxiliary projector with 

 stable power output is available, F' can be deter- 

 mined by measuring the output of the receiver when 

 exposed to a pulse from the standard projector. If the 

 output of the auxihary projector is not accurately 



