For visual analysis, the data acquired on shipboard was later transcribed in 

 the laboratory from the magnetic tape to paper strip chart using a six-channel 

 recording pen galvanometer. This procedure is illustrated by the block dia- 

 gram in Figure C-2. The three hydrophone channels were transcribed from 

 magnetic tape Channels 1, 2, 3 to the paper along with the acoustical synchron- 

 ization and clock pulses . By proper selection of narrow -band filters and by re- 

 playing each channel on the magnetic tape three times, the signals received at 

 the different frequencies were separated and recorded on the paper independently. 

 The local attenuators were adjusted to maintain reasonable signal levels on the 

 paper, their settings being written on the galvanometer paper whenever a change 

 was made. 



The shipboard (Rexburg) calibrations for each channel appeared with the appro- 

 priate frequency as each channel was transcribed. Thus, it was possible to cal- 

 culate a recording sensitivity for each channel, at f , in the form x db re 1 volt 

 per mm pen deflection. Accordingly, with a knowledge of the hydrophone sensi- 

 tivity, an overall channel sensitivity, S(f ), can be derived in the form y db re 

 1 M-b per mm pen deflection (i.e., press/mm), where f = 3000 cps on Channel 1, 

 f = 1300 cps on Channel 2, and f = 700 cps on Channel 3 . 



To determine the sensitivities, S(f), on these channels at the other two frequen- 

 cies, the local oscillator and VTVM shown in Figure C-2 provided local calibra- 

 tions at all three frequencies on each channel in terms of the voltages, V(f) that 

 produced full scale pen deflection. The values of V(f) at all frequencies, obtained 

 during transcription, are given in Table C-2. It was assumed that the sensitivity 

 of each receiving channel in the shipboard data acquisition and recording system 

 was constant over the frequency range 700 to 3000 cps, and hence that changes 

 in S(f) with frequency in any channel occurred during the transcription process . 

 Accordingly with the receiving hydrophone sensitivities, H(f), known for all chan- 

 nels and frequencies, and, using the V(f) to take account of the variation of trans- 

 cription amplifier gain with frequency, it was possible to calculate the channel 

 sensitivities S(f) for all frequencies . 



If S(f) is the channel sensitivity at frequency f in db re 1 nbar per mm pen 

 deflection 



S(f Q ) is the observed value of S(f) where f Q is the frequency of the Rexburg 

 calibration, i.e., 



f Q is 3000 cps for Channel 1 



1300 cps for Channel 2 



700 cps for Channel 3 



H(f) is the hydrophone sensitivity at frequency f in db re 1 volt per Mbar 



V(f) is the local calibration voltage giving full scale pen deflection at fre- 

 quency f and at a standard local attenuator setting 



C-4 



2rtlmr 2B.HittIcJnr. 



