28 SHOR [chap. 2 



kept well below this level. Direct waves through the water from nearby shots 

 (and even from distant shots in areas of good sound propagation conditions) 

 fre(piently create hydro] )hone output voltage in excess of 1 V in the higher 

 frequencies ; it is, therefore, desirable to have a dynamic range in the pre- 

 am})lifier of 100 dB or more with as nearly linear response as possible. 



At the shipboard end of the cables the signal is fed to an input panel which 

 uses blocking condensers to eliminate the d.c. component from the pre-am])litier 

 signal and ])rovides test })oints for checking pre-amplifier voltage and for 

 inserting calibration signals. The signals from all hydrophones are then distri- 

 buted through step attenuators to the ami)litiers. V^arious versions of the 

 amplifying system have used cross-over networks dividing at 20 c/s, paralleled 

 am})lifiers with low-pass filters for the low-frequency trace, and narrow-band 

 amplitiers in parallel. The present system used at SIO feeds the signals through 

 a cross-over network which sends signals below 20 c/s to the "LF" amplifiers 

 in the block diagram (Fig. 5) and signals above 20 c/s to the "IF" amplifiers. 

 Signals near 20 c/s are strongly suppressed. The choice of 20 c/s for the cross- 

 over stems from two causes : first, the energy in the refracted arrivals lies in 

 greater part below this frequency, and secondly, this is the mechanical fre- 

 quency of most of the water-borne sound created by rotating machinery on the 

 shi})s that have been used. If ships with machinery of other speeds are used, 

 the cross-over frequency must be changed to keep the maximum attenuation 

 at the noise frequency. Signals are separated before amplification to reduce 

 the range of linear amplification needed to prevent intermodulation, which, if 

 present, w^ll cause the direct water wave (strongest in high frequencies) to 

 produce a spurious signal on the low-frequency traces, masking the bottom 

 refracted arrival on shots at certain ranges. The low-frequency signals are 

 given up to 40 dB of amplification, and then two outputs at levels 26 dB apart 

 are carried out for recording. The amplifiers are battery powered to reduce 

 hum ; great care must be taken to avoid ground loops and nearby sources of 

 electrical noise. High dynamic range, linearity, freedom from blocking after 

 overload, low internal noise, stability, good low-frequency response and 

 reproducibility of gain settings are features of these amplifiers. IF amplifiers 

 are in most respects identical with those used for the LF channels, except for 

 the addition of rejection filters for 60 c/s electrical pick-up. The outputs of 

 the IF's go both to the recorders at two amplitude levels and to high-frequency 

 (HF) amplifiers that pass only frequencies above 600 c/s. The HF output is 

 then rectified and recorded at two levels. 



Outputs of the amplifiers are recorded with 230 c/s galvanometers on a 17- 

 channel photographic oscillograph along with the radio-transmitted signal 

 from the shooting shij) and second marks from a chronometer. A secondary 

 time base is provided by time lines controlled by a 100 c/s fork. In addition, 

 a monitor record is taken with a pen-and-ink recorder using two pens operated 

 by galvanometers and one operated by a relay. This requires the use of a set 

 of auxiliary ])ower amplifiers. With ]iro])er switching the operator can record 

 on this the chronometer ticks, the firing mark from the radio, the low-frequency 



