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graphic science. Passive listening gear is the simplest sort of acoustic system 

 with which to observe. However, if it is to be used for any purpose save that of 

 listening to and evaluating qualitatively the sounds of nature it may be demand- 

 ing and complex. In the past ambient noise has been studied for the most part 

 with non-directional hydrophones. This study has now reached the state where 

 knowledge will be most readily advanced with directional systems. I shall dis- 

 cuss only the non-directional systems in the hope that the other acousticians will 

 be prepared to describe the sort of directional properties that can be achieved 

 and the complexity of instrumentation required for the various frequency ranges. 



Observations are frequently taken from shore, but usually some sort of 

 boat or ship is required to go where the problem is, or perhaps mierely to get 

 away from the excessive self noise of the shore. The first requirement on the 

 boat or ship is that it be capable of sitting quietly in the water. No refrigera- 

 tors, no galley range, no main generator can usually be tolerated in operation 

 while observations are in progress. Electric power for the electronic gear is 

 preferably obtained from storage batteries which power a small motor genera- 

 tor set which is very carefully shock mounted to isolate its vibrations from the 

 ship. In short the ship is de-commissioned except for this single power system. 

 When operating from small craft very light convertors or vibropack power sup- 

 plies similar to those used for automobile radios have been used. A small row- 

 iDoat has been used successfully to carry a complete amplifying and recording 

 systenn powered by a 6 volt storage battery, B batteries and a vibropack. 



For non-directional listening and also the simplest directional systems 

 the principal instruments are the hydrophone, an amplifier, earphones and usual- 

 ly a recorder. There is available an array of essentially non-directional hydro- 

 phones sensitive to the audible range of humans. In this frequency range the 

 workhorse of our group and many others is the Brush Development Company's 

 AX58 series. A higher quality, but somewhat more fragile instrunlent is the 

 Bell Telephone Laboratory's 5E. Both are World War II designs.* The AX58 

 types generate about one ten thousandth of a volt when in a plane sound wave 

 whose pressure is 1 dyne/cm . The 5E has about one third this response. 

 Their response to greater or lesser pressures is approximately linear over a 

 considerable range. They are both usable with suitable modification of their 

 pre-amplifiers from well below 10 cycles per second to about 10 kilocycles. 

 Above this frequency their directional properties make them unsuitable for most 

 quantitative work. At higher frequencies various hydrophones have been de- 

 signed for non-directional reception. I am not thoroughly familiar with any of 

 them, but understand that the Brush Development Company makes a useful in- 

 strument. One word of warning about high frequency hydrophones as a class is 

 worth observing. Comparatively few of them are non-directional except in one 

 plane, and the designer of the particular observation must bear this carefully in 

 mind. 



If adequately quite pre-amplifiers and amplifiers are used with either 

 the AX 58 or the 5E the self noise of the listening system can be made somewhat 

 quieter than the quietest ambient noise thus far observed at sea between 30 cps 

 and 20 kc. So far as I am aware the same can be said for systems designed for 

 use above 20 kc. However, the use of small assemblages of sensitive elements 

 in the hydrophone to avoid directionality in this range makes for a less sensitive 

 hydrophone, which, in turn, means a relatively higher self noise except where 



* - Several hydrophones are being produced by various companies. These 

 are just beginning to be used by The Woods Hole Oceanographic Institution but at 

 this writing we can give no evaluation of them for underwater acoustics as ap- 

 plied to oceanographic research. 



