102 



any event acoustical instruments in general have rather complicated perform- 

 ance characteristics, and must be understood to a considerable degree by any- 

 one who assays to interpret results from them. 



Even where the design of an electronic system is adequate a continuing 

 and vexing problem is assuring stable, reliable performance. This is well 

 known to many more people than have any real understanding of electronics. As 

 a result we are all properly suspicious of any information that has passed 

 through a vacuum tube circuit. This sort of suspicion runs riot on occasion, 

 and can be quite dangerous scientifically, causing the investigator to ignore 

 significant data on the plea that it was "some sort of electronic difficulty". This 

 is an understandable attitude, but nevertheless, it is usually the result of not 

 understanding one's tools. However, if an electronic system has been properly 

 designed and tested for a specific job and is maintained as is required by the 

 nature of it, it can be quite free of spirits and spooks. As a final note of sus- 

 picion, it is usually well to include circuits in the system that can test its re- 

 liability in the field. 



In what follows acoustical instruments will be discussed in groupings 

 according to the general type of problem in which they are commonly used. I 

 shall try to describe in a general way the state of the art in each area, indicating 

 the sort of instruments that are readily available to oceanographic research. 

 These general groupings are: passive listening, acoustic ranging, sound trans- 

 mission studies and echo-sounding and echo-ranging. I have had little acquaint- 

 ance with acoustic laboratory techniques as opposed to broader scale field ob- 

 servations. Consequently, I hope this interesting field will be discussed by the 

 other physicists later. 



PASSIVE LISTENING INSTRUMENTATION 



This technique is employed whenever the primary object of study is eith- 

 er man-made or natural sound, rather than the transmission characteristics of 

 the medium. Broadly speaking this is the study of ambient noise; that is to 

 say, the sounds heard at any given location caused by human activity or other 

 natural agents. It may be used to detect the presence of and even locate ships, 

 noise-making animals, submarine landslides, distant storms, approaching rain 

 squalls, and seismic activity within the Earth, thus providing a basic observa- 

 tional tool in these several parts of oceanographic science. The quantitative 

 study of ambient noise intensities caused by breaking waves, rain on the water, 

 and other sounds caused by meteorological effects provides data which may well 

 contribute to the study of energy interchange between the atmosphere and the 

 ocean. 



Until now most of the effort in this technique has gone into naval prob- 

 lems such as the detection of enemy shipping. The ambient noise of the sea 

 forms the noise, background which limits naval detection just as the self noise 

 limits an amplifier, and consequently some attention has been devoted to the 

 spectrum of ambient noise in a variety of locations typical for naval operations. 

 A by-product of this work during the last war was a study of the sound-making 

 habits of a small number of noise-making fish, crustaceans and cetaceans in 

 their natural habitat. These observations have provided a jumping off place for 

 scientific work. We know the order of magnitude of noise caused by moderate- 

 ly high winds, and we have a good impression of how quiet the sea may be on a 

 very calmi day, but this knowledge is confined to the portion of the spectrum bet- 

 ween 100 cps and 10 kc. We also know something about the characteristic 

 sounds of some noise-making sea animals. Beyond this we know very little. 

 This is the case largely because very few people are using this tool in oceano- 



