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ACOUSTIC INSTRUMENTATION AS A TOOL IN OCEANOGRAPHY 



J. B. Hersey 



INTRODUCTION 



Acoustic waves travel readily in sea water whereas all electromagnetic 

 waves are rapidly attenuated. Consequently, except for rather short range ex- 

 amination, as is now achieved in underwater photography and television the only 

 means presently developed, or likely to be available, for perceiving objects or 

 events at considerable distances underwater is by sound. The sound perceived 

 may be the natural sound generated by the object or event, as a sound-making 

 fish or a submarine landslide, or it may be an echo from the object under study. 

 The techniques described above have to do with sensing the presence of objects 

 in the water or with analyzing the characteristics of things or events by the 

 sound generated or re-radiated by them. The physical properties and structure 

 of the water itself can be studied by analyzing the alteration of the intensity and 

 character of the sound received from a known source as the relative position bet- 

 ween source and receiver is varied. In addition there are various laboratory 

 techniques for studying the physical properties of sea water. In some instances 

 these are capable of working at sea, thus providing the oceanographer with a 

 potentially valuable tool. This area of the art of underwater sound is the least 

 developed outside of the laboratory. 



The Nature of Acoustic Data - Virtually all acoustic observations underwater 

 are made with the aid of a transducer which transforms a part of the energy 

 from the acoustic wave to electrical energy in a circuit which, in turn, feeds 

 some sort of electronic device. This drives some sort of presentation device 

 which may be any of several types of recorder, a momentary visual display on 

 an oscilloscope, or earphones or loud speaker. In common with most problems 

 of instrumentation, systems for recording or analyzing acoustical data have a 

 more or less vaguely limited range of usefulness for various problems. For 

 example, the directional discrimination of a hydrophone varies with the frequen- 

 cy to such a degree that it is totally satisfactory for a given prescribed job only 

 over a rather limited part of the spectrum. On the other hand, the same hydro- 

 phone is appreciably directional over a nnuch broader spectral range and may 

 well be usable for certain other problems so long as this variation is tolerable. 

 Again, it is usually not practical to build an amplifier that has a constant gain 

 over the whole range of frequencies of interest. Certain departures from such 

 a "flat" response may be acceptable and often are desirable for some problems 

 but not for others. All types of recorders distort the electrical signal fed to 

 them by the amplifier. The nature and degree of the distortion must be known 

 in greater or less detail for proper interpretation of the results. The list of 

 these properties of acoustical instruments could be extended considerably. 

 However, it is reasonable to expect that for a given problem instruments can be 

 designed to serve the needs of the problem within limits that can be known. In 



