INTRODUCTION 



through them, and reduce the maximum range on a 

 deep submarine to much less than the maximum 

 range on the same submarine at periscope depth. The 

 maximum range at which two surface ships can ob- 

 tain echoes from each other gives, by itself, no infor- 

 mation on the maximum range that can be expected 

 on a deep submarine. Thus, use of the bathythermo- 

 graph is required to estimate the approximate maxi- 

 mum range obtainable on a submarine at evasive 

 depths. Such an estimate is useful not only in the 

 choice of spacing between antisubmarine vessels but 

 also in evaluating the desirability of detaching escort 

 vessels from a convoy to hunt a submarine reported 

 sighted some distance away. When sound conditions 

 are good, detaching antisubmarine vessels is less likely 

 to endanger the convoy and more likely to sink the 

 submarine than when sound conditions are bad. 



Information on sound transmission conditions is 

 also usefvd in the choice of submarine tactics. A sub- 

 mariner is free to choose his depth of operation, and 

 one of the factors influencing this choice is the maxi- 

 mum range at which he is likely to be detected at 

 each depth. In any case, the behavior of the sub- 

 marine may be influenced by knowledge of the maxi- 

 mum range at which detection may be expected. At 

 times, transmission conditions are so severe that the 

 submarine cannot be detected even at 500 yd; such 

 conditions, if they can be readily and reliably identi- 

 fied, provide opportunity for unusually aggressive 

 action. 



1.2 



NATURE OF SOUND 



Historically, the various types of physical phe- 

 nomena were first defined in terms of the human 

 senses. Physics was di\'ided into the fields of (I) me- 

 chanics (dealing with touch and displacements ef- 

 fected by human muscle power), (2) light (dealing 

 with the perception of objects by the eye), (3) sound 

 (pertaining to hearing), (4) heat (dealing with the 

 .sensations of heat and cold), and other similar fields. 

 Gradually, as the causes of the nerve stimuli became 

 understood, the subject matter of physics was re- 

 grouped; classification in accordance with physiologi- 

 cal perception was gradually replaced by classifica- 

 tion according to the physical nature of the phenom- 

 ena studied. Thus, optics became more and more a 

 subdivision of the theory of electricity and magnet- 

 ism, while heat and sound came to be treated as sub- 

 divisions of mechanics. The theory of heat is con- 

 cerned with random motions of many particles. In 



contrast, sound is concerned with the formation and 

 propagation of \abrations, primarily in a fluid,'' at 

 frequencies both within and above the range of audi- 

 bility. This definition is purely arbitrary, dictated by 

 practical considerations, and may be ambiguous 

 under certain circumstances. Nevertheless, it is gen- 

 erally accepted. 



The physics of sound is usually called acoustics. 

 Although a major part of the work in acoustics deals 

 with sound perceptible by the human ear (the acous- 

 tics of rooms, the physiology of sound, and similar 

 subjects), inaudible sound, consisting of mechanical 

 vibrations above the range of frequencies perceived 

 by the ear, has come to play an important role in 

 subsurface warfare. In this volume on the proper- 

 ties of sound in the ocean, more than half of the dis- 

 cussion will be devoted to the propagation of super- 

 sonic sound, that is, sound at frequencies well above 

 those which can be heard. 



1.2 



1 Sound as Mechanical Energy 



It must be understood that sound energy is a form 

 of mechanical energy. The particles of a fluid in 

 which sound is traveUng are set in motion and tem- 

 porary stresses are produced which increase and de- 

 crease during each vibration. The motion of the indi- 

 vidual particles gives the fluid kinetic energy while 

 the stresses induce potential energy. In acoustics, the 

 sum of these two kinds of energy is called sound 

 energy or acoustic energy. It is not always easy to 

 separate the acoustic energy from other forms of 

 mechanical energy possessed by the fluid. 



A fluid obtains acoustic energy by some kind of 

 energy transformation. As an illustration, consider a 

 tuning fork in air. When this tuning fork is struck 

 with a rubber hammer, its two prongs are set in 

 rhythmic vibratory motion. The vibrating prongs of 

 the tuning fork produce compressions and rarefac- 

 tions in the surrounding air by pushing the adjacent 

 air mass away and then permitting it to rush back. 

 These alternating compressions and rarefactions are 

 propagated through the air and may be detected as 

 sound by a suitable instrument, such as the human 

 ear or a microphone. The original source of energy 

 was the rubber hammer, which had kinetic energy of 

 translation. This energy was transformed, by means 

 of a collision, to vibratory energy in the tuning fork. 



'' The term fluid, as used in physios and chemistry, means 

 any Uquid or gaseous substance. Thus air and water are fluids, 

 but ^teel is not. 



