621 HYDROGRAPHIC MANUAL PaGE 558 



aqueous sound ranging and detection, but in these respects a thorough understanding 

 of the subject is of primary importance. 



For the most successful use of R.A.R., not only are experimental investigations 

 essential but the physical laws governing the propagation of sound should be under- 

 stood. These physical laws are therefore briefly reviewed in 621. The propagation 

 of sound through an ideal water medium of homogeneous physical properties is dis- 

 cussed in 622, for the physical laws of sound can thus be more easily illustrated and 

 more clearly understood. Such a medium is not necessaril}^ hypothetical, for such 

 conditions are occasionally approached in actual practice. In 623 is considered prop- 

 agation through water whose physical characteristics are of a heterogeneous nature, 

 more nearly approaching the actual conditions most frequently encountered in sub- 

 aqueous soimd ranging. The treatment is often necessarily from a theoretical view- 

 point when the actual facts have not been fully disclosed by experimental investigations. 



In R.A.R. explosive sources of sound are used in order to permit detection at 

 great distances. Most of the energy from such explosions is in the low frequency 

 range and it is sounds of such frequency that are here considered. Sounds of higher 

 frequency having other underwater applications may follow different modes of prop- 

 agation. 



621. Physical Laws of Sound 



Sound is a form of energy transmitted through an acoustic medium by virtue of 

 the elastic properties of the medium. Acoustic energy is transmitted through an elastic 

 medium by bemg passed from one particle to another in the direction of propagation. 

 When a sound passes through a medium, the particles in the medium at any given pomt 

 along the path undergo a minute, purely local, longitudinal displacement. This local 

 particle motion must be clearly distinguished from the longitudinal wave motion, 

 which is the forward travel of the sound energy from particle to particle through the 

 medium. 



The particles in a medium through which a sound wave is passing are represented 

 by vertical lines at the top of figure 121. At A and C, where the lines are spaced most 

 closely, maximum instantaneous pressure of the medium is represented — the particles 

 or molecules are grouped more closely than in the undisturbed medium, and the medium 

 is said to be condensed. At B, where the vertical lines are farthest apart, minimum 

 instantaneous pressure of the medium exists — the particles or molecules are grouped 

 farther apart than in the undisturbed medium and the medium is said to be in a state 

 of rarefaction. At these points of condensation and rarefaction as the wave travels 

 forward, the pressure is increased and decreased above and below the normal pressure 

 of the undisturbed medium. Wliere the wave form is sinusoidal, as illustrated, these 

 points of maximum condensation and rarefaction are always separated from each other 

 by one-half the wave length, and they move forward with a, constant velocity in a homo- 

 geneous medium. The velocity of the forward travel is a function of the elasticity and 

 density of the medium (see 63). 



Sound waves are defined by certain characteristics of the disturbing force and of 

 the medium. These characteristics, some of which are interrelated, are: phase, 

 frequency, wave length, and intensity. The phase of a sound wave at any instant 

 and at any given point is based on the position of the particle relative to some reference 

 such as its maximum displacement. Thus in figure 121, A and C are in the same phase. 

 The frequency depends on the sound producing source; for example, if the source is 



