6223 



HYDROGRAPHIC MANUAL 



Page 564 



Velocity=V2 



Figure 124.— Total refraction of a sound wave. 



medium, and Vi and V2 are the values of the velocity of sound in the first and second 

 medium, respectively. 



As explained in 6221, when a sound wave in water encounters the surface boundary, 

 only a very small percentage of the incident energy is transmitted through the boundary 

 into the air. This is because the difference between the acoustic resistances of air and 

 water is very large — the ratio being 1 : 3,750. Even at normal incidence only about 0.12 



percent of the incident energy will be trans- 

 mitted into the atmosphere. This fact is 

 substantiated in subaqueous sound ranging, 

 for bomb explosions below the surface cannot 

 be heard on board ship, but vibrations from 

 the explosions are transmitted through the 

 water and can be felt tlu-ough the hull of the 

 ship. This also accounts for the fact that 

 the sound of the propeller of a ship is not 

 heard above the surface. 



There is generally no transmission through 

 the bottom boundary at the angle of incidence 

 of a sound wave that is normally encountered 

 in subaqueous sound ranging. Normally the 

 angle of incidence is greater than the critical 

 angle and hence total reflection occurs. The change in the direction of propagation 

 caused by refraction due to the pressure gradient, as ex-plained below, increases the angle 

 of incidence of the effective path slightly. 



In an ideal water medium the velocity of sound would increase linearly from surface 

 to bottom because of the linear increase in hydrostatic pressure. This is sufficient to 

 increase the velocity 1 meter per second for every 30 fathoms increase in depth. This 

 constant increase in velocity causes the sound wave to be refracted in the arc of a circle, 

 concave upward. The maxim lun distance that a refracted sound wave can travel 

 without reflection in a fluid medium of uniform temperature, is determined by the 

 refracted path that is tangent to the bottom. This is also the shortest path, for any 

 other part of the sound wave will be reflected from either the surface or the bottom 

 between the same source and point of reception, and hence in either case will have 

 traveled a greater distance, arriving later than the part that followed the tangentially 

 refracted path. 



Therefore, for distances shorter than the limit determined by the tangentially 

 refracted ray, the first sound to arrive will not have been reflected at all ; but for distances 

 beyond this limit, the refracted sound which is reflected one or more times from the 

 upper surface only will precede all others. 



However, at a considerable distance from the source, unreflected sounds and 

 sounds which are reflected only from the water surface, will be of reduced intensity 

 due to the interference phenomenon (6223). Because of this, and because in practice 

 the sensitivity of the receiving system is necessarily limited by the background noises, 

 the first sound to be detected may not be that which actually arrives first. 



6223. Interference of Sound 



Although a single source of sound is used in subaqueous sound ranging, reflections 

 from the surface and bottom cause propagations in the medium in various directions 



