SONIC WAVE 



ceans, ranging in frequency from the low 40-cps 

 moans of the fin whale to the echolocation 

 clicks of the bottlenose porpoise, which have 

 high energy between 20,000 and 30,000 cps and 

 may have minor components in the region of 

 200,000 cps. Eecent investigations of mider- 

 water sounds emitted by seals and sea lions in- 

 dicate that several pinnipeds produce sharp 

 clicks ranging from about 100 to at least 30,000 

 cps, with principal frequencies between 2,000 

 and 12,000 cps. 



sonic wave — &ee sound wave. 



soniferous fishes — See sonic fishes. 



soniferous marine animals — See sonic marine 

 animals. 



soniferous marine mammals — See sonic marine 

 mammals. 



sonobuoy — A free floating or anchored device 

 that includes a buoy with radio telemetering 

 equipment and a hydrophone suspended be- 

 neath. Sound signals received at the liydro- 

 phone are transmitted to a nearby receiver for 

 analysis. 



sonodivers — Untethered, unmanned, submersible 

 vehicles, launched from ships for recording 

 ambient noises and supplementary informa- 

 tion capable of sampling noise at depths from 

 100 to 20,000 feet over a frequency range of 10 

 to 50,000 cps. 



sonoprobe — A low frequency echo sounder which 

 generates sound waves and records their re- 

 flections from one or more sediment layers 

 beneath the sediment/water interface. (2) 

 See subbottom reflection. 



sound — 1. Tlie periodic variation in pressure, 

 particle displacement, or particle velocity in an 

 elastic medium. See sound velocity. 



2. A long arm of the sea which forms a chan- 

 nel between an island and the mainland or a 

 sea and ocean. It is usually wider and longer 

 than a strait. (2) 



3. To dive suddenly toward deep water, 

 usually from the surface or upper water levels, 

 as a fish or .whale when hooked or harpooned, 

 or a submarine attempting to escape detection or 

 attack by enemy craft. 



4. To measure or ascertain the depth of water 

 as with sounding lines. (61) 



sound absorption — See absorption. 



sound channel — The region in the water column 

 where sound velocity first decreases to a mini- 

 mum value with depth and then increases in 

 value as a result of pressure. Above the mini- 

 mum value sound rays are bent downward, 

 and below the minimum value sound rays are 

 bent upward ; the rays are thus trapped in this 

 channel. Sound traveling in a deep channel can 

 be detected thousands of miles from the sound 



source. 



sound channel axis — The depth at which mini- 

 mum sound velocity occurs. 



sound energy density — The sound energy density 

 at a point in a sound field is the sound energy 

 contained in a given infinitesimal part of the 

 medium divided by the volume of that part of 

 the medium. (6) 



soundhead — An enclosure containing the trans- 

 mitting projector and the receiving hydrophone. 



sounding — 1. The measurement of the depth of 

 water beneath a ship. 



2. In geophysics, any penetration of the nat- 

 ural environment for scientific observation. (5) 



sounding datum — The plane to which soundings 

 are referred. See also chart datum. (61) 



sounding line — See lead line. 



sound intensity — 1. At a point the average rate 

 of sound energy transmitted in a specified direc- 

 tion through a unit area normal to this 

 direction at the point considered. (6) 



2. In practice, considered as the square of the 

 sound pressure. 



sound pressure — Tlie instantaneous pressure at 

 a point in a medium in the presence of a sound 

 wave, minus the static pressure at that point. 



sound pressure level — Twenty times the loga- 

 rithm to the base 10 of the ratio of the pressure 

 of sound to the reference pressure, in decibels 

 at a specific point. The reference pressure shall 

 be explicitly stated. 



sound velocity — The rate of travel at which 

 which sound energy moves through a medium, 

 usually expressed in feet per second. 



The velocity of sound in sea water is a fimc- 

 tion of temperature, salinity, and the changes 

 in pressure associated with changes in depth. 

 An increase in any of these factors tends to in- 

 crease the velocity. Sound is propagated at a 

 speed of 4,742 feet per second at 32 °F, one at- 

 mospheric pressure, and a salinity of 35 per 

 mille. 



sound wave — (also called acoustic wave, sonic 

 wave). A mechanical disturbance advancing 

 with finite velocity through an elastic medium 

 and consisting of longitudinal displacements of 

 the ultimate particles of the medium, that is, 

 consisting of compressional and rarefactional 

 displacements parallel to the direction of ad- 

 vance of the disturbance; a longitudinal wave. 

 (5) 



source level — The sound output of a source as 

 expressed in decibels relative to 1 dyne per 

 square centimeter at a distance of 1 yard from 

 the sound source. 



source region — The extensive area of the oceans 

 where a water mass acquired its basic charac- 

 teristics. 



South Atlantic Current — An eastward flowing 

 current of the South Atlantic Ocean that is con- 

 tinuous with the northern edge of the West 

 Wind Drift. (5) 



152 



