B 



back bay — A small bay into which streams drain 

 and \yliich is connected with tlie sea through 

 passes between ban-ier islands. (2) 



backbeach — See backshore. 



backdeep — An oceanic depression on the concave 

 side of an island arc. (2) 



background — Ever-present effects in physical ap- 

 paratus above which a phenomenon must mani- 

 fest itself in order to be measured. Background 

 can take various forms, depending on the nature 

 of the measurement. In electrical measure- 

 ments of radioactivity and nuclear phenomena, 

 the term usually refers to those undesired counts 

 or currents that arise from cosmic rays, local 

 contaminating radioactivity, insulator leakage, 

 amplifier noise, power-line fluctuations, etc. In 

 nuclear work and photographic emulsions, the 

 term refers to developable grains unrelated to 

 the tracks under investigation. (41) 



background count — The evidence or effect on a 



detector of radiation, other than that which it is 



■ desired to detect, caused by any agency. (63) 



background noise — Noise made up of sound from 

 a variety of sources, other than the desired sig- 

 nal, as indicated by the block diagram below : 



BACKGROUND 

 NOISE 



_ AMPLIFIED 

 " NOISE 



r SELF NOISE 



CIRCUIT HOISE I 



HYDROPHONE MOTIONI 



INCISE FROM OWHSHiFl 



AMBIENT 

 NOISE 



SEA NOISE 



: 



BIOLOGICAL NOisTI 



TRAFFIC NOISE 



CLASSIFICATION OF BACKGROUND NOISE 



background radiation — 1. Radiation arising 

 from radioactive material other than the one di- 

 rectly under consideration. 



2. In the ocean environment when measuring 

 fission products or radioactive tracers, the 

 background radiation (radioactivity) is that 

 radiation (radioactivity) contributed by cosmic 

 rays and the natural radioactive constituents of 

 sea water. 

 (70) 



backing — 1. According to general internationally 

 accepted usage, a change in wind direction in a 

 counterclockwise sense (for example, south to 

 southeast to east) in either hemisphere of the 

 earth; the opposite of veering. 



2. Accordmg to widespread usage among 

 United States meteorologists, a change in wind 

 direction in a counterclockwise sense in the 

 Northern Hemisphere, clockwise in the Southern 

 Hemisphere ; the opposite of veering. 

 (5) 



back radiation — See counterradiation. 



back reef — The region behind a reef, between it 

 and the land. Variously applied to reef flat, 

 lagoonal deposits, or terrestrial deposits connec- 

 ting the reef and the land. (2) 



backrush — The seaward return of water following 

 uprush of the waves. See backwash. (61) 

 {See figure for surf zone.) 



backscattering — The part of the reflected sound 

 energy that returns to the transducer ; equiva- 

 lent to reverberation. 



backscattering — The part of the reflected sound 

 scattering cross section of an object is an area 

 equal to .^tt times the product of the square of a 

 unit distance and the square of the sound pres- 

 sure scattered by the object back in the direction 

 from which the sound has come as observed at 

 unit distance from the acoustic center of the 

 object, divided by the square of the sound pres- 

 sure of the plane wave incident on the object. 

 The unit of the cross section is the square of the 

 unit distance. (6) 



backshore — That part of a beach which is usually 

 dry, being reached only by the highest tides, 

 and, by extension, a narrow strip of relatively 

 flat coast bordering the sea. See foreshore. 

 (68) {See figure for shore profile.) 



back-slope — The gentler sloping side of a ridge ; 

 in contrast with escarpment, the steeper slope. 

 "(2) 



backward scatterance — The ratio of the radiant 

 flux scattered through angles 90 to 180 degrees 

 from a beam, to the incident flux. (8) 



backward scattering coefficient — The coefficient 

 which relates to backward scatterance. Unit 

 of measurement is m"^. (8) 



backwash — (also called hackrmh). Water or 

 waves thrown back by an obstruction such as a 

 ship, breakwater, cliff, etc. (61) 



backwater — 1. Water turned back by an obstruc- 

 tion, opposing current, etc. (68) 



15 



