vs frequency. These plots are overlaid on standard ambient noise curves 

 to show the relationship of the measured noise to expected natural ambient 

 sea noise. Such plots for each of the fourteen platforms are shown in 

 Figures 5 through 9. They show tonal components (also called spectrum 

 lines) as vertical lines with a dot at the top showing the sound pressure 

 level of that component. In general, line components are produced by a 

 repetitive mechanical process of very stable cyclic repetition rate, such 

 as a rotating machine (engine, electric motor, turbine, pump, etc.). All 

 platforms show such lines, as might be expected from their extensive use 

 of rotating machinery. Also on each plot is a dashed curve showing the 

 spectrum level (level in a one-hertz-wide band) of the continuous soectrum 

 portion of the noise. A continuous spectrum is generated by a series of 

 non-cyclic, or random events. Such noise generally covers a broad band 

 of frequencies; hence, it is often called broad-band noise. The normal 

 ambient noise in the sea is generated by breaking waves and is dependent 

 on sea state, and therefore wind speed. The spectra of such natural broad- 

 band noises for each of four selected sea-state conditions are plotted on 

 each graph to serve as a reference to natural sea noise which is familiar 

 to animals. Also shown are curves of the continuous spectrum noise (at 

 lower frequencies) generated by the cavitating propellers of ships. The 

 level of such noise depends on the number of cavitating ships in the general 

 vicinity (shipping density). Note that the three curves are labeled heavy, 

 moderate, and light. It is likely that the area off Santa Barbara would 

 be characterized as "heavy" shipping. The two sets of ambient noise curves 

 (sea state, and shipping) are widely used in underwater acoustics for pre- 

 diction of oceanic noise (Ref. 2). 



F-6 



