RODERICK: FORWARD SCATTERED LOW-FREQUENCY SOUND FROM THE SEA SURFACE 



7 50 Hertz transmitted signal. Note that the sidebands are displaced 

 equally from the carrier. The spectrum obtained at the transmitted 

 frequency of 1,500 Hertz shows significantly more energy in the side- 

 band frequencies. (All vertical scales are 5 dB per division.) 



The spectra shown in Figure 8 are for two consecutive pulses 

 reflected from the surface and separated in time by 3 minutes. Note 

 again that the sideband frequencies are displaced symmetrically about 

 the carrier and peaked at the frequency of maximum energy on the 

 surface. 



For a wind speed of 35 knots, the reflected spectra (Figure 9) 

 have their first-order sidebands peaking at approximately 0.07 Hertz. 

 It can be seen that the carrier frequency is suppressed for the 1,500- 

 Hertz case; thus, almost all the received energy is contained in the 

 scattered frequency components. The ocean spectra recorded for this 

 wind speed of 35 knots are also peaked at 0.07 Hertz. 



In a recent JASA article, Vertner Brown and George Frisk (1974) 

 reported on Doppler spectrum measurements conducted in the open 

 ocean in the frequency range of 100 to 500 Hertz. The statistics of 

 the sea surface were measured simultaneously with acoustic data by 

 a surface-sensing buoy. The acoustic spectra are compared with the 

 surface-wave spectra at each of the transmitted frequencies in Figure 

 10. For small surface roughness, the acoustic spectra contain the 

 discrete carrier frequency component with sidebands symmetrically 

 positioned about the carrier. For moderate roughness, marked 

 asymmetry in the acoustic spectra and strong spectral components 

 that are not prominent in the surface spectra are found. 



Harry DeFerrari and Nghiem-Phu (1974) published the scattering 

 functions of various acoustic arrivals over a propagation path of 



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