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



As mentioned previously, low- amplitude sinusoidal waves were 

 created on the water surface of an anechoic tank. A micrometer was 

 supported above the surface and lowered to measure the wave height. 

 The surface wavelength was measured from two wave-height sensors and 

 the results compared well to the dispersion equation for gravity 

 waves. With knowledge of acoustic parameters and geometry, predic- 

 tions can be made for the scattered field. The spectrum on the upper 

 left of Figure 5 is the amplitude frequency spectrum of an acoustic 

 signal reflected from a calm surface. The other three spectra are 

 the returns, measured in the specular direction, from a surface of 

 4.5 Hz with waves propagating in directions equivalent to up-wind, 

 down-wind, and cross-wind. The wave heights for the three cases 

 were the same. Note that in each spectrum the sideband frequencies 

 are symmetrical about the carrier and displaced from the carrier by 

 the surface frequency, 4.5 Hz. The sideband frequencies for the 4.5- 

 Hz surface waves were scattered within 3 degrees of the specular 

 direction and the energies in the sideband frequencies are identical 

 regardless of the direction of surface wave propagation. 



Looking only in the specular direction, it is not possible to 

 observe the effects of the spatial scattering of the sidebands (see 

 Figure 6) . Placing a hydrophone 10 degrees off specular and toward 

 the backscatter direction, the Doppler shift was measured at a sur- 

 face frequency of 6.0 Hz. The surface waves were propagating in a 

 direction equivalent to up-wind and, as expected, an up-Doppler was 

 obtained. There are no lower sideband frequencies and the spectrum 

 consists of the first- and second-order sidebands. Reversing the 

 direction of the surface waves, we obtain a down-Doppler , as shown 

 in the bottom spectrum. The received signal consists of just the 

 lower sidebands. The wave heights for these two cases were not the 

 same. 



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