Contributions of environmental quantities related to the noise source field and the propagation path are derived from the comparison of measurements of the wind-dependent noise by omnidirectional receivers at a fixed North Sea station with shipborne measurements in the Baltic Sea. The influence of propagation loss on the wind-dependent shallow water noise appears to be only marginal, even at extremely different sea areas. The quantity governing the noise spectrum level under uncontaminated conditions is the wind speed at the sea surface for which the second power law relation has been verified between 50 Hz and 20 kHz and above a "threshold" wind speed of • 5 kts. Neither the characteristic height of the sea waves nor the wind turbulence at the reference height are relevant to the noise production, but both may indicate wind profile changes which originate an essential portion of the noise variability for a given wind speed. Further deviations from the second power law are attributed to a bubble layer effect under storm conditions reducing or enhancing the high frequency noise level thus yielding a spread of the average spectrum level of more than 20 dB.
Transverse horizontal cross-coherence lengths as a function of signal frequency were measured by a hydrophone array 80 m long under homogeneous winter conditions with heavy sea state. The decrease of the normalized coherence length with the acoustic wavenumber is shown as an example of sound propagation parallel to the wave crests and is compared with published reverberation measurements. The cross-coherence length is interpreted in a simple model computation as the reciprocal of a medium-dependent angular uncertainty.
Shallow water propagation measurements in the thermocline-halocline refractive sound channel of the Baltic Sea are utilized to estimate the absorption coefficient in low salinity water of 8 ppt (8.0 pH, 4 *C) for 0.5-10 kHz. The lower margin of the present data which are derived from transmission loss by subtraction of cylindrical spreading can be represented by the Francois and Garrison formula of 1982. However, measurements of the boron relaxation frequency indicate that this agreement does not necessarily support the assumed salinity dependence of the relaxation frequency but might be produced by appropriate scattering loss of the refractive channel.
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