Underwater rain noise : sources, spectra and interpretations

Abstract: An overview over the most important results obtained over the past of noise produced in water by impacts of single droplets, artificial rain and real rain is presented. Two distinct sources form the major contributors to rainfall produced noise in the sea. The raindrop impact and the pulsations of entrained air bubbles. The impact sound is produced by raindrops of all sizes, while the bubble pulsations are connected with regular entrainment of small gas bubbles, leading to the characteristic peak spectral level between 14 and 16 kHz, and by irregular entrainment of larger gas bubbles produced by impact of large raindrops. The noise spectra are discussed and interpretations of their relations to the raindrop size and impact mechanisms are given. Relations between noise spectra and rainfall rates are discussed and some still unsolved problems are pointed out, particularly related to estimation of light and moderate rainfalls using underwater sound measurements

Contemporary aspects of the theory and application of nonlinear acoustics

The foundations of nonlinear acoustics may be traced nearly 250 years back in time, but only the last 50 years have shown an increasing number of attempts to exploit the research results in nonlinear acoustics. Based upon the fundamental equations of fluid dynamics, the second-order acoustic equations may be derived which can be reduced to a compound equation describing several of the most important and fast developing areas of research in nonlinear acoustics. The relations between this compound equation and Burgers' equation, Korteweg-DeVries equation, the K-Z-K equation, Westervelt's equation and the general second-order wave equation are discussed in depth. Finally, it is shown how the derivatives of the compound equation can be applied to nonlinear acoustic research related to materials characterisation by use of the B/A-ratio, to underwater acoustics by use of the parametric acoustic array and to focused, high-power ultrasonic fields

Scattering of plane acoustic waves at elastic particles with rough surfaces

A comprehensive theoretical and numerical study oj the influence oj surface roughness oj elastic particles in water on the scattering oj ultrasonic waves has been carried out. For near spherical shape oj the particles and with small rms-roughness heights a perturbation method has been developed. In this method the first-order perturbation contribution predicts the contribution to the incoherently scattered acoustic field due to surjace roughness, and the second-order perturbation contribution predicts the change in the coherent field and will satisfy the requirement oj energy conservation. The second-order perturbation contribution is evaluated using the concept ojform function, while the first-order perturbation to the total scattered acoustic field is evaluated using the scattering cross-section. As a junction oj the ka-value and jor different rms-roughness heights a numerical study oj the jorward and backward scattering from rough, elastic particles has been carried out and a substantial roughness influence on the scattered field has been verified. Same experimental resultsfrom measurements oj scattering jrom glass and cast iron spheres have given evidence to the numerical predictions

Underwater rain noise : sources, spectra and interpretations

An overview over the most important results obtained over the past of noise produced in water by impacts of single droplets, artificial rain and real rain is presented. Two distinct sources form the major contributors to rainfall produced noise in the sea. The raindrop impact and the pulsations of entrained air bubbles. The impact sound is produced by raindrops of all sizes, while the bubble pulsations are connected with regular entrainment of small gas bubbles, leading to the characteristic peak spectral level between 14 and 16 kHz, and by irregular entrainment of larger gas bubbles produced by impact of large raindrops. The noise spectra are discussed and interpretations of their relations to the raindrop size and impact mechanisms are given. Relations between noise spectra and rainfall rates are discussed and some still unsolved problems are pointed out, particularly related to estimation of light and moderate rainfalls using underwater sound measurements