Bubble density was estimated with a subharmonic acoustic wave generated in bubbly water. The subharmonic acoustic wave can be easily generated due to the nonlinearity of bubbly water if the frequency of primary acoustic wave is double of the bubble resonance frequency and the driving acoustic pressure amplitude exceeds a certain threshold value. The frequency of primary acoustic wave was varied from 200 kHz to 500 kHz while the bubble resonance radius at subharmonic frequency was from 12 um to 28 um. The pressure level of the subharmonic acoustic wave linearly increased as the driving acoustic pressure amplitude increased. With the subharmonic pressure level, the bubble density was estimated from nonlinear bubble oscillation equation [Yu. A. Ilinskii and E. A. Zabolotskaya, J. Acoust. Soc. Am. 92, 2837–2841 (1992)]. The estimated bubble densities were also compared with those from a linear conventional acoustic bubble sizing method. Bubble sizing with subharmonic acoustic wave seems to be easily utilized for the diagnosis of sandy sediment with bubbles.
Nonlinear acoustic responses of water-saturated sediments are very important to understand nonlinear phenomena of gassy ocean sediments. Especially, the second harmonic, the sum, and the difference frequency acoustic waves in water-saturated sediments can provide practical criteria to estimate the nonlinear parameters of gassy sediments. In this paper, the difference frequency acoustic waves in water-saturated sandy and muddy sediments were observed in a water tank experiment using a pulse transmission technique. Their pressure levels were 10 dB higher than the background noise level at a maximum driving pressure of source acoustic transducer. The experimental results were compared with theoretical estimations of the parametric acoustic array. The nonlinear parameters of water-saturated sandy and muddy sediments were estimated as a few tens in their comparison. The nonlinear parameters were also compared with those obtained with the second harmonic acoustic wave. [Work supported by MOCIE (Ministry of Commerce, Industry and Energy).]
Generation of subharmonic and difference frequency acoustic waves in a water-saturated sandy sediment was investigated. Subharmonic frequency acoustic wave was observed to be generated due to the nonlinearity of water-saturated sandy sediment when the fundamental frequency acoustic wave exceeded a certain threshold pressure amplitude. Pressure spectrum level of the subharmonic frequency acoustic wave linearly increased as the driving acoustic pressure amplitude increased. The pressure level was 10 dB higher than the background noise level. Generation of the difference frequency acoustic wave was also observed by the collinear acoustic waves with two different fundamental frequencies. The pressure level of the difference frequency acoustic wave was 5 dB higher than the background noise level. It seems very useful to evaluate the nonlinear parameter of water-saturated sandy sediment without disturbing the sediment. Such nonlinear acoustic responses of water-saturated sandy sediment can be utilized for the diagnosis of marine gassy sandy sediment.
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