Some of the authors of this publication are also working on these related projects:ocean reverberation, seabed geo-acoustic model and seabed scattering View project International cooperation on ocean acoustics View project Naturally occurring internal solitary wave trains (solitons) have often been observed in the coastal zone, but no reported measurements of such solitary waves include low-frequency longrange sound propagation data. In this paper, the possibility that internal waves are responsible for the anomalous frequency response of shallow-water sound propagation observed in the summer is investigated. The observed transmission loss is strongly time dependent, anisotropic and sometimes exhibits an abnormally large attenuation over some frequency range. The parabolic equation (PE) model is used to numerically simulate the effect of internal wave packets on low-frequency sound propagation in shallow water when there is a strong thermocline. It is found that acoustic transmission loss is sensitive to the signal frequency and is a "resonancelike" function of the soliton wavelength and packet length. The strong interaction between acoustic waves and internal waves, together with the known characteristics of internal waves in the coastal zone, provides a plausible explanation for the observed anomalous sound propagation in the summer. By decomposing the acoustic field obtained from the PE code into normal modes, it is shown that the abnormally large transmission attenuation is caused by "acoustic mode-coupling" loss due to the interaction with the internal waves. It is also shown that the "resonancelike" behavior of transmission loss predicted by the PE analysis is consistent with mode coupling theory. As an inverse problem, low-frequency acoustic measurements could be a potential tool for remote-sensing of internal wave activity in the coastal zone.

Due to the difficulty of direct measurement, there is a need to develop inverse techniques for remote sensing bottom geoacoustic parameters in the lowan mode measurements are extended to extract acoustic attenuation and speed in a horizontally stratified bottom in shallow water as a function of frequency and depth. The computational and experimental results show that, for a limited frequency band, we can find an equivalent depth profile of sea-bottom acoustic attenuation with a linear frequency dependence that simulates the effect of nonlinear frequency dependence (without depth structure) on some field characteristics, such as the attenuation rate of individual mode, the frequency response of long-range sound propagation, and the amplitude ratio of mode 2 to mode 1. However, the resultant equivalent negative gradient for the sea-bottom attenuation is too strong to be accepted in light of available data. The conclusion is that nonlinear frequency dependence of the acoustic attenuation in the upper sedimentary layer is required to explain many aspects of shallow-water sound propagation.

Hamilton’s seabed geo-acoustic model, which is widely accepted, predicts that the attenuation of sound in marine sediments increases linearly with frequency over the full frequency range of interest in ocean acoustics (a few hertz to megahertz). However, Biot-Stoll’s physics-based seabed geo-acoustic model predicts that the bottom attenuation should exhibit non-linear frequency dependence, particularly in sandy bottoms. Since the publication of previous papers [Zhou, J. Acoust. Soc. Am. 78, 1003–1009 (1985); Kibblewhite, ibid. 86, 716–738 (1989)], more low-frequency field data, collected from different coastal zones around the world, have shown inconsistencies with the often-used linear frequency dependence. This paper attempts to support the non-linear frequency dependence of sea bottom attenuation through a review of shallow-water acoustic field measurements, with a special emphasis on the 50–1000 Hz frequency range. The relevant measurements include bottom reflection loss, sound transmission loss, dispersion analysis, vertical coherence of both propagation and reverberation, normal-mode spatial filtering, and optimum frequency and transition range of sound propagation. A non-linear frequency dependence of equivalent bottom acoustic attenuation, derived from these measurements, will be introduced. [Work supported by ONR and NNSF of China.]

The optimum frequency for acoustic propagation in shallow water is controlled by a number of physical effects and environmental parameters. This article concentrates on the effect of a nonlinear frequency dependence of the sea-bottom attenuation on the optimum frequency. Experimental data on low-frequency acoustic propagation in shallow water are presented, for which, over the frequency range where the optimum frequency should occur, no apparent optimum frequency is observed. If there is an optimum frequency for these sea areas, it is much lower than predicted by existing theories. It is demonstrated that the nonoccurrence, or lowering, of the optimum frequency can be easily explained if the sea bottom is assumed to have a nonlinear frequency dependence. Excellent agreement between theory and experiment is achieved for a site in the Yellow Sea for three different seasons using values for sediment attenuation (with nonlinear frequency dependence) and sediment sound speed which were measured independently at the same site [J. X. Zhou, J. Acoust. Soc. Am. 78, 1003-1009 (1985) ]. The sea-bottom attenuation used for the calculations was proportional to frequency to about the 1.8 power.

The hydrolysis of earth‐abundant AlIII has implications in mineral mimicry, geochemistry and environmental chemistry. Third‐order nonlinear optical (NLO) materials are important in modern chemistry due to their extensive optical applications. The assembly of AlIII ions with π‐conjugated carboxylate ligands is carried out and the hydrolysis and NLO properties of the resultant material are studied. A series of Al32‐oxo clusters with hydrotalcite‐like cores and π‐conjugated shells are isolated. X‐ray diffraction revealed boundary hydrolysis occurs at the equatorially unsaturated coordination sites of AlIII ions. Charge distribution analysis and DFT calculations support the proposed boundary substitution. The Al32‐oxo clusters possess a significant reverse saturable absorption (RSA) response with a minimal normalized transmittance up to 29 %, indicating they are suitable candidates for optical limiting (OL) materials. This work elucidates the hydrolysis of AlIII and provides insight into layered materials that also have strong boundary activity at the edges or corners.