Low-frequency Sound Propagation Research Articles

Low-Frequency Sound Attenuation in the Indian Ocean

With the support of the French Navy, a low-frequency sound propagation experiment was conducted to obtain attenuation coefficients in the Indian Ocean for the frequency range 200&–10 000 Hz. The experiment was conducted along a 500-km track, which was characterized by a broad sound channel with an axis depth of 250 m. The results are compared with previous measurements in the Red Sea and Atlantic Ocean.

Sound propagation in critical mixtures

A modification of the Fixman's theory for the low frequency sound propagation in binary critical mixtures is proposed. The results obtained are significantly different from those derived by a straight forward transcription of the Kawasaki theory.

Measurements of the acoustic target strengths of fish in dorsal aspect, including swimbladder resonance

The need for measurements of the acoustic target strength of fish is discussed. The phenomenon of swimbladder resonance of small deep ocean fish is well known and is a useful means of estimating their sizes. For larger commercial fish in shallower seas the resonant frequency is much lower and resonance is very difficult to observe in the field. A method of observing and measuring the swimbladder resonance of a captive live fish in controlled conditions is described, and results on several gadoids are given. Reasons for the observed resonant frequencies being higher than predicted are given; the damping of resonance is high, which is expected. Application of these results to acoustic sizing at sea appears remote. They are relevant, however, to studies of low-frequency sound propagation, and the experimental technique is offered as a useful tool in physiological studies involving swim-bladder function. Measurements at higher frequencies in the diffraction and geometrical regions are also presented, resulting in an empirical equation for target strength as a function of length of the fish and wavelength. It is believed that this equation is useful for acoustic fish sizing using echo sounders at sea. The swimbladder is the major scatterer over the whole frequency range.

Model for Low-Frequency Sound Propagation in Deep Water

Low-frequency sound-propagation loss in deep water between near-surface sound sources and hydrophones exhibits short- and long-term fluctuations with range. A model of the sound field based on a product relationship between the two types of fluctuations is discussed. Long-term fluctuations are described by a deterministic range function obtained from the application of simple ray-tracing techniques. A random variable is used to describe the short-term fluctuations. The effect of multipath interference on both types of fluctuations is considered. The deterministic range function, and the probability density functions and spectra describing the short-term fluctuations are discussed using experimental results. The experimental work was carried out using source and hydrophone depths of 25 and 400 ft, respectively. Source signals were provided by TNT charges and a 230-Hz CW projector.

Two-particle Green’s function and the derivation of the Boltzmann equation — II

The theory has been used, which expresses the response to the external field through the set of three functionsF m , introduced in an earlier work (1). These functions satisfy a system of coupled linear integral equations which has been derived from a version of the Bethe-Salpeter equation. The approximations in kernels are considered, which are applicable for description of the high-frequency and low-frequency sound propagation. Finally, forωτ≪1, the single integral equation of the linearized Boltzmann type is derived and the kernel of this equation is found to be identical with the Boltzmann one.

Ray Analysis of Low-Frequency Sound Propagation in Shallow Water

The problem of propagation of low-frequency sound in a system consisting of shallow water overlying several thin layers above a basement is approached through a ray theory in which the first two bottom layers are lumped together with the water layer. This effective structure is assumed on the basis of expected good transmission through a thin layer (less than one wavelength thick) of impedance not very different from those of its bounding media. The required reflection coefficient at the lower boundary of the lumped layer, expressed as a function of range and the order of the image source of a ray, is taken in the form given by Abeles for reflection of plane electromagnetic waves from a layered system. The theory yields calculated transmission curves whose rates of fall-off and fine structure agree reasonably well with experimental results.