No abstract

There is substantial interest in the analytical and numerical modeling of low-frequency, long-range atmospheric acoustic propagation. Ray-based models, because of their frequency limitations, do not always give an adequate prediction of quantities such as sound-pressure or intensity levels. However, the parabolic approximation method, widely used in ocean acoustics and often more accurate than ray models for frequencies of interest, can be applied to this type of acoustic propagation in the atmosphere. Modifications of an existing implicit finite-difference implementation for computing solutions to the parabolic approximation are discussed. A locally reacting ground surface is used with one- and two-parameter impedance models, while a nonreflecting boundary condition is used to handle the upper boundary. Relative sound-pressure level calculations are performed for a number of flow resistivity values in both homogeneous and nonhomogeneous atmospheres. Comparisons to experimental data are made which suggest this modeling approach can be useful in the study of these types of propagation problems.

Under the assumption that deep-water ocean noise is a superposition of uncorrelated plane waves, the covariance can be expressed in terms of the noise directivity. We propose a simple directivity function that agrees with experimental results and use it to study the covariance for the case of vertical receiving points. We investigate covariance versus spacing for zero time delay and various directivities and bandwidths. When almost all of the noise arrives from the horizontal, the zero-delay covariance is always positive; when most of the noise arrives from overhead, it is negative for some spacings. We present contours of constant covariance as a function of spacing and time delay for various directivities and a narrow frequency band. We also examine covariance for single-frequency noise. Vertical noise with steering towards the vertical and horizontal noise with steering towards the horizontal are found to give the same covadance values. We examine maximum covariance over time delay and also that delay for which maximum covariance occurs. Numerous comparisons are made between our predictions and published experimental results. Finally, covariance for horizontal receiving points is considered.

The effects of sound-speed and current variations induced by a mesoscale cyclonic eddy on short-range propagation are considered. A parametric eddy model is used t0• determine acoustically relevant eddy environmental effects, so that eddy-acoustical effects can be determined for eddies of arbitrary size, strength, and position. Approximations to sound-speed and current structures are used to investigate eddy effects on the three-dimensionality of rays and on ray types. The influence of current and soundspeed variations on travel time is examined, and accurate expressions for per-ray phase variation are obtained. Examples are presented illustrating effects of source-receiver position and orientation on perray phase shifts and relative phase spreading of arrivals. Also, general results are presented which illustrate the variations of eddy-acoustical effects as functions of source-receiver range and of eddy size and strength.

The effect of currents on the acoustic pressure field in an underwater sound channel is investigated. Based on fundamental fluid equations, model equations are formulated for sound pressure while including nonuniform currents in the source–receiver plane. Application of parabolic-type approximations yields a collection of parabolic equations. Each of these is valid in a different domain determined by the magnitudes of current speed, current shear, and depth variation of sound speed. Under certain conditions, it is possible to interpret current effects in terms of an effective sound speed. Using this effective sound speed in an existing numerical code, we examine sound speed in a shallow water isospeed channel with a simple shear flow and a lossy bottom. It is found that even small currents can induce very substantial variations in relative intensity. The degree of variation depends upon current speed, source and receiver geometry, and acoustic frequency. Particular emphasis is placed on intensity-difference predictions in reciprocal sound transmissions in the presence of an ocean current.