Mean Sound Speed Research Articles

A new method of measuring in vivo sound speed in the reflection mode.

This paper describes a new method for in vivo sound measurement in the reflection mode. The mean sound speed between the reflector and linear array transducer is measured using the following three parameters: time of flight, time of flight difference, and distance between two receiver elements. To detect time of flight, the system delay-line time compensator is adjusted to obtain the sharpest reflector image. This method was evaluated in vivo in human livers, specifically 26 normal, 27 cirrhotic, and 15 fatty livers. The mean sound speed between diaphragm and the transducer was obtained. The measured sound speed was significantly higher in cirrhotic livers and lower in fatty livers.

Production of Sound by Turbulent Flow over an Embedded Strut

An analysis is made of the sound produced during low-Mach-number turbulent flow over a thin, two-dimensional rigid strut embedded in a compliant layer. The layer is interposed between the flow and a plane rigid wall, and is modelled by ahomogeneous ‘blanket’ of uniform thickness, whose properties are represented by those of a fluid of uniform mean density and sound speed. The strut is transverse to the mean flow and projects normally from the wall to an arbitrary distance, but does not make physical contact with the fluid. Sound is produced by the diffraction by the strut of the intensive convective pressure fluctuations of the boundary layer. Numerical predictions are made of the radiation into the fluid, and of the extent to which the diffracted field stands out against the background noise of the turbulent boundary layer.

Generalized parabolic wave equation and field‐moment equations for random media having spatial variation of mean sound speed

The wave equation for propagation of sound waves through random oceanic inhomogeneities is expressed in a special coordinate system that follows the ray path of the deterministic wave that would propagate in the absence of randomness. The ratio of the full wave field in the presence of random inhomogeneities to the deterministic wave field is shown to satisfy the parabolic wave equation with acoustic wavelength depending on position along the ray. This approximation requires that the local sound speed be only slightly perturbed from its local deterministic value, although it may be very much different from the sound speed at a distant point on the deterministic ray. No matter how tortuous the path of the deterministic field, this parabolic equation holds provided that the angles between the rays of the full field and rays of the deterministic field are small. The equations for moments of the field (mean field, mutual coherence function, fourth moment) have a form very similar to the case of uniform mean s...

Propagation through refracting ocean sediments

A transport theoretic analysis of propagation through highly anisotropic randomly perturbed sediment layers has previously been developed for the case of constant mean density and sound speed. [J. Acoust. Soc. Am. 12, 937–946 (1982); preliminary results reported in J. Acoust. Soc. Am. Suppl. 1 70, S1 (1981)]. The present paper extends this work to include the effects of refraction. The effect of variations in the mean density and sound speed upon the mean field and incoherent intensity is discussed. [Work Supported by ONR Contract No. N00014-76-C-0056 and NAUSEA Code 63D.]

Fluctuations of resolved acoustic multipaths at long range in the ocean

A phase‐coded signal centered at 220 Hz with 64‐ms resolution was transmitted at 10‐min intervals between an acoustic source moored at 2‐km depth and a bottom‐mounted receiver at ∼3‐km depth and at ∼900‐km range. A variety of fluctuation statistics (pulse and CW) are computed for three resolved multipaths from a 32‐day record. The fluctuations are compared with theoretical predictions based on internal wave scattering. Some measurements agree with theory; measurements that disagree differ by amounts that are statistically significant but not large. Uncertainty of the internal wave energy level could account for some discrepancies. Some theoretical results are very sensitive to small changes of the second derivative of the mean sound speed profile. There are substantial fluctuations at frequencies below those that can be accounted for by internal waves.

Influence of mean sound speed on acoustic transmission through an internal wave

A major purpose of this paper is to investigate the influence of mean sound‐speed structures on sound transmission in the presence of a single‐frequency internal wave. A cw signal is transmitted through a shallow ocean over a range that is small compared to the wavelength of the internal wave. A constant value of the Brunt–Vaisala frequency is assumed, and this value is taken as a parameter of the model. The total field associated with refracted/bottom‐reflected rays is studied, and the effect of the internal wave on total‐field phase and transmission loss is examined. Then, the maximum variation of the phase is investigated for different mean sound‐speed structures and internal‐wave amplitudes. This variation shows a general downward trend, as bottom sound speed increases, and an oscillatory behavior possibly due to rapid changes in the rate of change of phase with respect to mid‐depth sound speed. A simple mathematical model is constructed to explain the dominant decreasing trend. Phase variation is shown to vary linearly with bottom sound speed and to be proportional to wave amplitude, range, and acoustic frequency. It is inversely proportional to ocean depth.

Survey Techniques for Acoustic Positioning Arrays

the problem of defining the relative and geographic coordinates of an acoustic sea-floor transponder array is examined. Effects of refraction to slant ranges of 10 km are shown to be compensated for by the use of a harmonic mean sound speed. Ranges measured with an acoustic transponder are compared to those measured by a tellurometer. Methods of Cloverleaf measurement of acoustic transponder depth, Baseline Crossing measurement of transponder spacings, and an iterative technique for determination of array coordinates are described. All three methods are used to determine the configuration of a transponder triad in deep water near Antigua. The most efficient method of establishing relative and geographic coordinates of an array of sea-floor acoustic markers is identified and discussed.