A simple four-panel transducer capable of producing a beam with a screw dislocation along its axis was constructed and evaluated. A screw dislocation in a wavefront is characterized by a phase dependence about the dislocation axis that varies as exp(−imφ), where m is an integer and φ is the angle about the axis. At the axis, the phase is indeterminate and as a result there is a corresponding null in the pressure magnitude. The screw dislocation in the transducer beam is along the beam axis and is found to exist in both the far- and near-fields of the transducer. This null then clearly indicates the axis of the beam at all distances and has the potential to be used as an aid in the alignment of objects in sonar experiments or other similar applications. The helicoidal wave is also shown to possess axial angular momentum. A related transducer was summarized previously [J. Acoust. Soc. Am. 103, 2971 (1998)] and is also discussed here for the purposes of comparison.

12b. DISTRIBUTION CODE ABSTRACT (Maximum 200 words)During the sediment acoustics experiment (SAX99) independent sound speed and attenuation measurements were made in a well-characterized sandy sediment. These measurements covered a broad frequency range that made it possible to test both Biot theory and Buckingham's more recent grain-to-grain attenuation model. Biot theory was able to model sound speed well but was unable to predict attenuation measured above 50 kHz. A series of measurements was made in the laboratory to test the hypothesis that this modeling deviation was due to scattering within the sediment. The measured attenuation in the glass bead sediments exhibited the same frequency dependence observed in the SAX99 data, indicating that scattering is not the relevant attenuation mechanism. A sediment model is proposed which is a hybrid of the Biot and Buckingham theories. This hybrid model is able to predict both attenuation and sound speed in the SAX99 sediment and in the laboratory sediments. The implications of the success of the hybrid model are discussed. Running Title: Sound progagation in glass bead sediments. SUBJECT TERMS AbstractDuring the sediment acoustics experiment (SAX99), a number of independent sound speed and attenuation measurements were made in a well-characterized sandy sediment. These measurements covered a broad frequency range and made it possible to test both Biot theory and Buckingham's more recent grain-to-grain attenuation model. While Biot theory was able to model the sound speed well, it was unable to predict the attenuation measured above 50kHz. This paper presents a series of measurements made in the laboratory on a simple glass bead sediment to test the hypothesis that this deviation was due to scattering within the sediment. The sediments used were saturated with two fluids with different viscosities in order to shift the dispersion into the frequency range of the measurement system. The measured attenuation in the glass bead sediments exhibited the same frequency dependence as observed in the SAX99 data indicating that scattering is not the relevant attenuation mechanism. A sediment model is proposed which is a hybrid of Biot and Buckingham's theories. This hybrid model is able to model both the attenuation and sound speed in the SAX99 sediment and in the laboratory sediments. The implications of the success of this hybrid model are also discussed.

Unlike most common solids, "plastic" polymer solids typically have shear and Rayleigh wave phase velocities less than the speed of sound in water. Subsonic Rayleigh waves on smooth objects in water are not classified as leakey waves and it is necessary to reexamine backscattering mechanisms. Also the intrinsic material dissipation of the Rayleigh wave can be significant. Backscattering by acrylic or polymethlmethacrylate (PMMA) spheres in water is analyzed and measured in the region ka = 1.5-7 and it is found that prominent low-lying resonance peaks of the form function f exist. The peaks can be modeled with quantitative ray theory as the result of coupling of subsonic Rayleigh waves with sound through acoustic tunneling. The most prominent maximum of f=5.63 occurs at ka = 1.73 and is associated with the quadrupole (or n=2) partial wave. In addition to explaining the scattering, the target strength is found to be sufficiently large that such spheres may be useful for passive low frequency targets.

Some properties of acoustical traveling waves with helicoidal or "spiral-like" wavefronts are analyzed. These include orbital angular momentum flux and potentially useful imaging or scattering properties for the discrimination of axisymmetric objects from other objects. Measurements are summarized for a simple singleelement ultrasonic transducer for generating approximately helicoidal beam-waves in water.

A spiral wave front source produces an acoustic field that has a phase that is proportional to the azimuthal angle about the source. The concept of a spiral wave front beacon is developed by combining this source with a reference source that has a phase that is constant with the angle. The phase difference between these sources contains information about the receiver's azimuthal angle relative to the beacon and can be used for underwater navigation. To produce the spiral wave front, two sources are considered: a "physical-spiral" source, which produces the appropriate phase by physically deforming the active element of the source into a spiral, and a "phased-spiral" source, which uses an array of active elements, each driven with the appropriate phase, to produce the spiral wave front. Using finite element techniques, the fields produced by these sources are examined in the context of the spiral wave front beacon, and the advantages of each source are discussed.