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Dec 1986

Volume 80, Issue S1, pp. S1-S128

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back to top Session HHH. Underwater Acoustics XI: Scattering
Contributed Papers
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Some concepts in the statistical energy analysis (A)

G. Maidanik

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S127-S127 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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In a recent presentation an attempt was made to introduce basic concepts in the development of the elements of the statistical energy analysis (SEA). [G. Maidanik, J. Acoust. Soc. Am. Suppl. 1 79, 511 (1986)]. The analytical definitions and manipulations were exercised on a simple dynamic system so that they could not clutter unduly the conceptual understanding of SEA. Moreover, although SEA is devised to deal mostly with the dynamic behavior of interacting structural system, many of the basic concepts that underlie SEA can be explained and understood employing a single noninteracting dynamic system. The modal approach was used and the division of the energy stored in the dynamic system into a resonant and a nonresonant part was derived. The significance of this division was briefly discussed. This paper discusses more recent concepts such as the division of stored energy into a direct and a reverberant part.
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Sound scattering by fluid cylinders of finite length (A)

T. K. Stanton

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S127-S127 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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The scattering of incident plane waves by penetrable fluid finite‐length circular cylinders for all frequencies is described. The results appear to be valid for all frequencies for lengths much greater than the diameter of the cylinder and limited to low frequencies (ka ≪l) when the length is comparable to the diameter. By neglecting end effects, we are able to approximate the volume flow per unit length of the scattered field of the finite cylinder by that of the infinite cylinder. The solution is obtained by integrating this volume flow along the length of the cylinder. This approximation restricts the solution to geometries where the direction of the incident wave is normal or near‐normal to the lengthwise axis of the cylinder. The results are compared to those from other theories and to data. Under the same limiting conditions that the other theories were derived, the solution in this paper is identical to those. There was good to excellent agreement between this solution, when appropriately truncated to include just the first few modal terms, and lead cylinders and preserved shrimp. Finally a simple heuristic “high‐pass” model is also derived and shown to pass through the data and asymptotically approach the rigorous solution at extreme values of ka. [Work supported by ONR.]
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Necessary conditions for superresonant interaction between monopole scatterers (A)

I. Tolstoy

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S127-S127 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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Systems of precisely spaced bubbles, balloons, or air‐filled thin shells in water, in full spaces, or near elastic boundaries, insonified at frequencies close to the fundamental resonance value ω0 (“bubble frequency”) and interacting via multiple scatter may develop true resonant modes or superresonances (SRs). Under SR conditions the pressure amplification relative to the incident field will be of order (kRa)−2, as opposed to (kRa)−1 for single scatter at ω0, kR being the wavenumber in water and a the scatterer radius. For simple configurations this leads to theoretical SR amplifications between 103 and 5 × 103. It is shown here that, in order to observe the SR effect, spacings and volumes must be controlled to ½ or better. Typical pair spacings are of order ⅓ λacoust or (roughly) multiples thereof. When a bubble/balloon pair is near a thin plate the basic spacing is about one flexure‐wave wavelength and SR amplification becomes very sensitive to the direction of flexure mode arrivals, tending to vanish entirely for angles intermediate between broadside and endfire incidence. [Work supported by ONR.]
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Transient and steady‐state target resonance vibrations extracted from their acoustic echoes (A)

G. C. Gaunaurd and C. Y. Tsui

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S127-S127 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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Scattering experiments carried out decades ago [W. Angeloff and F. Abbot, NEL TR‐1273 (Mar. 1965)] are examined in the light of modern theoretical advances. This places the early observations on solid theoretical foundations and permits a reliable and understandable measurements program. This description is achieved by the suitable sampling and processing of the returned signals. It was found that pulses backscattered from targets are‐divided into three parts, an initial transient region, a steady‐state region, and a final transient regime corresponding to the re‐radiation of the stored energy by the structure. If the backscattered pulse is sampled in the steady‐state or free‐vibration region, one obtains the usual backscattering cross section of the body. However, if the sampling is done within the final transient regime, near the tail end of the pulse, then the “backgrounds” of the resonance scattering theory (RST) are automatically suppressed and one obtains, experimentally, the “spectrogram” of the body, with its modal resonances completely isolated. In this case, if one sweeps the angle θ around the body with the frequency fixed at the value of any resonance frequency, one then obtains “rosetta” patterns, which yield graphic multipole interpretations of the resonant modes excited. Theory and experiments match well, and we exhibit many examples.
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Nonlinear distortion of the bubble pulse in a layered inhomogeneous ocean (A)

David Epstein

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S127-S127 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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Many years ago, the author analyzed a representative deep‐fired long‐range underwater explosion waveform [J. Acoust. Soc. Am. 35, 800 (1963)]. The most striking feature was the so‐called “double shock” formation, e.g., a strong resemblance between the shock wave and bubble pulse in both amplitude and shape. The phenomenon is attributed to the combined effects of detonation depth and waveform propagation. In a previous paper the characteristics of an explosion waveform at the source, as function of shot depth was established [Proc. 11th ICA, Paris (1983), Vol. 1, pp. 345–348]. Weak shock theory was used to study nonlinear distortion of the bubble pulse as it propagates in an homogeneous, unbounded, dissipationless medium [Proc. 10th Int. Symp. Nonlinear Acoust., Kobe, Japan (1984), pp. 79–82]. It was found that the deepest fired shots suffer the greatest distortion but that, in general, extremely long range is required for shock wave formation. However, focusing due to inhomogeneous structure may substantially increase pulse strength thereby enhancing nonlinear effects. Here the effect of inhomogeneous layering on the range required for shock wave formation is investigated.
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Resonance identification through bistatic scattering for elastic spheres and spheroids (A)

H. Überall, M. F. Werby, S. H. Brown, and J. W. Dickey

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S128-S128 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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Free vibrations of a submerged elastic object, resonantly excited by an incident acoustic wave or wave train, occur in the characteristic fashion of one of an infinite series of allowed eigenvibrations (normal modes for objects of separable geometry) if the excitation takes place at the eigenfrequency of that resonance. While such resonances appear in the backscattering cross section plotted versus frequency, interfering with the contribution of specular echoes, only resonance frequencies and resonance widths can be determined from such a plot but not the resonance order. For targets of separable geometry it has been shown both experimentally [G. Maze and J. Ripoche, J. Acoust. Soc. Am. 73, 41 (1983)] and theoretically [M. F. Werby and H. Uberall, submitted to J. Acoust. Soc. Am.] that bistatic observations can determine the resonance order after subtraction of the specular background, and the dipole resonance of the Rayleigh wave has been theoretically identified in this way. The method is here applied to bistatic scattering from elastic spheroids, as analyzed by the T‐matrix formalism and computer code. [H. Überall is supported by the David W. Taylor Naval Ship Research and Development Center, and by the Office of Naval Research.]
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Amplitude and spectral characteristics of low grazing angle backscattering (A)

Paul D. Koenigs

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S128-S128 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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Acoustic backscattering from the sea surface at grazing angles in the region of 5 deg is affected by shadowing. Under these conditions the equations presented by Bass et al. [IEEE Trans. Antennas Propag. AP‐16 (5) (Sept. 1968)] may be combined to form a single equation. The agreement between experimental data near 1 kHz and the unified equation is good. The frequency shift of the backscattered acoustic energy is dependent on wave height and period through the horizontal orbital particle velocity. The frequency spread of backscattered energy is still dependent on the same particle velocity but is diminished because the wave troughs are preferentially insonified. The experimentally determined values of backscattering strength compare favorably with other reported low‐frequency, low grazing angle data.
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