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May 1984

Volume 75, Issue S1, pp. S1-S93

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back to top Session OO. Physical Acoustics V: Waveguides and Propagation
Contributed Papers
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Stability analysis of a Stokes boundary layer in a waveguide having slowly varying height (A)

Charles Thompson

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S77-S77 (1984); (1 page)

Online Publication Date: 12 Aug 2005

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The stability of an acoustic disturbance in a two‐dimensional waveguide having slowly varying height will be investigated. The enclosed fluid is assumed to be both viscous and compressible. It is shown that the dynamic behavior of the enclosed fluid can be parameterized by three small parameters, ϵ, 1/R, and 1/S, where ϵ is the ratio of the typical duct height H0 to the wall wavelength L0, 1/S is the ratio of the oscillatory particle displacement U0/ω to the typical duct height H0, and 1/R is the ratio of the oscillatory boundary layer thickness lv to the typical duct height H0. Special attention will be paid to waveguides with cross sections that are small compared to an acoustic and/or wall wavelengths. A stability analysis will be presented in the amplitude range where ϵ2R/S2  =  0(math).
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Propagation of axisymmetric free waves in a circular cylindrical shell (A)

D. H. Trivett

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S77-S77 (1984); (1 page)

Online Publication Date: 12 Aug 2005

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This paper presents the results of an investigation of free wave propagation in a circular cylindrical shell in vacuum, using the linear theory of elasticity. The study has resulted in identifying a previously unreported low‐phase velocity axisymmetric mode. This new mode is a low‐frequency branch of the first mode in a cylindrical shell and exists below the cutoff frequency of the second axisymmetric mode. The physical properties of the two low‐frequency branches are discussed and numerical results are presented along with preliminary data verifying the existence of the new mode. In addition, the results are used to demonstrate that present thin shell theory does not adequately describe the behavior of thin shells at low frequency.
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Some interesting modes of propagation in a large elastic sample (A)

M. Paul Hagelberg

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S77-S77 (1984); (1 page)

Online Publication Date: 12 Aug 2005

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The availability of a large sample of high‐quality steel with carefully finished surfaces has made possible time resolution of propagation modes that ordinarily overlap. The resolved echo trains clearly demonstrate several interesting modes of propagation. These observations serve not only to illustrate important features of wave propagation in an elastic medium but also to show the importance of recognizing these features when interpreting results from experiments performed in such media.
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Low‐frequency vibrational modes of fluid‐loaded thin spherical shells (A)

L. H. Green, Roger H. Hackman, D. H. Trivett, and L. Flax

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S77-S77 (1984); (1 page)

Online Publication Date: 12 Aug 2005

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The low‐frequency form function for plane‐wave scattering by a fluid‐loaded thin spherical shell is characterized by the high Q monopole or “bubble” resonance. As the shell thickness is increased, the monopole resonance is observed to shift to higher frequency and broaden (i.e., the Q of the resonance decreases). With a further increase in thickness, high Q resonances appear on top of the spectrum of the monopole resonance. These low‐frequency, high Q resonances, first reported by Diercks and Hickling [J. Acoust. Soc. Am. 41, 380–393 (1967)] have never been satisfactorily explained. In this paper we present the results of our investigation, based upon the linear theory of elasticity, of these high Q resonances. The dependence of the resonances on frequency, and shell and fluid parameters are presented along with the elastic stresses and displacements in the shell. Dispersion curves are generated from numerical solutions and a physical explanation for the strong coupling to the fluid medium is obtained.
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Acoustic surface wave measurements on live bottlenose dolphins (A)

W. M. Madigosky, G. F. Lee, F. Borkat, R. Kataoka, and J. Haun

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S77-S77 (1984); (1 page)

Online Publication Date: 12 Aug 2005

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Measurements of the velocity and attenuation of acoustic surface waves propagating on the outer surface of live bottlenose dolphins (Tursiops truncatus) were made as a function of frequency and location on four different animals. Two miniature accelerometers were attached to the skin and a surface wave was launched using an electromagnetic shaker driven by a noise source. A dual channel FFT spectrum analyzer and computer analyzed and computed the data [see J. Acoust. Soc. Am. 73, 1374 (1983)]. Surface wave velocities varied from 4 to 14 m/s over the frequency range 100–1000 Hz. The attenuation appeared to follow the α = Af1.0 law, where, A ≃ 1.5 dB‐s/m. Generally, the wave speed and attenuation were independent of the propagation direction (anterior, posterior, dorsal, or ventral) except near the dorsal fin. Additional measurements of the shear wave velocity were obtained on just the epidermis plus dermis section of the skin. These velocities were found to be two or three times higher and dependent on the direction of the propagation.
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Transient Rayleigh wave transmission across periodic surfaces (A)

Jacques R. Chamuel and Gary H. Brooke

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S77-S78 (1984); (2 pages)

Online Publication Date: 12 Aug 2005

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Sharp attenuation of 10‐Hz Scholte wave components has been observed in a region of the Canadian Arctic. The presence of shallow nearly periodic bottom ridges and surface features is believed to be the cause of the sharp attenuation of these low‐frequency components (G. H. Brooke). Experimental investigations using ultrasonic models (Chamuel) have been carried out to study the transmission of transient Rayleigh waves across periodic surface grooves. Attenuation, dispersion, and time delay effects have been measured and related to groove dimensions and spatial distribution. The energy of the first Bragg frequency component is partly transmitted over a wide time window. Strong low‐frequency components with a wavelength equal to 4 times the spatial wavelength of the grooves are transmitted. The effects of the number of grooves, and the groove depth and spacing on filtering and delaying the transmitted Rayleigh wave pulse will be described. The propagation of broadband transient Scholte waves across periodic surface grooves and ridges is being investigated. [Work sponsored by DREP.]
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Impedance formulation and clipping technique in the fast field program (A)

R. F. Richards, S. W. Lee, N. Bong, and Richard Raspet

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S78-S78 (1984); (1 page)

Online Publication Date: 12 Aug 2005

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The propagation of a sound wave in a layered media excited by a point pressure source can be formulated into a Fourier integral. A powerful method for evaluating this integral is to use the Fast Fourier Transform, resulting in the so‐called “Fast Field Program (FFP)” [F. R. DiNapoli and R. L. Deavenport, J. Acoust. Soc. Am. 67, 92–105 (1980)]. In the existing scattering matrix formulation, the FFP requires the multiplication of matrices containing exponential factors exp (+γh) and exp (−γh), where γ is the attenuation constant along the vertical direction of a layer, and h is the layer's thickness. These factors often exceed the computer's capability in handling large/small numbers, thus resulting in erroneous FFP solutions. In the present paper, we describe a new formulation of the FFP that is inherently numerically stable and is completely free from the difficulty mentioned above. The central step in our formulation is to calculate the equivalent impedance for each layer in succession starting from the top/bottom layers toward the source. This technique results in terms containing (γh) rather than (−γh), which goes to unity smoothly as γh → ∞. In addition, we introduce a “clipping technique.” For a given horizontal wavenumber, it removes layers that are not physically significant by terminating the preceding layer at its characteristic impedance.
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Interface wave mode propagation in clad rod acoustic waveguides (A)

Susan J. Hanna and Richard O. Claus

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S78-S78 (1984); (1 page)

Online Publication Date: 12 Aug 2005

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The propagation of acoustic waves on the cylindrical boundary between the core and cladding materials of a clad rod waveguide is described. Typically, axisymmetric torsional, axisymmetric radial‐longitudinal, and core‐guided shear modes may propagate within a rod of cylindrical cross section if the velocity of plane shear waves in the cladding material exceeds the velocity in the material of the core [R. N. Thurston, J. Acoust. Soc. Am. 64, 1 (1978)]. If instead the elastic constants of the core and cladding materials of the rod are reversed so the material with the slower shear wave speed is on the outside, no modes are supported within the core but an interface wave can exist on the core‐cladding boundary. In this paper the transmission properties of an all glass clad rod with a suitable core‐cladding elastic constant relationship to support non‐attenuating interface waves are discussed. The resulting improved freedom from spurious responses allowed by this single mode operation is discussed. [Work supported by NASA.]
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Tangential component of an acoustic surface pulse (A)

Richard O. Claus

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S78-S78 (1984); (1 page)

Online Publication Date: 12 Aug 2005

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Measurements of the in‐plane components of the surface particle displacement caused by a simulated Heavyside step function load source on the same surface of a large glass block are described and compared with theory. These components were measured by attaching a small rectangular solid crystal with reflecting surfaces to the block and observing the tangential motions of the normal surfaces of the crystal using stabilized optical interferometry. Although the bandwidth of the detected signals is limited by the length of the crystal compared to an acoustic wavelength, and the observable motions are influenced by the mass loading of the surface by the crystal, good agreement between theory and measurements is obtained. These data combined with similar optical measurements of the normal component [F. R. Brekenridge, C. E. Tschiegg, and M. Greenspan, J. Acoust. Soc. Am. 57, 626 (1975)] yield full localized three‐dimensional displacement information. [Work supported by NASA.]
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Mode shapes and propagation characteristics for waves propagating in nonuniformly lined ducts (A)

P. O. Vaidya

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S78-S78 (1984); (1 page)

Online Publication Date: 12 Aug 2005

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It is well established that the sound propagation in a duct, lined in a nonuniform fashion, either in the circumferential or the axial direction, cannot be described by means of separable modes. However, under these circumstances nonseparable modes can be used. In this paper it is shown that for some specific nonuniform boundary conditions, closed form solutions can be obtained. The mode shapes of the nonseparable modes have been obtained. It has been shown that these modes can be used to convert energy from lower modes to higher modes, which are easier to attenuate.
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The propagation of spinning modes through nearly choked inlets (A)

P. G. Vaidya

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S78-S78 (1984); (1 page)

Online Publication Date: 12 Aug 2005

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It is well known that sound cannot propagate upstream against a supersonic flow. However, even at subsonic speeds, in excess of the Mach number of 0.6, considerable attenuation has been observed in practice. To explain this, an equation, for the propagation of spinning modes in ducts carrying flow with axial and radial gradients, has been derived. The equation has been used to show that when a wave passes through a convergent‐divergent nozzle, the net result is an attenuation of sound. Alternative mechanisms for the sound reduction in practical flows are also discussed.
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