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May 1990

Volume 87, Issue S1, pp. S1-S164

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back to top Session M. Engineering Acoustics III: Sound Interaction and Analysis
Contributed Papers
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Application of the transfer function method to acoustic measurements in a duct with flow (A)

Luc Mongeau and Donald E. Thompson

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S34-S34 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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The acoustic noise propagating as plane waves in a cylindrical duct with a mean flow was measured following a transfer function method [J. Y. Chung and D. A. Blaser, J. Acoust. Soc. Am. 68, 907 (1980)]. A special facility was used. The cylindrical duct consisted of two Plexiglas sections having different inside diameters connected by a streamlined fiberglass transition. The downstream end of the duct is connected to a fan, while an anechoic termination, screens, flow straighteners, and a bell‐mouth are located in that order at the upstream end. The facility ensures a very steady uniform pipe flow and enables the acoustic measurements to be carried out simultaneously in low‐velocity and higher velocity mean flows with a velocity ratio of 4. The maximum Mach number achieved in the smaller duct is around 0.08. Excellent agreement between the acoustic power measured in the large section and in the small section is achieved at all Mach numbers. This successful calibration entitles the facility to be used for a study of the importance of flow noise on the flush‐mounted microphones and the influence of a turbulent boundary layer, obtained by tripping the flow, on the accuracy of the acoustic intensity calculation. [Work sponsored by ONR.]
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Effect of a backing plate on the transfer function of a flow noise reduction coating (A)

Sung H. Ko

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S34-S34 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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A theoretical model was developed to evaluate the transfer function of an elastomer layer bonded to a steel plate of finite thickness. The elastomer layer is designed to reduce pressure fluctuations due to turbulent flow over the elastomer (or other types of excitation). The effectiveness of the elastomer layer in discriminating against flow noise depends on the transfer function of the composite structure. Theoretical analysis is made using the theory of elasticity and pertinent boundary conditions. Due to the backing structure, the transfer function is affected by the flexural vibration of the plate. The contribution of the flexural wavenumber to the transfer function is discussed and numerical results are presented.
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The damped acoustic pressure response of a thin diaphragm in close proximity to an arbitrarily shaped backplate (A)

Wallace R. A. George

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S34-S34 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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It is assumed that the damped behavior of the diaphragm is controlled by the motion of the gas in the space between the diaphragm and the neighboring backplate (i.e., the diaphragm to backplate distance is small but varying). The diaphragm could, in general, be a membrane type (as in typical capacitance microphones) or a plate type. The modeling is done for plate diaphragms, although the methods can, in general, be easily applied to membrane diaphragms. An explicit normal‐mode expansion solution for the nonlinear fourth‐order PDEs describing this plate‐diaphragm's motion is discussed. The expansion is carried out in terms of the normal modes of the unbacked diaphragm. The nonlinear coupled fourth‐order backspace pressure is derived in terms of the diaphragm displacement. The first explicit relations for the spatially dependent damped natural frequencies and associated damping factors for the diaphragm response are derived. The effect of the static gas‐film pressure and thickness and a criterion for truncating the infinite sum solution are discussed.
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Why acoustic sources are unique and how they can be identified from the fields they produce (A)

P. E. Doak

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S34-S34 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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The general Green formula solution of the inhomogeneous linear differential system for the general problem of acoustic motion in a uniform medium in an arbitrary space‐time domain is presented. “Acoustic sources” are thereby seen to be exclusively the specified inhomogeneous terms of the differential system. The problem solution is seen to exist and to be unique; formal existence and uniqueness are retained even when nonanalytic generalized function type terms are present. The acoustic source distribution and the field it produces are seen to be in a one‐to‐one, and thus unique, relationship. The complete set of two, mutually exclusive, types of source distributions, active and reactive, is identified. An active distribution can be precisely reconstructed from knowledge of its farfield. A reactive distribution produces a zero field everywhere outside the region in which it is nonzero. Explicit results for one‐dimensional examples are presented to provide some physical insight into why this one‐to‐one source/field relationship exists and how acoustic sources can be identified in practice.
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Active three‐dimensional sound‐field cancellation (A)

Jun Lan, Vijay K. Varadan, and Vasundara V. Varadan

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S34-S35 (1990); (2 pages)

Online Publication Date: 13 Aug 2005

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This paper introduces a new method for three‐dimensional active sound‐field cancellation. Analytical considerations include the problem of an incident plane wave interacting upon a rigid sphere as the scattering object. An array with elements consisting of a set of rings perpendicular to the sphere's pole axis is arranged on the surface of the sphere. This discussion will present an analytical technique for cancellation of the scattered sound field to arbitrary extent over either the whole space or the partial area of interest, as long as the number of the array elements is sufficient. Further considerations include a fixed and a variable incident plane‐wave direction with respect to the pole axis of the sphere.
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Longitudinal axial resonances in the form function for backscattering from a fluid spherical shell (A)

Steven G. Kargl and Philip L. Marston

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S35-S35 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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Reverberations of longitudinal waves between the inner and outer surfaces of a hollow shell can strongly affect the backscattering amplitudes at frequencies associated with a thickness resonance. The phenomena are studied for the idealized case of vanishing shear stresses by letting the material of the shell be an inviscid fluid; the surrounding fluid is water. An exact partial‐wave series (PWS) gives the form function f (θ,ka) and plots of ∣f(θ = π,ka)∣ display a structure from resonances. Here, ka = 2πa/λ is the size parameter, where a is the outer radius of the shell and λ is the wavelength in water of the incident acoustic wave. The resonant structure is also recovered in a geometrical calculation of f (π,ka), which sums the amplitudes associated with rays multiply reflected within the curved shell. The geometric synthesis demonstrates that the effects of curvature are essential to modeling f (π,ka). The analysis gives the geometric divergence factors of successive internal reflections. In addition to numerical comparisons with the PWS, the geometrical synthesis is tested by considering several limiting cases. These limiting cases correctly give results anticipated from elementary considerations. [Worked supported by ONR.]
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Ray synthesis of the form function for backscattering from hollow elastic spherical shells including leaky Lamb waves and longitudinal axial resonances (A)

Steven G. Kargl and Philip L. Marston

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S35-S35 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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A ray synthesis [P. L. Marston, J. Acoust. Soc. Am. 88, 25–37 (1988)] is employed for describing the form function f (θ = π,ka) for backscattering from a hollow, elastic spherical shell. The synthesis of f contains a component associated with a specular reflection fsp and contributions from leaky Lamb waves. The contribution fl, of the l th leaky Lamb wave is expressed in a Fabry‐Perot form. The present synthesis differs from previous results by including the effects of longitudinal axial resonances on fsp. A novel ray synthesis of fsp, which sums amplitudes associated with multiply reflected rays within the shell, indicates a significant resonance effect near the condition kt/h = nπ (n = 1,2,…,), where kL is the longitudinal wavenumber of the shell's material and h is the thickness of the shell. The ray synthesis shows that the curvature of the shell is essential to the modeling of longitudinal resonances. The ray synthesis of f is compared with the exact partial‐wave series representation for a 440c stainless steel shell where the inner‐to‐outer radii ratio is b/a = 0.838. The synthesis works well down to ka = 7. The first longitudinal resonance occurs at ka ≈ 77 and inclusion of its effect on fsp improves the ray synthesis of f in this region of ka. [Worked supported by ONR.]
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Critical angle scattering: An asymptotic series approximation (A)

Cleon E. Dean and Philip L. Marston

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S35-S35 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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The critical light scattering angle at 82.8° from an air bubble in water locates the transition from partial to total reflection from elementary geometrical optics (EGO). A physical‐optics approximation (POA) has been applied to analogous critical angle phenomena for sound reflected from a fluid sphere [P. L. Marston and D. L. Kingsbury, J. Acoust. Soc. Am. 70, 1488–1495 (1981)]. The present research gives a novel asymptotic series approximation for scattering amplitudes at the critical angle that is an improvement over both EGO and the POA. The series describes the breakdown of EGO at the critical angle: The leading correction to the scattering amplitude relative to the perfect reflection amplitude is found to be O[(ka)−1/4], where a is the radius of the reflector and the wavelength is 2π/k. A modified Watson transformation was applied to the exact Mie partial wave series for the case of light scattered from a bubble. A series was derived for the resulting integral special functions which is asymptotic for large ka [C. E. Dean, Ph.D. dissertation, Washington State University (1989)]. The series was confirmed by comparison (as a function of ka) with smoothed Mie computations for bubbles. It clarifies a relationship between scattered irradiance and bubble radius that has been used in optical studies of bubble dynamics and suggests how to improve the POA for critical angle reflection of sound. [Work supported by ONR.]
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Localization of scatterers by swept‐frequency imaging (A)

Kent E. Eschenberg

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S35-S35 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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A method has been developed that determines the location of a scatterer by exploiting the frequency dependence of the scattered field. Nearly any object will have a range of frequencies over which its scattered field is a simple function of frequency. An estimate of this function has been constructed and tested through simulation for scattering by a fiat plate and a pair of point sources. This algorithm has been incorporated into the swept‐frequency imaging method and exercised for a variety of noise‐free simulations. Results indicate that this method can offer high resolution when the scatterer and bistatic geometry are suitable, or poor resolution in other cases.
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Localization of sources by two pattern‐match methods (A)

Kent E. Eschenberg

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S35-S35 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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Two methods for the localization of sources were developed and applied to a simulation of point sources located near and behind barriers. Both methods are extensions of the concept of a “fictitious scatterer” and the approach used by acoustical holography and beamforming. The pattern‐match point of view leads directly to a straightforward, though computationally demanding, method for dealing with cases where the radiation passes through nonisotropic but known disturbances. A second variation of pattern match has been explored that uses the mismatch, rather than the match, between the receiver and processing signal. Results for noise‐free simulations of one and two point sources, in free space and near a barrier, indicate that these methods can extract information not available through holographic methods. Some situations may also lead to a number of false detections, indicating that the pattern‐matching methods may need to be combined with more robust imaging in actual application.
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Neural network classification of laser welds from acoustical signals (A)

Dave F. Farson, Kirk T. Kern, and Kenneth S. Fang

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S35-S36 (1990); (2 pages)

Online Publication Date: 13 Aug 2005

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A neural network was used to classify laser welds based upon their acoustical signatures. During laser welding of metal, a plasma (a region of ionized gas) is present above the surface of the material. Characteristics of the process cause this plasma to fluctuate over time, generating an airborne acoustical signal. In this work, the signals from laser welds were detected by a microphone, digitized in ⅙‐s time slices and Fourier transformed. The 2048‐point power spectrum of the transformed signal was input to backpropagation networks of variable size. It was possible to train the backpropagation networks to classify laser welds as full or partial penetration based upon their acoustical signature (in a full penetration weld, the molten zone fully penetrates through the thickness of the metal being welded). Furthermore, it was also found that backpropagation networks with two hidden layers could learn to classify weld signals more efficiently than single hidden layer networks. The ability to classify welds as full or partial penetration will be useful in real‐time process control algorithms to be developed in future work.
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Finite‐difference approximations for transient acoustic waves in a pipe (A)

Sanford Davis

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S36-S36 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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A new formulation using compact higher‐order finite differences is used to compute the propagation of a sound pulse in a pipe. The sound is generated by the arbitrary motion of a piston, and the pulse is free to radiate at its open end. After many reflections, conventional finite differences suffer severe phase distortion, which results in a scrambled signal. The new formulation is shown to maintain phase integrity. An expression for the open‐end impedance that is appropriate for transients is used to compute the farfield radiation.
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Comparison of four high‐precision pitch extraction methods (A)

Haixiang Liang and Ingo R. Titze

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S36-S36 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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The accuracies of four pitch extraction methods are introduced and compared for high‐precision perturbation measurement. They include combinations of peak‐picking, zero‐crossing, and waveform‐matching strategies. The effects of sampling frequency, signal‐to‐noise ratio, pitch frequency, and signal modulation on the accuracies are explored. It is shown that methods with the waveform‐matching technique give the best results for most cases. For near‐sinusoidal stimuli (e.g., EGG signals), the zero‐crossing technique gives higher accuracy than the peak‐picking technique. At a low signal‐to‐noise ratio, a higher sampling frequency does give better results for the waveform‐matching technique, but the accuracy of the peak‐picking method may reduce as sampling frequency increases. As the signal‐to‐noise ratio increases, the effect of sampling frequency on accuracy diminishes when interpolation is used. A variety of waveforms are currently being tested to check the suitability of these methods for different applications. [Work supported by NIH.]
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