bluuebunny/crossref_metadata_2025 huggingface dataset:
Neodymium iron boron and professional audio loudspeakers
Authors: Warren Daniel M.
Published: October 2005

Neodymium iron boron, a high reminence, high coercivity permanent magnet material, has been in use in the audio industry for several years in the form of small (approximately 1-cm-diam) disks in high-performance microphones. Dropping prices and continuing enhancements in material properties, reducing thermal demagnetization and increasing residual magnetic flux density, have made neodymium more attractive for use in professional audio loudspeakers, where magnent size can reach 4 in. in diameter and almost 1/2 in. thick. While still considerably more expensive than the more commonly used ferrite ceramic magnets, neodymium magnet structures can be smaller and have a higher flux density than can be practically attined with ceramics. A specific example of loudspeaker motor structure design using neodymium—the Architectural Acoustics Neo Series Acoustical Components from Peavey Electronics—will be presented. Acoustical implications of high-force loudspeakers on enclosure design will also be discussed. [Work supported by Peavey Electronics Corporation.]
Near-field and far-field measurments of a ribbon tweeter/midrange
Authors: Butler Alexander L. , Butler John L.
Published: October 2005

It is common practice to make near-field pressure measurements on woofers as a means for avoiding low-frequency reflections from surroundings which can occur during far-field measurements. There is also a problem with making far-field measurements on ribbon tweeters. This is due to the long length of the ribbon compared to the wavelength which results in an extended near-field distance in the higher-frequency region and the requirement of a very distant far-field microphone. As a result of this measurement problem, a technique has been developed for near-field measurements of ribbon tweeters. This technique is based on the Helmholtz integral equation evaluated on the axis of the tweeter. In the case of a rigid baffle only the pressure gradient measurements are needed over the surface of the ribbon. This paper will present the results of measurements of the pressure gradient as well as the pressure distribution in the vicinity of the ribbon and present a simple technique for predicting the far-field from these near-field measurements.
Improvement of sound quality through accurate acoustic diagnostic boundary element analysis
Authors: Trevelyan J. , Adey R. A. , Niku S. M.
Published: October 2005

The boundary element method (BEM) has long been used effectively in the prediction of sound-pressure levels in acoustic cavities and surrounding vibrating bodies. While this is an important capability in the design of structures for noise constraints, it does not point the engineer to the appropriate remedial action if the noise is found to be excessive. This paper presents a discussion of ‘‘acoustic diagnostic analysis,’’ which is an analytical extension to the BEM which traces the source of any noise to its root cause. The impact of a design change can be seen immediately through the use of sensitivity computations, which clearly show the most effective modifications which can be made for noise reduction. The technique works by calculating the contributions made by each portion of the surface area to the sound-pressure level at an internal point. This allows an accurate evaluation of the contribution made from each part of the panel to the noise experienced at that point. The paper discusses some issues involved in performing the boundary element analysis for sound-pressure level prediction and calculating the panel contributions accurately. The paper also describes recent extensions to the method which allow acoustic diagnostic analysis of multidomain problems, and also how more accuracy can be obtained for problems including sound absorbing materials. Both of these developments have important consequences, in particular in automotive noise reduction. A test example shows how the technique locates the structural panels which are the major contributors to a noise problem, and which are therefore the panels which offer the most noise reduction effect for a given remedial action.
A numerical approach to scattering by many scatterers of various sizes
Authors: Imhof Matthias G.
Published: October 2005

A numerical method is derived for solving wave fields in the presence of many scatterers with different sizes. Three different scattering regimes are distingished: a≪λ, a<λ, and a≳λ, where λ denotes the wavelength and a is the maximal diameter of the particular scatterer. In the case of a≪λ, the Rayleigh approximation can be used where the scattered wave field hardly depends on the geometry of the scatterer. For a≳λ the scattered fields are calculated using MMP expansions [M. G. Imhof, J. Acoust. Soc. Am. 97, 754–763 (1995)], which yields the full waveform solution by evaluating the boundary conditions. Scatterers with a<λ are too large for the Rayleigh approximation to hold or for the scattered field to be independent of the geometry. On the other hand, the scatterers are too small to justify the MMP approach. Thus, the scattered fields are expanded into a series of Hankel functions where only the first few orders are used. Their weights are found in the least-squares sense by Galerkin’s method. As an example, results are presented for many elliptical scatterers of different sizes.
Finite element models for sound absorbing materials
Authors: Bolton J. Stuart , Kang Yeon June
Published: October 2005

Many noise control treatments feature porous materials that are either fibrous or foamlike. Analytical methods are available to model the behavior of these materials in simple geometries: e.g., infinite planar or cylindrical geometries. Realistic noise control treatments, however, are of finite size, are often multilayered, and may have textured surfaces. If treatments of that type are to be designed optimally, numerical models of the materials must be used. Note also that when a surface is nonlocally reacting, the sound fields within the treatment and in the adjacent space must be solved for simultaneously. For both these reasons, a number of finite element formulations have been developed to model sound absorbing materials. The first models treated the absorbing medium as an effective fluid, and were used to model extended reaction fibrous materials. More recently, finite element models for elastic porous materials, i.e., foams, that are based on the Biot theory have been developed. The main features and capabilities of these various finite element models will be discussed in this presentation. In addition, a number of examples of their use will be given: e.g., the optimal design of a foam wedge for sound absorption and transmission control.
An improved iterative Born technique for ultrasound mammography
Authors: Liu Lujiang , Broschat Shira L.
Published: October 2005

In ultrasound mammography the objective is to image tumors about 4 or 5 mm in size. Thus the resolution of the reconstructed object profile is critical. Because of the strong refraction caused by fat lobes in the breast, simple pulse echo methods are inadequate, and a full inverse imaging technique is needed. The iterative Born method is an inverse technique that has been used in ultrasound imaging. However, the calculation cost is very high, and parallel computation is limited to the forward problem. In this work, a method is presented that improves the performance of the iterative Born approach, giving better resolution at less cost. The dependence between the incident source angle and the reconstructed object profile is explored, and an algorithm is presented that is realizable on a parallel computer for both forward and inverse calculations. Four single sources are evenly spaced around the object to be imaged and are used to reconstruct the object simultaneously; the results from the four separate sources are then averaged. The cost of this method is 20% less than the iterative Born method. In addition, this method is robust to noise since averaging helps to cancel noise. [Work supported by NSF.]
Development of a portable, focused-beam ultrasonic scanner for the NDI of adhesively bonded aircraft fuselage skin structures
Authors: Patton Thadd C. , Hsu David K.
Published: October 2005

In keeping with the requirements of the air carrier maintenance community, a closed-cycle, water-coupled, focused-beam ultrasonic method for the NDI of adhesively bonded aircraft fuselage structures has been developed. This approach, known as the ‘‘dripless bubbler’’ technique is the combination of focused-beam immersion ultrasonics with protable ultrasonic scanners. Because a focused ultrasonic beam is used during the scan, the spatial resolution is much greater than that obtainable with conventional flat contact transducers. The improved spatial resolution is necessary for the detection of localized corrosion pits and surface roughness associated with active corrosion sites in aluminum skin structures. The dripless bubbler allows for the capability to ultrasonically scan the exterior of an aircraft over surface protrusions such as button-head rivets in any orientation without the problem of uncontained couplant water. Recently the dripless bubbler has been involved in a technology deployment and transition program as part of the FAA-National Aging Aircraft Research Program. The result of this technology transfer will be a fully functional prototype scanner incorporating the dripless bubbler technique for rapid, large area NDI of aircraft bonded structures. Results will be presented on the development history of the dripless bubbler along with its current design and verification process. [Work supported by FAA.]
Optical detection of ultrasound with a high-power pulsed laser
Authors: Nakano Hidetoshi , Matsuda Youichi , Shin Shoichirou , Nagai Satoshi
Published: October 2005

An optical system has been developed for detection of laser-generated ultrasound on rough surfaces at a distance over 1 m. A long pulse Nd:YAG laser is used to prove samples. The laser provides a light pulse of 60 μs in duration and 60 mJ in energy with a single frequency. The scattered light from the sample is focused into a multimode fiber and introduced to the modified confocal Fabry–Perot interferometer which has a wave plate in the cavity. The wave plate gives the different optical lengths of the cavity depending on the beam polarization. By adjusting the difference of the optical lengths, the ultrasound signals detected from the orthogonally polarized beams can be out of phase. Taking the difference of the signals produces the valance output which is less sensitive to intensity modulations of the pulsed laser. Longitudinal and shear waves have been observed in several metallic samples without signal averaging.
Harmonic generation measurements as a function of frequency in a carbon-fiber-epoxy-resin composite
Authors: Elmore Paul , Breazeale M. A.
Published: October 2005

Harmonic generation in a carbon-fiber-epoxy-resin composite has been measured as a function of frequency between 4.0 and 8.5 MHz. The amplitudes of both the fundamental and the generated second harmonics were measured absolutely with a capacitive receiver at room temperature. Phase velocities also were measured. Dispersion is seen in the conventional nonlinearity parameter, but not in the phase velocity. The composite sample is highly attenuative and may have additional nonlinear terms that are not yet accounted for theoretically. Since the conventional nonlinearity parameter is defined for a lossless solid, data interpertation is unclear at present.
Numerical study on particle velocity and sound pressure by circular flat transducers
Authors: Imamura Tohru
Published: October 2005

During years of investigation on the ultrasonic near field, only the sound pressure, namely, the spatial distribution of velocity potential, has been studied. In this report, particle velocity and acoustic impedance density of the ultrasonic field by circular flat transducers are derived and computed together with sound pressure. Sound pressure is proportional to the velocity potential of the ultrasonic field. Its particle velocity is the space differential of the velocity potential, and the acoustic impedance density is the quotient of the sound pressure by the particle velocity. On the axis of the transmitting circular flat transducer, the phase delay of the sound pressure has peculiar leaps. But, acoustic impedance density has constant leaps from −π/2 to π/2, where the amplitude is zero. The mean value over a receiving coaxial circular flat transducer is also computed changing the ratio of the radius (a) of the circular flat transducer to the wavelength (λ) of the ultrasonic wave. Mean amplitudes of sound pressure, particle velocity, and acoustic impedance density are tabulated with the normalized distance (zλ/a2) in the computing precision of 0.1%. The mean amplitude of the z component of the particle velocity is always less than 1.0 and seems to be an appropriate response for the ultrasonic system of a pair of circular flat transducers.
Numerical investigations of modified Greenspan viscometers
Authors: Mehl James B.
Published: October 2005

Modified Greenspan viscometers are being studied with the objective of better understanding of inertial and dissipative duct-end effects for application to Greenspan viscometers. Greenspan viscometers are double Helmholtz resonators comprised of a circular duct coupling two concentric cylindrical chambers. Previous investigations [J. B. Mehl, J. Acoust. Soc. Am. 97, 3327 (1995)] of the acoustic fields and eigenfrequencies of viscometers with ducts terminating flush with the chamber ends have been extended to ducts which extend into the chambers a distance D. Numerically determined end effects are compared with analytic results for radiation from baffled and unbaffled open ducts. When D=0 the inertial end correction agrees well with the correction for a baffled open end; when D is greater than a few duct diameters it approaches the value for an unbaffled termination. In contrast, the resistance of the duct end (defined for the limiting case of a small viscous penetration length) is not sensitive to D. The numerical model permits easy investigation of the effects of rounding and chamfering corners. [Work supported by the Office of Naval Research.]
Speech intelligibility prediction from calculated C50 values
Authors: Marshall L. Gerald
Published: October 2005

The measured 50-ms early-to-late sound energy ratio, C50, is one of the indices used to quantify speech intelligibility in enclosed spaces. A calculated C50 could be useful for predicting intelligibility in hypothetical situations for both unamplified speech and speech reinforcement systems. The equation used for the C50 calculations may also be used for source-directivity evaluations.
Internal waves and tides in shallow water
Authors: Plueddemann Albert J. , Lynch James F.
Published: October 2005

The well-known Garrett and Munk description of the internal wave spectrum synthesizes many observations into a spectral model that reproduces the general features of the mid-latitude, deep-ocean internal wave field. The general applicability of this ‘‘kinematic’’ spectral model is related to the dynamic balance of the internal wave field, but the model itself includes no dynamics and is meant to represent only the average or steady-state spectrum, away from the direct influence of sources, sinks, or boundaries. In shallow water, the kinematic model is typically not appropriate, since some or all of the simplifying assumptions about the wave field may be invalidated. Variable topography, strong tidal currents, fronts, and ice cover (at high latitudes) are the principal reasons for these complications, and result in an internal wave field which varies both spatially and temporally from the steady-state kinematic description. The most straightforward way to address the inadequacy of steady-state models for shallow water acoustics experiments is direct observation of the internal wave field at the site. Profiles sufficient to define the ‘‘mean’’ or background density gradient along with time series of temperature and horizontal velocity are the most common observations.
Upcoming field experiment examining media effects on synthetic aperture sonar—Experimental considerations and preliminary design
Authors: Williams Kevin L. , Ewart Terry E. , Henyey Frank S. , Rouseff Daniel , Reynolds Stephen A. , Grochocinski James
Published: October 2005

Ocean variability imposes limitations on performance of synthetic aperture sonar (SAS) systems. An experiment to examine these limitations will be carried out in conjunction with the Coastal Mixing and Optics program. The talk begins with a description of the environment anticipated, based on historical data. Using this environmental information, the effects of refraction, internal waves, and turbulence are shown. Acoustic wavefront curvature caused by internal waves is a primary concern. These results have driven the design of the experimental apparatus. An overview of this apparatus is presented. [Work supported by ONR.]
The internal waves effect on the vertical noise directionality in shallow water
Authors: Yang T. C. , Tielbuerger Dirk
Published: October 2005

Acoustic environment is known to influence the vertical directionality of the ambient noise. For example, in shallow water with a downward refractive (summer) sound-speed profile, the noise vertical directionality is expected to show a deep (≳10 dB) null in the horizontal direction for wind-generated noise below 1 kHz. This ‘‘horizontal null’’ is a consequence of the fact that low-order modes are weakly excited by the noise sources which are located near the ocean surface. This phenomenon has been experimentally observed and is not expected to occur in an upward refractive (winter) sound-speed profile environment. It is noted, however, that when internal waves are present, the deep null may disappear or sufficiently weaken due to the ‘‘strong’’ mode coupling effect caused by the scattering of the noise field from the internal waves. In the internal wave fields, the conversion of higher-order modes (generated by the surface noise sources) to low-order modes by the internal waves can fill in the deep (≳10 dB) null which would otherwise be expected (theoretically). Numerical calculations of the vertical noise directionality at <500 Hz for typical shallow water environments with rock (low loss) and sediment (lossive) bottoms are presented and the potential effects of internal waves on the ambient noise directionality are investigated. Practical situations are discussed for which the smearing of the ‘‘horizontal null’’ in the noise vertical directionality can be experimentally verified. a)On leave from Forschungsanstalt der Bundeswehr fur Wasserschall-und Geophysik, Klausdorfer Wef 2-3=24, 24148 Kiel, Germany.
Time-domain modeling of finite-amplitude sound in relaxing fluids
Authors: Cleveland Robin O. , Hamilton Mark F. , Blackstock David T.
Published: October 2005

Two similar time-domain computer algorithms are described, one that solves an augmented Burgers equation and the other an augmented KZK equation. Both codes include the effects of nonlinearity, absorption, and dispersion (both thermoviscous and relaxational). The Burgers code also accounts for geometrical spreading and inhomogeneity of the medium. The KZK code accounts for diffraction in directive sound beams [Lee and Hamilton, J. Acoust. Soc. Am. 97, 906–917 (1995)]. The novel feature of the codes is that they run exclusively in the time domain, even for the calculation of absorption and dispersion due to multiple relaxation phenomena. For a plane step shock in a monorelaxing fluid, the results from the Burgers code are compared with an analytical prediction of Polyakova etal. [Sov. Phys. Acoust. 8, 78–82 (1962)]. Numerical results are also compared with frequency-domain calculations for a plane harmonic wave in a thermoviscous, monorelaxing fluid. Both comparisons reveal excellent agreement. New results include calculations showing the effect of relaxation on the propagation of a pulsed, diffracting, finite-amplitude sound beam. [Work supported by NASA, the Office of Naval Research, and the IR&D Program at ARL.]
Finite amplitude sound beam propagation in dense gases
Authors: Cox Edward A. , Kluwick Alfred
Published: October 2005

Current studies indicate that fluids with high specific heats may prove useful in a number of technological applications. These fluids have been shown to exhibit a range of new phenomena—the most prominent example probably being the existence of negative shocks (shocks across which the normal stresses decrease). Of particular theoretical interest is the possibility of propagating waves with mixed nonlinearity where for example positive and negative shocks propagate as stable structures within a single wave pulse. Here existing results for essentially plane waves are extended to include the effects of signal variations in directions transverse to the propagation direction. An extended Khoklov–Zabolotskaya–Kuznetsov (eKZK) is derived which models a propagating finite-amplitude sound beam. The extension to mixed nonlinearity predicts the formation of postitive and negative shocks and the existence of sonic shocks (where the shock speed is equivalent to the convected sound speed upstream or downstream of the shock). Numerical results are presented illustrating the combined effects of nonlinearity and diffraction for both time harmonic and pulsed radiators. An analysis of these results is given in terms of propagating characteristic surfaces.
Underwater explosion shock wave due to a buried charge
Authors: Powell Alan
Published: October 2005

The explosive removal of off-shore oil platform structures has an environmental impact on marine life because of the shock waves from the buried charges. For an in-water detonation the similarity parameter (W1/3/R) is well known (W= charge weight, R=radius to observation point), with peak pressure pmax= const.×(W1/3/R)α, α=attenuation coeff.≊1.13 (Arons, 1954). Combining the similarity method with ray theory gives, for a point distance a above the bottom due to a charge buried distance b below it, pmax=const.×(W1/3/R)ε{(a+b)/b}ε−α with ε=bottom attenuation coeff. Now pmax is a function only of (W1/3/R) only on horizontal planes (a+b)/b= const.; moreover for given R, the pressure increases with height a above the bottom. Full-scale measurements by Connor (1990) confirm these findings, also yielding ε≊1.99. The pressure now depends more acutely on charge weight, pmax∼W2/3; but more weakly on burial depth than pmax∼1/b2.6 as sometimes assumed, e.g., in deep water a&R≫b, pmax∼1/b0.86. Constant peak pressure contours follow R=const.×cos(ε−α)/αθ, θ=angle to vertical.
A powerful acoustical source for seismology
Authors: Lebedev Andrey V. , Sutin Alexander M.
Published: October 2005

To get information about the ‘‘entrails’’ of the Earth it is necessary to have a powerful source of seismic waves. Standard equipment for this purpose is too heavy and rather unstable. In this work the possibility of powerful electrodynamic hydroacoustic transducer usage for seismology is investigated theoretically. To match the transducer with earth, a tube containing water is used. The hydroacoustical transducer is placed inside the tube. This transducer produces waves that are transformed to seismic waves. The open end of the tube is a monopole-type source of primary compression waves. This source is a resonant one, and, according to calculations, the Q factor is of the order 10. Working resonance frequency of this system is defined by h≂λ0/2 condition, where h is the height of the tube, and λ0 is the wavelength of sound waves in water filling the tube. Such design of the source permits the exclusion of Rayleigh wave generation which is very important for volume tomography of the Earth. Varying the height of the water column can easily tune the frequency of radiation.
Interfacial waves in a finitely strained layered elastic half-space
Authors: Sotiropoulos Dimitrios A. , Sifniotopoulos Christoforos G.
Published: October 2005

Propagating and standing interfacial waves between a surface layer and an underlying half-space, both under finite strain, are examined. The media are compressible nonlinear elastic and homogeneously pre-strained with their principal axes of pre-strain aligned, one axis being normal to the planar interface. For arbitrary strain energy functions and propagation along a principal axis of pre-strain, the dispersion equation is obtained. A low-frequency wave speed is subsequently obtained in explicit form yielding nonpropagation parameter conditions which for a specific state of stress hold at any frequency. The high-frequency limit of the dispersion equation yields the secular equation for interfacial waves between two half-spaces. It is then found that equal-density compressible materials may allow propagation and that compressible materials with equal shear wave velocities parallel to the interface may filter interfacial waves, even under isotropic in-plane stretching. For an arbitrary layer thickness as compared to the wavelength, material and pre-strain parameter conditions are also derived for the existence of standing waves as solutions of the bifurcation equation, a limiting case of the dispersion equation.
Local temporal variance of Wigner’s distribution
Authors: Hughes David H.
Published: October 2005

Lamb wave resonances on submerged spherical shell are analyzed via the local temporal variance of the signal Wigner distribution. The local resonance widths are obtained by approximating the local resonance contribution to the farfield backscattering form function of a particular Lamb wave as a resonance of the single Breit–Wigner type. The half-width of the resonance is approximately proportional to the inverse of the square root of the local temporal variance evaluated at resonance. This discrete distribution is compared against the continuous half-width distribution as rendered by the ray acoustic approximation for the particular Lamb wave under study. [Work supported by ONR.]
The effect of internal waves on rays that turn sharply in deep and shallow water
Authors: Henyey Frank S. , Ewart Terry E. , Reynolds Stephen A. , Macaskill Charles
Published: October 2005

Conditions for which an acoustic ray turns in a strong thermocline occur in both shallow and deep water. Internal wave induced fluctuations in the acoustic propagation are expected to be particularly large for such rays. A shallow-water example is the Synthetic Aperture Sonar experiment of summer, 1996, modeled with historical data for the site. A deep-water example is the upper path at the MATE experiment done a number of years ago. Conditions of validity of the Markov approximation are unlikely to hold under such conditions. In particular, the range derivative of the correlation length tangent to the ray greatly exceeds unity for the MATE upper path. Other approximations may also be inaccurate. A program to investigate standard approximation schemes for the MATE upper path has begun, whose purpose is to try to improve theories for such conditions, and to serve as a comparison case for the shallow-water experiment. Various statistical quantities are to be calculated in simulations, and compared to the same quantity calculated with standard approximations.
Modal sensitivity of acoustic interactions with internal waves in shallow water
Authors: Zhou Ji-Xun , Zhang Xue-Zhen
Published: October 2005

Analyses of individual acoustic mode interactions with internal waves should permit both qualitative and quantitative understanding of the influences of internal waves on multimode signals in shallow water. In this report, some experimental results taken in the Yellow Sea during the summer with strong thermoclines, will be reported. Numerical simulations will be used to analyze the potential internal wave interactions suggested by these data. (a) A multimode signal obtained at a single hydrophone and the first acoustic mode obtained by a mode filter array, both from a fixed 442-Hz cw source, exhibit very different fluctuation characteristics. (b) The transmission loss (TL) measured with explosive sources indicates a strong source/receiver depth dependence. (c) TL is, generally, anisotropic and can be abnormally large over some frequency range. It has been shown that the acoustic mode-coupling induced by internal waves can be an important loss mechanism for sound propagation during the summer [Zhou etal., J. Acoust. Soc. Am. 90, 2042–2054 (1991)]. The observed fluctuations can be explained in terms of the propagation characteristics of individual modes to the presence of internal waves. In so far as the mode-coupling and both the intensity and arrival time fluctuations are concerned, certain acoustic modes are more sensitive to internal waves than others. The characteristics of acoustic-internal wave interaction strongly depend on signal frequency, thermocline profile, seabottom properties, source/receiver depths and distance. [Work supported by ONR.]
Modeling the temporal coherence of pulse propagation in shallow water with internal wave and rough boundary scattering
Authors: Tappert F. D.
Published: October 2005

For fixed source and receiver geometry, the broadband UMPE ocean acoustic model is used to make Monte Carlo predictions of pulse propagation in shallow water with internal wave and rough boundary scattering. The internal waves are modeled with a broad spectrum of excitation, and evolve slowly in geophysical time tg according to their intrinsic dynamics. For each realization of a field of internal wave fluctuations, assumed frozen at time tg, a time series of complex demodulates of the received acoustic signal is computed by Fourier synthesis. The square modulus of this function yields the intensity envelope time series, A(t), and many peaks of A(t) correspond to ray arrivals and travel times that are spread by scattering. By advancing tg and repeating the pulse transmission simulation, the function A(t,tg) is computed and displayed as a ‘‘waterfall’’ plot. Examination of these plots gives quantitative information about the temporal stability and coherence of multipath arrivals in shallow water. Examples are presented in three distinct geographical areas: Florida Strait, New Jersey Shelf, and the Yellow Sea. [Work supported by ONR.]
Evidence for coastal generation of internal tides from deep-water acoustic observations
Authors: Flatté Stanley M. , Bracher Charles
Published: October 2005

In the summer of 1989 a 1000-km acoustic experiment in the Pacific showed a semidiurnal variation in pulse travel time that was coherent in depth and varied systematically across timefronts. The effect of a single-plane-wave, gravest mode, M2, internal tide has been examined by ray tracing and has been found to show the same characteristic variation if the internal tide is coming from the direction of the Gulf of Alaska. In the process of fitting this specific acoustic data, an analysis technique for low-mode internal tides has been developed that can be applied generally to any acoustic tomography experiment.
Acoustic mode coupling induced by internal wave fields in shallow water
Authors: Finette Steven , Tielbuerger Dirk , Wolf Stephen
Published: October 2005

The problem of the propagation of acoustic waves in a stochastic ocean waveguide, for which the sound-speed variability within the water column is treated explicitly as a random variable, is addressed. The sound speed is composed of a deterministic, time-independent profile and two time-dependent stochastic components representing a (linear) background Garrett–Munk internal wave field and (nonlinear) internal wave soliton packets. A high-angle elastic parabolic equation method is used to compute single-frequency realizations of the pressure field using this representation of the sound-speed fluctuations. Transmission loss and scintillation index measures are estimated for both the full field and its modal decomposition at various ranges from the acoustic source, for different source depths and for both flat and sloping bottoms. These measures are incorporated in the analysis of acoustic modal coupling induced by the internal wave fields as a function of range; results support a recent prediction [D. Creamer, submitted to J. Acoust. Soc. Am.] that the scintillation index increases exponentially with range due to the competition between mode coupling and mode stripping found in shallow water waveguides. a)Permanent address: Forschungsanstalt der Bundeswehr für Wasserschall-und Geophysik, Klausdorfer Weg 2-24, 24148 Kiel, Germany.
Time discrimination in a monotonic, isochronous sequence
Authors: Friberg Anders , Sundberg Johan
Published: October 2005

In acoustic communication timing seems to be an exceedingly important aspect. The just noticeable difference (jnd) for small perturbations of an isochronous sequence of sounds is particularly important in music, in which such sequences frequently occur. This article reviews the literature in the area and presents an experiment designed to resolve some conflicting results in the literature regarding the tempo dependence for quick tempi and relevance of music experience. The jnd for a perturbation of the timing of a tone appearing in an isochronous sequence was examined by the method of adjustment. Thirty listeners of varied musical background were asked to adjust the position of the fourth tone in a sequence of six, such that they heard the sequence as perfectly isochronous. The tones were presented at a constant interonset time that was varied between 100 and 1000 ms. The absolute jnd was found to be approximately constant at 6 ms for tone interonset intervals shorter than about 240 ms and the relative jnd constant at 2.5% of the tone interonsets above 240 ms. Subjects’ musical training did not affect these values. Comparison with previous work showed that a constant absolute jnd below 250 ms and constant relative jnd above 250 ms tend to appear regardless of the perturbation type, at least if the sequence is relatively short.
Localization of real and virtual sound sources
Authors: Bronkhorst Adelbert W.
Published: October 2005

Localization of real and virtual sound sources was studied using two tasks. In the first task, subjects had to turn their head while the sound was continuously on and press a button when they thought they faced the source. In the second task, the source only produced a short sound and the subjects had to indicate, by pressing one of eight buttons, in which quadrant the source was located, and whether it was located above or below the horizontal plane. Virtual sound sources were created using head-related transfer functions (HRTFs), measured with probe microphones placed in the ear canals of the subjects. Sound stimuli were harmonic signals with a fundamental frequency of 250 Hz and an upper frequency ranging from 4 to 15 kHz. Results, obtained from eight subjects, show that localization performance for real and virtual sources was similar in both tasks, provided that the stimuli did not contain frequencies above 7 kHz. When frequencies up to 15 kHz were included, performance for virtual sources was, in general, poorer than for real sources. Differences between results for real and virtual sources were relatively small in the first task, provided that individualized HRTFs were used to create the virtual sources, but quite large (a factor of 2) in the second task. The differences were probably caused by a distortion of high-frequency spectral cues in the HRTFs, introduced by the probe microphone measurement in the ear canal.
Effects of panel boundedness on sound transmission problems
Authors: Takahashi D.
Published: October 2005

A theory for sound transmission through a panel of finite width and infinite length mounted in a rigid baffle is formulated in order to discuss the effects of the panel boundedness on the sound transmission loss. The results calculated from a rigorous solution are presented as the frequency characteristics curves. However, it seems to be difficult to evaluate the sound insulation efficiency of the panel using the rigorous results, because of their violent peaks and dips owing to resonance. A method for averaging the response over any frequency band is discussed. The averaged response corresponds to the actual transmission measurement with a bandlimited excitation and allows one to evaluate the transmission characteristics from a viewpoint of practical noise control. The effects of the panel boundedness including a comparison with an infinite panel are also discussed by means of this method.
Advanced time domain wave-number sensing for structural acoustic systems. Part III. Experiments on active broadband radiation control of a simply supported plate
Authors: Maillard J. P. , Fuller C. R.
Published: October 2005

The present work gives further developments and experimental testing of a new time domain structural sensing technique for predicting wave-number information and acoustic radiation from vibrating structures. Most structure-borne active sound control approaches now tend to eliminate the use of microphones located in the far field by developing sensors directly mounted on the structure. In order to reduce the control authority and complexity required to minimize sound radiation, these sensors should be designed to provide error information that is solely related to the radiating part of the structural vibrations, e.g., the supersonic wave-number components in the case of planar radiators. The approach discussed in this paper is based on estimating supersonic wave-number components coupled to acoustic radiation in prescribed directions. The spatial wave-number transform is performed in real time using a set of point structural sensors with an array of filters and associated signal processing. The use of the sensing approach is experimentally demonstrated in the time domain LMS active control of broadband sound radiated from a vibrating plate. Comparisons of the control performances obtained with the wave-number sensor and error microphones in the far field show that only a few point sensors are required to provide accurate radiation information over a broad frequency range. The approach demonstrates good broadband global control of sound radiation.
Resonant transmission of a three-dimensional acoustic sound beam through a solid plate in air: Theory and measurement
Authors: Anderson Michael J. , Martin Philip R. , Fortunko Christopher M.
Published: October 2005

A quantitative model has been developed to explain the prominent features of the resonantly enhanced transmission of a three-dimensional airborne sound beam through a solid elastic plate. A quantitative model of this phenomenon is important in order that air coupling can be exploited for applications, such as the determination of elastic properties of plate-shaped materials. In our model, the beam axis is aligned to exploit resonant coupling with the A0 Lamb mode of the plate, and a finite-sized transducer is used to measure transmission of the beam through the plate. The interaction of airborne and liquid beams with a solid plate differs significantly. Because of the difference in densities between air and the solid plate material, the resonant coupling mechanism for airborne beams is very sensitive to the incidence angle of individual plane waves in the angular beam spectrum, approximately 1000 times more sensitive than if the coupling fluid is water. It was found that the elastic waves excited in the plate by the incident beam propagate in a sector limited by a half-angle determined from the incidence angle of the beam and the beam ka product. Measurements are presented that validate the model predictions of transmission loss caused by the presence of a plate. Specifically, over the range of frequencies 160–180 kHz, measured transmission loss caused by the presence of a 0.8-mm-thick aluminum and 1.40-mm-thick PMMA plate were within 1–2 and 2–5 dB of model predictions.
A conceptual model for acoustic microcavitation
Authors: Madanshetty Sameer I.
Published: October 2005

A conceptual model is developed to explain microcavitation events at low megahertz frequencies and low duty cycles, facilitated by smooth, spherical, submicrometer sized particles in clean water. The extremely high levels of accelerations associated with high-frequency acoustic fields, the active detector field, can, according to this model, ‘‘coax’’ such smooth particles to nucleate cavitation events at reduced thresholds. This conceptual model should help further experimentation exploring the phenomenon of ‘‘acoustic coaxing’’ for microcavitation.
A loaded thermoacoustic engine
Authors: Olson J. R. , Swift G. W.
Published: October 2005

Measurements and analysis of the performance of a thermoacoustic engine driving a dissipative load are presented. The effect of the load can be explained qualitatively using a simple low-amplitude approximation and quantitatively by invoking a more accurate low-amplitude numerical solution. The heater power Q̇ and hot-end temperature TH are found to be simple functions of the load impedance and the unloaded values of Q̇ and TH.
Ultrasonic assessment of Poisson’s ratio in thin rods
Authors: Nagy Peter B. , Kent Renee M.
Published: October 2005

It is shown in this paper that longitudinal guided modes propagating in a thin rod can be exploited to determine the axial Poisson’s ratio of the material. The feasibility of two versions of the guided wave method have been studied. The first technique determines Poisson’s ratio from the low-frequency dispersion of the axial guided mode in the free rod. It is well known that the velocity of this mode in a thin rod decreases with increasing frequency by a small amount that is proportional to the square of Poisson’s ratio. The second technique is based on measuring the excess attenuation of the same mode due to leaky losses when the rod is immersed in water. The leaky attenuation, which is caused solely by the normal component of the surface vibration, is also proportional to the square of Poisson’s ratio in the specimen. The main goal was to verify the results of this novel leaky-attenuation technique by comparing them to those of the more conventional first technique. The experimental results indicate that both techniques can be used to determine Poisson’s ratio in thin (0.3–1.5)-mm-diam metal wires in the 200 kHz–2 MHz frequency range. The measured Poisson ratios were generally higher than the corresponding values in isotropic materials, which is indicative of the significant anisotropy characteristic of such specimens.
Prewhitening for improved detection by matched-field processing in ice-ridging correlated noise
Authors: Zala Cedric A. , Ozard John M. , Wilmut Michael J.
Published: October 2005

Correlated ambient noise at an array can severely impair the ability of matched-field processing and conventional processing to detect and localize an acoustic source. When components of the correlated noise field are stationary, however, their effects can be reduced through the technique of prewhitening the data covariance matrix. Presented here are the results of a series of simulations in which prewhitening was applied to synthetic data generated for a correlated noise field. This study is relevant to prewhitening in noise fields that arise from distant localized biological sources, distant shipping, or ice ridging at low frequencies where the noise source remains spatially stable for hours. The present simulations modeled noise from ice ridging in the Arctic. In these simulations, a slanted linear array spanning the water column of a range-independent environment was used to enable full 3-D localization. The ability of prewhitening to enhance signal detection for low signal-to-noise cases was demonstrated for realistic conditions of covariance estimation of both the noise and data matrices. Under conditions where the intensity ratio of the ice-ridging noise to spatially uncorrelated noise was 20 dB, the sensor signal level required for detection was found to be 16 dB lower if prewhitening was employed, even when the source was positioned directly below one of the ice ridges. These results were obtained for the Bartlett processor, realistic time–bandwidth products, and experimentally determined environmental conditions.
Time-harmonic acoustic Bloch wave propagation in periodic waveguides. Part III. Nonlinear effects
Authors: Bradley C. E.
Published: October 2005

The effect of nonlinearity in the propagation of time-harmonic acoustic Bloch waves in periodic waveguides is investigated theoretically and experimentally. It is found that a forward traveling fundamental Bloch wave generates both forward and backward traveling second harmonic Bloch waves, the amplitudes of which oscillate with distance. A local second harmonic field is also generated. An effective coefficient of nonlinearity for Bloch waves is identified and found to be strongly frequency dependent. Measurements of the second harmonic field amplitude show good agreement with the theory; both the oscillations in amplitude and the change in effective nonlinearity are clearly evident.
Analysis of the propagation of plane acoustic waves in passive periodic materials using the finite element method
Authors: Langlet Philippe , Hladky-Hennion Anne-Christine , Decarpigny Jean-Noël
Published: October 2005

The finite element approach has been previously used, with the help of the ATILA code, to model the scattering of acoustic waves by single or doubly periodic passive structures [A. C. Hladky-Hennion et al., J. Acoust. Soc. Am. 90, 3356–3367 (1991)]. This paper presents a new extension of this technique to the analysis of the propagation of plane acoustic waves in passive periodic materials without losses and describes with particular emphasis its application to doubly periodic materials containing different types of inclusions. In the proposed approach, only the unit cell of the periodic material has to be meshed, thanks to Bloch–Floquet relations. The modeling of these materials provides dispersion curves from which results of physical interest can be easily extracted: identification of propagation modes, cutoff frequencies, passbands, stopbands, as well as effective homogeneous properties. In this paper, the general method is first described, and particularly the aspects related to the periodicity. Then a test example is given for which analytical results exist. This example is followed by detailed presentations of finite element results, in the case of periodic materials containing inclusions or cylindrical pores. The homogenized properties of porous materials are determined with the help of an anisotropic model, in the large wavelength limit. A validation has been carried out with periodically perforated plates, the resonance frequencies of which have been measured. The efficiency and the versatility of the method is thus clearly demonstrated.
Ultrasonic radiative transfer in polycrystalline media: Effects of a fluid–solid interface
Authors: Turner Joseph A. , Weaver Richard L.
Published: October 2005

In previous derivations of the ultrasonic radiative transfer equation (URTE) for the modeling of multiply scattered diffuse ultrasound in polycrystalline media, the boundary conditions appropriate to experiments performed in a water bath were not used. In the present work, this fluid–solid boundary condition is discussed as it applies to the URTE. An interpolation scheme is developed that is consistent with the discrete ordinates method used for the solution of the URTE. Steady-state and time-dependent results are presented for the solution of the URTE with a fluid–solid boundary condition. The steady-state results show that diffusive behavior is exhibited nearer the specimen surface than before. The time-dependent results show the qualitative change one might expect from such a boundary condition.