Acoustic Dipole Research Articles

Structural and acoustic responses of a submarine hull due to propeller forces

The low frequency structural and acoustic responses of a simplified axisymmetric submarine model to fluctuating propeller forces along the submarine axis are investigated. The forces arise from a hydrodynamic mechanism and are transmitted from the propeller to the submarine hull through both the shaft and the fluid. Numerical models have been developed to simulate the strongly coupled structure–fluid interaction of a submerged vessel in the frequency domain. The structure is modelled using the finite element method, so that more complex features such as ring-stiffeners, bulkheads and the propulsion system can be taken into account. A simple, passive vibration attenuation system known as a resonance changer is included in the model of the propeller/shafting system. The surrounding fluid is modelled using the boundary element method. The influence and importance of model parameters such as structural stiffness and fluid loading effects are investigated. Due to the fluctuating propeller forces, the hull is excited by axial structural forces transmitted through the propeller/shafting system as well as by acoustic dipoles, where the dipoles are correlated to the structural forces in strength and direction. The acoustic dipole at the propeller also radiates sound directly to the far field of the surrounding fluid. It is demonstrated that the performance of the RC is negatively influenced at frequencies above the fundamental axial resonance of the hull by the effect of forces transmitted through the fluid. Another problem arises due to increased axial movement of the propeller, when the RC is optimised to minimise excitation of the hull via the propeller shaft. This results in an additional sound field that excites the submarine hull in a similar manner to the fluid forces that arise directly from the hydrodynamic mechanism.

Effects of boundary conditions on vibrating mode of acoustic logging dipole transducer

Acoustic transducer is an important part of acoustic well logging tool. In this paper, ANSYS software package is used to design acoustic dipole transducer and simulate vibrating mode of the dipole transducer in different mechanical boundary conditions. The results show that boundary conditions influence the number of vibrating mode in the same frequency band and the frequency value of the same vibrating mode. Several acoustic dipole transducers are designed according to the results of numerical simulation and laboratory measurements. The basic frequency of vibrating mode of experiment has good agreement with that of simulation. The numerical simulation plays a good guidance role in designing, producing and correctly installing the acoustic dipole transducer.

Numerical analysis of flapping sound generated by hovering insects.

Insect flapping sound is a consequence of flow disturbance generated by flapping wing motion. Spatial and temporal changes of pressures on the wing surface and vortex structures generated by the wing motion are considerable sources of the flapping sound. To analyze such mechanism in the sound generation, we have developed an integrated method combining computational fluid dynamics (CFD) techniques and acoustic analysis. Unsteady flows around a hovering insect are simulated by NS solution‐based CFD analysis with a multiblocked, moving‐overset grid technique. In the acoustic analysis, monopole and dipole sound sources generated by the wing motion are analyzed by using Ffowcs–Williams and Hawkings method. In this study, numerical analyses of the flapping sounds are carried out for three insects of hawkmoth, honeybee, and fruitfly. The CFD analysis provides a detailed picture of the flow fields around the hovering flyers where complicated vortex aspects and the induced pressure distributions on the wing surfaces show concrete evidence of the sound sources. The acoustic analysis further clarifies the characteristics of the flapping sounds induced by these sound sources, including the sound directivity and its spectrum distribution. These results effectively demonstrate the cause‐and‐effect relationship in sound generation and thus indicate the availability of this integrated CFD‐acoustic method.

Acoustic source characterization of impulsive Strombolian eruptions from the Mount Erebus lava lake

We invert for acoustic source volume outflux and momentum imparted to the atmosphere using an infrasonic network distributed about the erupting lava lake at Mount Erebus, Ross Island, Antarctica. By modeling these relatively simple eruptions as monopole point sources we estimate explosively ejected gas volumes that range from 1,000 m 3 to 24,000 m 3 for 312 lava lake eruptions recorded between January 6 and April 13, 2006. Though these volumes are compatible with bubble volumes at rupture (as estimated from explosion video records), departures from isotropic radiation are evident in the recorded acoustic wavefield for many eruptions. A point-source acoustic dipole component with arbitrary axis orientation and strength provides precise fit to the recorded infrasound. This dipole source axis, corresponding to the axis of inferred short-duration material jetting, varies significantly between events. Physical interpretation of dipole orientation as being indicative of eruptive directivity is corroborated by directional emissions of ejecta observed in Erebus eruption video footage. Although three azimuthally distributed stations are insufficient to fully characterize the eruptive acoustic source we speculate that a monopole with a minor amount of oriented dipole radiation may reasonably model the primary features of the recorded infrasound for these eruptions.

Acoustic characterization of an ultrasound surgical transmitter in the linear and nonlinear regime of working

The method for measurement of a derived acoustic power of an ultrasound surgical transducer has been suggested in the free field conditions. The pressure field of the transmitter, immersed in depth of quater wavelength and vibrating at the fundamental frequency (≈25kHz), has been measured with calibrated hydrophone at different excitation levels. In the linear regime, the transmitter has been theoretically described as an acoustic dipole, the source parameters have been found and good agreement between theoretical and experimental results is obtained. When transmitter is excitated at higher excitation levels, the nonlinear behaviour in loading medium appears, with strong cavitation activity. In the averaged power spectrum of the recorded acoustic pressure signal, is evident the presence of harmonics (n⋅f), subharmonics (f/q), ultraharmonics (n⋅f/q) of excitation frequency. The spatial pressure distribution of each discrete frequency component in the free acoustic field has been measured and its contribution to total acoustic power has been calculated. The total acoustic power in the cavitation noise signal is estimated integrating the averaged pressure power spectrum with appropriate contributions of each frequency component in the signal.

Sound generated by a jet-excited spherical cavity

An analysis is made of the sound generated by the impingement of a high-speed ventilating gas jet on the gas–water interface of a ventilated supercavity. A precise analytical problem is formulated and solved in which the supercavity is modelled by a spherical cavity of equal interior volume and the jet consists of a planar, radially symmetric flow from the centre of the sphere. The gas content of the sphere is assumed to be maintained at a constant mean level by steady exhaust into the water from the ‘rear end’ of the cavity. It is estimated that the damping of jet-induced oscillations by this outflow is negligible compared with conventional losses produced by acoustic radiation and by viscous and thermal dissipation. The sound radiated into the water is omnidirectional at low frequencies, but with increasing frequency it progressively exhibits the directional characteristics of an acoustic dipole with axis centred on the radial axis of the gas jet.

Hull vibratory forces transmitted via the fluid and the shaft from a submarine propeller

Disturbing forces due to a rotating propeller are transmitted to a ship or submarine hull via both the propeller shaft and the pressure field that is created in the surrounding water. This pressure field has hydrodynamic and acoustic components, because water is a compressible medium with a finite speed of sound. The work described in this article is a pilot study to investigate the combination of forces generated by the propeller motion that are transmitted to a submerged body. Fluctuating propeller forces are transmitted along the shafting system to result in structural excitation of the submarine hull. These forces are also transmitted to the surrounding fluid and result in an external hull pressure field, because they correspond to acoustic dipoles. Both types of hull excitation contribute to hull vibration and underwater radiated noise. To demonstrate the fundamental nature of the problem, the case of a propeller located on the principal axis of a finite cylinder is considered, which represents a typical submarine arrangement. The analysis in this work has been simplified by considering only stationary fluctuating forces at the propeller hub and by regarding the propeller, hull and shaft as rigid structures. The relative magnitudes and phases of forces transmitted by the fluid and propeller shaft change with frequency in the range containing both low multiples of propeller blade passing frequency and broadband random components owing to turbulent flow in the submarine wake. The effects of a finite speed of sound are found to be significant at low frequencies, causing changes in the hull forces that would be computed by assuming that water is incompressible. Results are presented describing the properties of acoustic dipole sources, which have both hydrodynamic and acoustic pressure fields that change in relative importance with distance from the source.

Localization and tracking of acoustic stimuli using millimetric artificial lateral line flow sensing arrays

We report on the successful development and implementation of acoustic dipole localization and tracking algorithms for millimeter-scale flow sensors arranged in an artificial lateral line configuration. Our previous work includes the development of an artificial lateral line system a novel, bioinspired flow sensing array. The artificial lateral line aims to mimic the lateral line sensory organ of many species of fish, an organ that allows the fish to perform near-field hydrodynamic imaging to navigate, capture prey, or align to a flow stream. Many of the stimuli that these fish sense can be modeled as near-field dipoles with widely understood acoustic signatures. In our previous work, we successfully developed an artificial lateral line using micro-flowsensors for applications such as dipole localization, wake tracking, and flow imaging. However, while the applications and the platform were both novel, potential uses were limited due to the size of the microsensors (and correspondingly the sensor arrays) as well as the limited distribution of the technology. In this paper, we show for the first time that a commercially available array can also be successfully used as an artificial lateral line for observing the above-mentioned phenomena, thereby enabling a new biomimetic millimeter-scale flow-sensing platform.

Vortex shedding and the voice source

The voice source is primarily an acoustic dipole produced by the fluctuating drag on the vocal folds [Zhao, Zhang, Frankel et al., J. Acoust. Soc. Am. 112, 2134–2146 (2002)]. In this paper the unsteady drag is determined theoretically in terms of the vorticity shed from the vocal folds. The principal source of acoustic energy is glottal-jet vorticity lying within an axial distance downstream of the glottis of less than about the glottal width. The vortex drag dipole is equivalent to the volume velocity source traditionally assumed to be located at the glottis. In addition, there exists a true, but weaker fluctuating volume source associated with volumetric changes of the vocal folds region. The relationship between the voice source dipole and sibilant fricative dipoles will be discussed. Our findings will be presented in a largely qualitative manner, so that the few equations used in the presentation can be understood physically. [Work partially supported by NIDCD-004688.]

Heart sounds as a result of acoustic dipole radiation of heart valves

Heart sounds are associated with impulses of force acting on heart valves at the moment they close under the action of blood-pressure difference. A unified model for all the valves represents this impulse as an acoustic dipole. The near pressure field of this dipole creates a distribution of the normal velocity on the breast surface with features typical of auscultation practice: a pronounced localization of heart sound audibility areas, an individual area for each of the valves, and a noncoincidence of these areas with the projections of the valves onto the breast surface. In the framework of the dipole theory, the optimum size of the stethoscope’s bell is found and the spectrum of the heart sounds is estimated. The estimates are compared with the measured spectrum.

Observations of the near-field structures of the glottal flow

The three-dimensional glottal jet was measured downstream from a self-oscillating physical model of the vocal folds using several techniques, including grid-based hotwire anemometry, high-speed flow visualization, and particle image velocimetry. Coherent flow structures were extracted from the spatio-temporal data using principal component analysis (PCA). The glottal jet resembled planar jets through static slitlike orifices. The direction of the jet core axis oscillated in the medial-lateral direction. Also, there was a positive correlation between the dynamics of shear layers around the laminar jet core and flow structures in the transitional region. This finding was similar to the dynamics of planar jets where jet flapping was observed previously. This can be understood in terms of an antisymmetric array of large-scale vortices being convected downstream. This antisymmetric mode of coherent vortex structures acts as an acoustic dipole that could effect the dynamics of the shear layers in the laminar core region. Thus, our observations help to understand the dynamics of the acoustic near-field of the glottal oscillator and suggest a possible feedback mechanism for source-tract coupling.

On the Time Dependence of Field in the Near-Field Zone of a Nonstationary Multipole

The subject of consideration is the time dependence of the field in the near-field zone of an (acoustic or electric) dipole generating a nonmonochromatic signal smoothly varying with time. It is shown that this dependence is approximately the same at different points in the near-field zone; i.e., the smooth signal does not lag in time, as it might with regard to a finite velocity of wave propagation. The absence of delay is a result of interference between two identical but differently damped and delayed copies of one signal that are emitted from two monopoles constituting the dipole. The same is true for an arbitrary electroneutral (as a whole) set of nonstationary charges (in particular, for a nonstationary quadrupole, octupole, etc.).

Energy radiated by a point acoustic dipole that reverses its uniform velocity along its rectilinear path.

This work extends a mathematical approach developed recently for monopoles to describe the sound energy radiated by a rectilinearly moving dipole that changes direction along its trajectory. Although the dipole travels with constant speed, it undergoes acceleration by reversing its direction during a finite time interval along its path. This work determines the joint angular and frequency distribution of the radiated energy, its angular distribution, and the total radiated energy output. Results for the radiated energy are systematized by expressing the radiation integrals in terms of hypergeometric functions. This procedure simplifies the evaluations, particularly at low Mach numbers, and permits the comparison of results to the earlier monopole case.

The acoustic emission from a nonlinearly oscillating charged drop

A charged drop oscillating in a compressible medium is capable of emitting acoustic waves. According to calculations in the constant drop volume approximation linear in the oscillation amplitude, the most intense acoustic emission corresponds to the dominant mode. An acoustic dipole emission related to the excitation of a translational mode is only revealed by calculations in the second order of smallness with respect to the oscillation amplitude, in the case when two adjacent modes are present in the spectrum of initial deformations.

ON THE SCATTERING OF SOUND BY A RECTILINEAR VORTEX

A re-examination is made of the two-dimensional interaction of a plane, time-harmonic sound wave with a rectilinear vortex of small core diameter at low Mach number. Sakov [1] and Ford and Smith [2] have independently resolved the “infinite forward scatter” paradox encountered in earlier applications of the Born approximation to this problem. The first order scattered field (Born approximation) has nulls in the forward and back scattering directions, but the interaction of the wave with non-acoustically compact components of the vortex velocity field causes wavefront distortion, and the phase of the incident wave to undergo a significant variation across a parabolic domain whose axis extends along the direction of forward scatter from the vortex core. The transmitted wave crests of the incident wave become concave and convex, respectively, on opposite sides of the axis of the parabola, and focusing and defocusing of wave energy produces corresponding increases and decreases in wave amplitude. Wave front curvature decreases with increasing distance from the vortex core, with the result that the wave amplitude and phase are asymptotically equal to the respective values they would have attained in the absence of the vortex. The transverse acoustic dipole generated by translational motion of the vortex at the incident wave acoustic particle velocity, and the interaction of the incident wave with acoustically compact components of the vortex velocity field, are responsible for a system of cylindrically spreading, scattered waves outside the parabolic domain.

Acoustic monopoles, dipoles, and quadrupoles: An experiment revisited

A simple and inexpensive demonstration of acoustic monopole, dipole, and quadrupole sources utilizes four 4-in. boxed loudspeakers and a homemade switch box. The switch box allows the speakers to be driven in any combination of phase relationships. Placing the speakers on a rotating stool allows students to measure directivity patterns for monopole, dipole, and quadrupole speaker combinations. Stacking the speakers in a square, all facing the same direction, allows students to aurally compare the frequency and amplitude dependence of sound radiation from monopoles, dipoles, and quadrupoles.

Low frequency passive noise control of a piston structure with a weak radiating cell

The concept of a weak sound radiating cell is proposed to reduce the low-frequency radiated noise from structures. The cell consists of two mechanically coupled surfaces such that, when placed on a vibrating structure, the response of the two surfaces are nearly out-of-phase and nearly of the same strength over a wide frequency range. This structure response leads the cell to behave as an acoustic dipole and thus a poor sound-radiating source. The control of low-frequency structurally radiated noise is then achieved by covering the structure with an array of these weak radiating cells, i.e., surface treatment. Thus the surface treatment essentially transforms the response of the structure to that of a distributed array of dipoles yielding a low sound radiating structure. A theoretical model of a single weak radiating cell applied to a simple piston structure was developed and experimental verification was performed. Overall sound power level reductions of over 6 dB were experimentally achieved between 400 and 1600 Hz with maximum reductions of over 30 dB at discrete frequencies.

A Lloyd-mirror model to estimate the scattering strength of fish near rough ocean boundaries

It is well known that a point scatterer located near the sea surface creates an acoustic dipole and that one near the ocean bottom is in general described by a series of higher-order moments (e.g., a double monopole in the simplest case of a hard, flat bottom). This Lloyd-mirror effect has been coupled with a resonant fish scattering model of Love [R. H. Love, ‘‘Resonant acoustic scattering by swimbladder-bearing fish,’’ J. Acoust. Soc. Am. 64, 571–580 (1978)] to develop a simple model which predicts the backscattering strengths of a variety of fish species known to occur (during at least part of a diurnal cycle) in near-boundary layers. The model is applied to and compared with measurement results from Critical Sea Test 7 (CST-7 Phase 2) to demonstrate that fish can be significant contributors to low-grazing-angle scattering. The general implications of the model are that observed enhancements in backscattering strength over levels predicted by first-order rough-surface scattering theories may, in some cases, be primarily due to backscatter from near-surface fish (rather than from bubbles or the sediment volume). [Work supported by ONR.]

Control of Low Frequency Structurally Radiated Noise with an Array of Weak Radiating Cells: An Experimental Study

The concept of a weak sound radiating cell to reduce low frequency radiated noise from structures is proposed. The cell consists of two mechanically coupled surfaces such that, when placed on a vibrating structure, the responses of the two surfaces are nearly out-of-phase and of equal strength over a wide frequency range. The cell thus behaves as an acoustic dipole and thus a therefore poor sound radiating source. The control of low frequency structurally radiated noise is achieved by covering the structure with an array of these weak radiating cells, i.e., surface treatment. Thus, the surface treatment essentially transforms the response of the structure to that of a distributed array of dipoles yielding a low sound radiating structure. For the implementation of the concept one surface of the cell is rigidly linked to the vibrating structure while the second surface is obtained by attaching a plate to the structure through a flexible mounting system. A dipole acoustic source is formed by designing the system so that the radiated sound from the plate is out-of-phase with the radiated sound from the rigidly linked surface. Experimental verification was performed by applying an array of weak radiating cells to a vibrating rectangular plate excited with white noise from 0-1600 Hz. An overall sound power level reduction of 10.2 dB was achieved between 100-1600 Hz with maximum reductions of 25 dB at discrete frequencies. These results demonstrate the potential of the weak radiating cell concept to reduce low frequency structurally radiated noise.

Reply to "Comments on 'Acoustic Scattering by an Air Bubble Near the Sea Surface'"

Ahtruct- The paper’s paradoxical result that the scattering form function of a bubble increases with decreasing distance from a pressurerelease surface is discussed. In this paper,’ Gaunaurd and Huang describe calculations of the effect of a nearby pressure-release surface on the spherical pulsation frequency of a gas bubble submerged in a liquid, and indicate good agreement between their values and those published by me some years ago [I]. Their comparison with my somewhat obscure data is gratifying; but a minor correction is required. The frequencies listed in their Table I1 as my “measurements” are actually calculated values. My paper does, indeed, list measured frequencies that are in reasonable agreement with the calculations, but those are not the ones cited by them. It is instructive to note that my simple calculation of the frequency of resonance of a bubble, which assumes that the surrounding liquid is incompressible, agrees with their more exact and complicated theory, at least for bubbles much closer than a wavelength from the surface. The values of the bubble “form function” shown in their Figs. 2 and 3(a) indicate that its peak value at resonance increases as the bubble moves closer to the free surface. This seems contrary to what might intuitively be expected, since a nearby free surface converts the bubble monopole into an acoustic dipole which radiates less efficiently. The apparent paradox is explained by noting that their theory does not take account of any mechanism limiting the magnitude of bubble pulsations at resonance except radiation damping, which is smaller for a dipole than for a monopole. In the absence of other sources of damping, the reduction in radiation damping due to a free surface might lead to a sufficiently large increase in the bubble pulsation amplitude at resonance, for constant incident sound pressure, to overcompensate for the reduced radiation efficiency, and thus result in a net increase in their calculated values of the form function for some directions. In a real gas bubble, however, thermal damping is much larger than radiation damping above 1 kHz; see, e.g., Devin [2]. If the resonant response of a bubble is controlled by thermal damping, its pulsation amplitude will not be affected by the presence of a free surface. Accordingly, the peak form function would then be expected to decrease with decreasing distance from the surface, contrary to the trend shown in their figures.