Acoustic Monopole Research Articles

Acoustic multipole source model for volcanic explosions and inversion for source parameters

SUMMARY Volcanic explosions are accompanied by strong acoustic pressure disturbances in the atmosphere. With a proper source model, these acoustic signals provide invaluable information about volcanic explosion dynamics. Far-field solutions to volcanic infrasound radiation have been derived above a rigid half-space boundary, and a simple inversion method was developed based on the half-space model. Acoustic monopole and dipole sources were estimated simultaneously from infrasound waveforms. Stability of the inversion procedure was assessed in terms of variances of source parameters, and the procedure was reliable with at least three stations around the infrasound source. Application of this method to infrasound observations recorded at Tungurahua volcano in Ecuador successfully produced a reasonable range of source parameters with acceptable variances. Observed strong directivity of infrasound radiation from explosions at Tungurahua are successfully explained by the directivity of a dipole source model. The resultant dipole axis, in turn, shows good agreement with the opening direction of the vent at Tungurahua, which is considered to be the origin of the dipole source. The method is general and can be utilized to study any monopole, dipole or combined sources generated by explosions.

Technology of predicting acoustic turbulence in the far-field flow

The paper presents a technology of predicting acoustic turbulence in a given field based on calculating the surface integral with delay. A specificity of the technology is that prediction of the farfield pulsations is carried out in the process of simulating the near-field flow. In doing so, we use all of the available discrete data for the numerical integration and avoid the necessity to store a huge amount of data on disks for the far-field estimation in the post-processing. The results on the test case simulating the acoustic monopole source in the subsonic flow are given.

Energy Radiation from an Impulsive Acoustic Monopole with Uniform Internal Structure, and from a Pair of Colliding Acoustic Dipoles

The energy radiation emitted by multipole sources is theoretically analyzed here in two basic novel configurations. First using an impulsive monopole source of radius R with uniform internal structure. The pressure field is affected by the entire spherical region r < R. Several methods are used. It is first obtained from the difference between the retarded and advanced solutions of the pertinent wave equation for this case. It is also later obtained from the retarded solution alone, confirming the first case. We also illustrate how to obtain the energy supplied to the fluid. The second configuration models two acoustic dipoles of the same strength on a linear collision course. The dipoles come from infinity and reverse directions after collision as they return to infinity. The radiated energy in this case is obtained when the collision occurs at the origin, and later when there is a finite stand-off separation between them. This second situation shows that: (a) less energy is then radiated than in the first, (b) that the radiated energy is then dominated by a τ–5 time decay law rather than a τ–3 law and (c) that in this case one can have more than one “time-duration factor.”

Radiation by a subwavelength‐sized acoustic monopole: Energy considerations and application to subwavelength time‐reversal focusing.

In this work, the radiation of pulsed acoustic waves by an acoustic monopole is theoretically studied based on energy consideration. Specifically, the instantaneous acoustic intensity and the instantaneous acoustic energy contained in a spherically symmetrical pulsed radiated field are calculated. In the case of a subwavelength‐sized acoustic monopole, it is shown that the radiation of a given amount of energy involves energy exchanges between the field and the source that are much larger than the radiated energy. It is also shown that even after the source has been turned off, the instantaneous acoustic intensity may be directed inwards, although the field cannot exchange energy with the source anymore. By time‐reversing the radiated field, we demonstrate that subwavelength refocusing requires a subwavelength energy exchanger, which is not only needed to absorb the incoming energy as well‐known, but importantly also to transiently provide energy to the field before reabsorbing it. In other words, we demonstrate in a homogeneous medium that the acoustic sink needed to perform subwavelength time‐reversal must transiently be an active source of energy.

On the impact noise of a drop falling on water

An analysis is made of the impact sound generated when a water droplet impinges on a nominally smooth air–water interface. Guo and Ffowcs Williams [ Journal of Fluid Mechanics 227 (1991) 345–355] derived a mathematical representation of the sound produced in the initial period of impact of duration ⪡ 1 μ s . The theory of this paper is a simple extension of their method that covers all of the effective life time ( ∼ 100 μ s ) of the impact sound pulse. At the acoustic wavefront the predicted pressure signature in the water consists of a large impulsive peak with the directivity of an acoustic monopole source. Behind this peak lower frequency components of the sound are strongly influenced by the effective pressure-release condition on the free surface of the water, and the radiation acquires the characteristics of a much weaker dipole field (with dipole axis normal to the water interface) accompanied by a rapid decrease in wave amplitude. The dipole pressure exhibits a single cycle oscillation before decaying to evanescence about 0.1 ms after the initial impact. Predictions of the acoustic pressure spectral level are shown to be consistent with measurements available in the literature. Applications of the theory are envisaged to situations where the entrainment of air bubbles by droplets is not important, so that the impact is the dominant source of sound—for example, in estimating the contribution to the self-noise of a supercavitating vehicle from ventilating gas containing a ‘spray’ of small droplets impinging on the cavity gas–water interface.

A new laser vibrometry-based 2D selective intensity method for source identification in reverberant fields: part I. Development of the technique and preliminary validation

The selective intensity technique is a powerful tool for the localization of acoustic sources and for the identification of the structural contribution to the acoustic emission. In practice, the selective intensity method is based on simultaneous measurements of acoustic intensity, by means of a couple of matched microphones, and structural vibration of the emitting object. In this paper high spatial density multi-point vibration data, acquired by using a scanning laser Doppler vibrometer, have been used for the first time. Therefore, by applying the selective intensity algorithm, the contribution of a large number of structural sources to the acoustic field radiated by the vibrating object can be estimated. The selective intensity represents the distribution of the acoustic monopole sources on the emitting surface, as if each monopole acted separately from the others. This innovative selective intensity approach can be very helpful when the measurement is performed on large panels in highly reverberating environments, such as aircraft cabins. In this case the separation of the direct acoustic field (radiated by the vibrating panels of the fuselage) and the reverberant one is difficult by traditional techniques. The first aim of this work is to develop and validate the technique in reverberating environments where the location and the quantification of each source are difficult by traditional techniques. The reverberant field is clearly challenging also for the proposed technique, affecting the achievable accuracy, mainly due to the fact that coherence between radiated and reverberated fields is often unknown and may be relevant. Secondly, the applicability of the method to real cases is demonstrated. A laboratory test case has been developed using a large wooden panel. The measurement is performed both in anechoic environment and under simulated reverberating conditions, for testing the ability of the selective intensity method to remove the reverberation.

A new laser vibrometry-based 2D selective intensity method for source identification in reverberant fields: part II. Application to an aircraft cabin

The selective intensity technique is a powerful tool for the localization of acoustic sources and for the identification of the structural contribution to the acoustic emission. In practice, the selective intensity method is based on simultaneous measurements of acoustic intensity, by means of a couple of matched microphones, and structural vibration of the emitting object. In this paper high spatial density multi-point vibration data, acquired by using a scanning laser Doppler vibrometer, have been used for the first time. Therefore, by applying the selective intensity algorithm, the contribution of a large number of structural sources to the acoustic field radiated by the vibrating object can be estimated. The selective intensity represents the distribution of the acoustic monopole sources on the emitting surface, as if each monopole acted separately from the others. This innovative selective intensity approach can be very helpful when the measurement is performed on large panels in highly reverberating environments, such as aircraft cabins. In this case the separation of the direct acoustic field (radiated by the vibrating panels of the fuselage) and the reverberant one is difficult by traditional techniques. The work shown in this paper is the application of part of the results of the European project CREDO (Cabin Noise Reduction by Experimental and Numerical Design Optimization) carried out within the framework of the EU. Therefore the aim of this paper is to illustrate a real application of the method to the interior acoustic characterization of an Alenia Aeronautica ATR42 ground test facility, Alenia Aeronautica being a partner of the CREDO project.

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.

Prediction method for tire air-pumping noise using a hybrid technique

Air-pumping noise from a car tire is investigated with a hybrid technique composed of three stages: (1) small-scale air-pumping noise generation process is modeled as a piston-like movement of the base-side of the tire groove and then numerically simulated; (2) the flow properties in the tire groove are used as air-pumping sources and noise propagation is simulated with emphasis placed on scattering process with full tire/road geometry; (3) the far-field acoustic pressure is predicted from a Kirchhoff integral method by using unsteady flow data in space and time which is provided by the computational fluid dynamics (CFD) calculation of full tire-road domain. The comparison of predicted results shows that the nonlinearity of the air-pumping noise generation mechanism affects not only noise characteristics in frequency domain but also in the directivity pattern. It seems that this approach can overcome the weakness of the acoustic monopole theory which stems from the usual assumption of a small amplitude acoustic wave equation while using nonlinear governing equation for the CFD calculation. Furthermore, through the use of a computational domain which covers tire and road surface, the geometric effects on air-pumping noise generation and propagation are taken into account in the source modeling process.

자동차 타이어의 Air-Pumping소음 예측을 위한 수치적 기법

The monopole theory has long been used to model air-pumped effect from the elastic cavities in car tire. This approach models the change of an air as a Piston moving backward and forward on a spring and equates local air movements exactly with the volume changes of the system. Thus, the monopole theory has a restricted domain of applicability due to the usual assumption of a small amplitude acoustic wave equation and acoustic monopole theory This paper describes an approach to predict the air-pumping noise of a car tyre with CFD/Kirchhoff integral method. The tyre groove is simply modeled as piston-cavity-sliding door geometry and with the aid of CFD technique flow properties in the groove of rolling car tyre are acquired.`rhese unsteady flow data are used as a air-pumping source in the next CFD calculation of full tyre-road geometry. Acoustic far field is predicted from Kirchhoff integral method by using unsteady flow data in space and time which is provided by the CFD calculation of full tyre-road domain. This approach can cover the non-linearity of acoustic monopole theory with the aid of Non-linear governing equation in CFD calculation. The method proposed in this paper is applied to the prediction of air-pumping noise of simply modeled car tyre and through the predicted results, the influence of nonlinear effect on air-pumping noise propagation is investigated.

Constant-beamwidth and constant-powerwidth wideband robust Capon beamformers for acoustic imaging

The standard Capon beamformer (SCB) is sensitive to the mismatch between the assumed and actual array steering vector, which occurs often in practice. Recently a robust Capon beamformer (RCB) was proposed by extending the SCB to the case of uncertain array steering vectors. In certain applications such as acoustic imaging, it is desirable that the beamwidth is constant across the frequency bins. This prevents future corrections for different frequencies and contributes to consistent sound pressure level (SPL) estimation, which means that for an acoustic wideband monopole source with a flat spectrum the acoustic image for each frequency bin stays the same. However, the beamwidth of RCB decreases with frequency, which can lead to inconsistent imaging results across the frequencies. In this paper two beamformers are proposed, namely a constant-beamwidth robust Capon beamformer (CBRCB) and a constant-powerwidth robust Capon beamformer (CPRCB), as extensions of RCB for consistent wideband acoustic imaging. Both CBRCB and CPRCB are more robust against array steering vector errors and finite sample size problems than SCB and have better resolution and interference suppression capability than data-independent beamformers. Moreover, they both can be efficiently implemented. The effectiveness of CBRCB and CPRCB is demonstrated via a number of simulated and experimental examples.

Efficient prediction methods for the micro-pressure wave from a high-speed train entering a tunnel using the Kirchhoff formulation.

The compression wave generated by a high-speed train emerging from the exit portal of a tunnel gives rise to an impulsive noise called a micro-pressure wave. In this study, new methods for the prediction of sonic-boom noise are proposed. The first method combines acoustic monopole analysis and the method of characteristics with the Kirchhoff method. The compression wave from a train entering a tunnel is calculated by an approximate compact Green's function, and the resultant noise at the tunnel exit is predicted by a linear Kirchhoff formulation. The second method couples the Kirchhoff formulation with the Euler equation, which is solved numerically for the generation and propagation of the compression wave. Numerical prediction of the compression wave, the propagation in the tunnel, and the micro-pressure wave obtained by the present methods are compared with measured data. The numerical results exhibit a reasonable agreement with the experimental data. The proposed methods in this study are shown to be very useful design tools for the nose shape of trains and the geometry of tunnels, and they can be utilized to minimize the pressure fluctuation in the tunnel and the corresponding booming noise.

Noise source models for fricative consonants

Hybrid source models for fricative consonants are derived based on aeroacoustic principles of sound generation employing vocal tract area functions obtained from magnetic resonance imaging data of voiced and unvoiced English fricatives. Results based on data from a male and a female subject indicate that a linear source-filter model is fairly adequate for capturing essential spectral characteristics of sustained voiced and unvoiced strident fricatives below 10 kHz. The hybrid source models employ a combination of acoustic monopole and distributed dipole sources, and a voice source in the case of the voiced fricatives. The number of sources, source locations, and spectral characteristics and the relative source levels are chosen based on an analysis-by-synthesis approach and are motivated by aeroacoustic theory of speech production. The resulting model is computationally efficient and can be readily used for synthesis.

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.

Sonic logging of compressional‐wave velocities in a very slow formation

Borehole sonic waveforms are commonly acquired to produce logs of subsurface compressional and shear wave velocities. To this purpose, modern borehole sonic tools are usually equipped with various types of acoustic sources, i.e., monopole and dipole sources. While the dipole source has been specifically developed for measuring shear wave velocities, we found that the dipole source has an advantage over the monopole source when determining compressional wave velocities in a very slow formation consisting of unconsolidated sands with a porosity of about 35% and a shear wave velocity of about 465 m/s. In this formation, the recorded compressional refracted waves suffer from interference with another wavefield component identified as a leaky P‐wave, which hampers the determination of compressional wave velocities in the sands. For the dipole source, separation of the compressional refracted wave from the recorded waveforms is accomplished through bandpass filtering since the wavefield components appear as two distinctly separate contributions to the frequency spectrum: a compressional refracted wave centered at a frequency of 6.5 kHz and a leaky P‐wave centered at 1.3 kHz. For the monopole source, the frequency spectra of the various waveform components have considerable overlap. It is therefore not obvious what passband to choose to separate the compressional refracted wave from the monopole waveforms. The compressional wave velocity obtained for the sands from the dipole compressional refracted wave is about 2150 m/s. Phase velocities obtained for the dispersive leaky P‐wave excited by the dipole source range from 1800 m/s at 1.0 kHz to 1630 m/s at 1.6 kHz. It appears that the dipole source has an advantage over the monopole source for the data recorded in this very slow formation when separating the compressional refracted wave from the recorded waveforms to determine formation compressional wave velocities.

Calculation of transient tube-wave signals in cross-borehole acoustics

Closed-form expressions are obtained for the transient acoustic pressure in a borehole, due to the action of a volume injection (acoustic monopole) source in another borehole in a typical cross-well seismic setting with a homogeneous isotropic solid formation. At the relatively low frequencies involved the acoustic wave motion inside a fluid-filled borehole, which may be surrounded by a structure of perfectly bonded circularly cylindrical solid shells, is dominated by tube waves. The excitation and propagation properties of the tube wave are modeled by regarding the borehole as an acoustic waveguide with a compliant inner wall. The corresponding elastic wave-field quantities at the outer borehole wall are evaluated through a plane-strain elastostatic transfer of the stress and the elastic displacement across the shell structure. For the radiation of the wave-field quantities into the formation, the elastodynamic Kirchhoff–Huygens integral representation is used. The acoustic pressure on the axis of the receiving borehole is evaluated with the aid of the fluid/solid acoustic reciprocity theorem. Various physical phenomena are described by the resulting expressions, including pre- and postcritical phenomena (conical waves) for slow formations, and tunnelinglike phenomena for proximate boreholes in fast formations.

Scattering of hydrodynamic pressure pulses by coating inhomogeneities

Sound generation by a deterministic, hydrodynamic pressure pulse in a fluid flowing over a compliant coating with an embedded void inhomogeneity is investigated. The inhomogeneity is represented by an acoustic monopole and its strength is found for several models of the pressure pulse. The far-field acoustic pressure is calculated for all models and is compared with the sound generated by a statistically stationary turbulent boundary layer. It is found that for certain parameters of the coating, the intensity of scattered sound generated by a deterministic pulse may exceed by 10–40 dB the intensity of sound estimated for the statistical turbulent boundary layer forcing.

Measurement of the sound produced by a tipping trough with fresh and salt water

Recent experiments confirm the production of sound by breaking waves at lower frequencies (30 to 500 Hz) with a dipole characteristic. The noise produced has a broadband characteristic associated with the impact and subsequent sounds that have discrete spectral characteristics. Breaking waves are known to produce bubble plumes and bubble clouds; the dynamic evolution of which provides a mechanism for sound production. Since the initial plume and cloud have appreaciable void fractions, compressible resonant oscillations of these structures as a whole or in parts are possible. These bubble plumes would act as compact acoustic monopole sources of sound and due to the pressure release surface would have an effective dipole characteristic. Sufficient energy exists in the initial breaking vorticity and turbulence to excite these regions and to explain measured source levels. These effects have been simulated with a tipping trough experiment that demonstrates the production of low-frequency sound from salt and fresh water tipping trough events. These experimental results are shown to be consistent with the theory of sound radiation from the collective oscillations of bubble plumes.

Directivity of an acoustic monopole in a floating ice plate and implications to Arctic ambient noise

Predictions were made of the directivity of higher‐frequency (1‐kHz) acoustic radiation into the water from a monopole source located in a floating ice plate (2.4 m thick). Theoretical calculations were conducted using the SAFARI propagation routine [H. Schmidt and J. Glattetre, J. Acoust. Soc. Am. 78, 2105–2114 (1985)] and by the method of steepest descents (P. J. Stein, “Monopole Source in a Floating Ice Plate,” MIT/WHOI Ph.D. thesis, 1986). The models show close agreement with each other and predict a strong downward directivity with most of the energy radiated at grazing angles greater than 70°. This implies that (1) higher‐frequency impulsive ice noises detected at an omnidirectional hydrophone are likely radiated from a relatively small area of ice directly above the receiver (the area increasing with receiver depth) and (2) the nonimpulsive (background) ambient noise at higher frequencies is dominated by radiation from ice events located within a horizontal range radius equal to roughly one water depth. Modeling also shows that nulls can occur for some radiation angles for particular source depths within the ice. Also, only small effects of a more realistic ice strength vertical profile over a uniform profile are seen. [Work supported by ONR.]

Active control of sound fields in elastic cylinders by multicontrol forces

An unstiffened cylindrical model was used to study the control of sound transmission into aircraft cabins by the use of multi-control forces applied directly to the cylinder wall. External acoustic monopoles were located on each side of the cylinder to approximate the propeller noise source. This allowed the study of a dual control system utilizing multi-control forces in conjunction with synchrophasing of the twin acoustic monopole sources. For acoustic resonant conditions within the cavity, a spatially averaged noise reduction of approximately 30 dB was achieved using the active control system for both in-phase and out-of-phase monopoles; however, effective reduction of the sound field was dependent upon judiciously positioning the control forces for optimal control of the sound field.