Showing papers on "Sound power published in 2013"

Experimental and numerical investigation of turbulence-airfoil noise reduction using wavy edges

Abstract: A passive leading-edge treatment based on sinusoidal serrations aimed at reducing turbofan interaction noise has been recently studied in the framework of a European project (FLOCON). The turbulence-airfoil interaction mechanism is achieved using a turbulence grid located upstream of an isolated NACA airfoil tested in the Institute of Sound and Vibration Research anechoic open jet wind tunnel. The experimental setup with several airfoils designed and manufactured by ONERA is first presented with main acoustic results, highlighting the sound power level reductions obtained for all studied flow speeds (about 3–4 dB reduction) without altering the aerodynamic performances (as shown by available measurements and Reynolds-averaged Navier–Stokes calculations). The experimental investigations are supplemented by numerical predictions in order to assess the acoustic performances of the serrations. The method described in the second part of the paper is based on a computational aeroacoustics code solving the nonli...

Systems, methods, and apparatus for producing a directional sound field

Abstract: A system may be used to drive an array of loudspeakers to produce a sound field that includes a source component, whose energy is concentrated along a first direction relative to the array, and a masking component that is based on an estimated intensity of the source component in a second direction that is different from the first direction.

Aeroacoustics of volcanic jets: Acoustic power estimation and jet velocity dependence

Abstract: [1] A fundamental goal of volcano acoustics is to relate observed infrasonic signals to the eruptive processes generating them. A link between acoustic power Π¯ and volcanic gas exit velocity V was proposed by Woulff and McGetchin (1976) based upon the prevailing jet noise theory at the time (acoustic analogy theory). We reexamine this approach in the context of the current understanding of jet noise, using data from a laboratory jet, a full-scale military jet aircraft, and a full-scale rocket motor. Accurate estimates of Π¯ require good spatial sampling of jet noise directionality; this is not usually possible in volcano acoustic field experiments. Typical volcano acoustic data better represent point measurements of acoustic intensity I¯(θ) at a particular angle θ from the jet axis rather than Π¯. For pure air jet flows, velocity-scaling laws currently proposed for acoustic intensity differ from those for acoustic power and are of the form I¯(θ)∼(V/c)nθ, where c is the ambient sound speed and nθ varies nonlinearly from ∼5 to 10 as a function of temperature ratio and angle θ. Volcanic jet flows are more complex than the pure air laboratory case, which suggests that we do not currently know how the exponent nθ varies for a volcanic jet flow. This indicates that the formulation of Woulff and McGetchin (1976) can lead to large errors when inferring eruption parameters from acoustic data and thus requires modification. Quantitative integration of field, numerical, and laboratory studies within a modern aeroacoustics framework will lead to a more accurate relationship between volcanic infrasound and eruption parameters.

Modal decomposition of exterior acoustic-structure interaction.

Abstract: A modal decomposition technique to analyze individual modal contributions to the sound power radiated from an externally excited structure submerged in a heavy fluid is presented. The fluid-loaded structural modes are calculated by means of a polynomial approximation and symmetric linearization of the underlying nonlinear eigenvalue problem. The eigenvalues and eigenfunctions of a fluid loaded sphere with and without internal structures are presented. The modal sound power contributions using both fluid-loaded structural modes and acoustic radiation modes are presented. The results for the resistive and reactive sound power obtained from the superposition of the individual modal sound power contributions are compared to the harmonic solution of the forced problem.

Aeroacoustic Measurements of a Scaled Half-Model at High Reynolds Numbers

Abstract: The measurement of airframe noise on small-scale models is well known and common practice in conventional wind tunnels. Since conventional wind tunnels cannot generally achieve full-scale Reynolds numbers, measurements during the development process of modern aircraft are often performed in cryogenic and/or pressurized wind tunnels which are capable of higher Reynolds number flows. Thus, the characteristics of the moving fluid are better adapted to the scale model. At the DLR Institute of Aerodynamics and Flow Technology the microphone array measurement technique was further developed to perform measurements in a cryogenic wind tunnel at temperatures down to 100 K. A microphone array consisting of 144 microphones was designed and constructed for this purpose. In this paper, acoustic array measurements performed in a cryogenic wind tunnel are described for various Reynolds numbers using a 9.24% Dornier-728 half model. Additionally, the background noise of the empty test section was measured within the range of the measurements performed on the Dornier-728 half model. Our results seems to indicate a Reynolds number dependency of the measured sound power for various sources.

Dynamics of a vertical cavity quantum cascade phonon laser structure

Abstract: Driven primarily by scientific curiosity, but also by the potential applications of intense sources of coherent sound, researchers have targeted the phonon laser (saser) since the invention of the optical laser over 50 years ago. Here we fabricate a vertical cavity structure designed to operate as a saser oscillator device at a frequency of 325 GHz. It is based on a semiconductor superlattice gain medium, inside a multimode cavity between two acoustic Bragg reflectors. We measure the acoustic output of the device as a function of time after applying electrical pumping. The emission builds in intensity reaching a steady state on a timescale of order 0.1 μs. We show that the results are consistent with a model of the dynamics of a saser cavity exactly analogous to the models used for describing laser dynamics. We also obtain estimates for the gain coefficient, steady-state acoustic power output and efficiency of the device.

Microstructural topology optimization with respect to sound power radiation

Abstract: The paper deals with the problem of topological design of microstructure with respect to minimization of the sound power radiation from a vibrating macrostructure. The macrostructure is excited at a single or a band of excitation frequencies by a time-harmonic mechanical loading with prescribed amplitude and spatial distribution. The structural damping is considered to be proportional damping. The sound power is calculated using a high frequency approximation formulation and thus the sensitivity analysis may be performed in a very efficient manner. The microstructure composed of two different solid isotropic materials is assumed to be identical from point to point at the macro-level which implies that the interface between the structure and the acoustic medium is unchanged during the design process. The equivalent material properties of the macrostructure are calculated using homogenization method and the bi-material SIMP model is employed to achieve zero-one design at the micro-scale. Numerical examples are given to validate the model developed. Some interesting features of acoustic microstructure topology optimization are revealed and discussed.

On aerodynamic noises radiated by the pantograph system of high-speed trains

Abstract: Pantograph system of high-speed trains become significant source of aerodynamic noise when travelling speed exceeds 300 km/h. In this paper, a hybrid method of non-linear acoustic solver (NLAS) and Ffowcs Williams-Hawkings (FW-H) acoustic analogy is used to predict the aerodynamic noise of pantograph system in this speed range. When the simulation method is validated by a benchmark problem of flows around a cylinder of finite span, we calculate the near flow field and far acoustic field surrounding the pantograph system. And then, the frequency spectra and acoustic attenuation with distance are analyzed, showing that the pantograph system noise is a typical broadband one with most acoustic power restricted in the medium-high frequency range from 200 Hz to 5 kHz. The aerodynamic noise of pantograph systems radiates outwards in the form of spherical waves in the far field. Analysis of the overall sound pressure level (OASPL) at different speeds exhibits that the acoustic power grows approximately as the 4th power of train speed. The comparison of noise reduction effects for four types of pantograph covers demonstrates that only case 1 can lessen the total noise by about 3 dB as baffles on both sides can shield sound wave in the spanwise direction. The covers produce additional aerodynamic noise themselves in the other three cases and lead to the rise of OASPLs.

Surface contributions to radiated sound power.

Abstract: This paper presents a method to identify the surface areas of a vibrating structure that contribute to the radiated sound power. The surface contributions of the structure are based on the acoustic radiation modes and are computed for all boundaries of the acoustic domain. The surface contributions are compared to the acoustic intensity, which is a common measure for near-field acoustic energy. Sound intensity usually has positive and negative values that correspond to energy sources and sinks on the surface of the radiating structure. Sound from source and sink areas partially cancel each other and only a fraction of the near-field acoustic energy reaches the far-field. In contrast to the sound intensity, the surface contributions are always positive and no cancelation effects exist. The technique presented here provides a method to localize the relevant radiating surface areas on a vibrating structure. To illustrate the method, the radiated sound power from a baffled square plate is presented.

A mechanism study of sound wave-trapping barriers.

Abstract: The performance of a sound barrier is usually degraded if a large reflecting surface is placed on the source side. A wave-trapping barrier (WTB), with its inner surface covered by wedge-shaped structures, has been proposed to confine waves within the area between the barrier and the reflecting surface, and thus improve the performance. In this paper, the deterioration in performance of a conventional sound barrier due to the reflecting surface is first explained in terms of the resonance effect of the trapped modes. At each resonance frequency, a strong and mode-controlled sound field is generated by the noise source both within and in the vicinity outside the region bounded by the sound barrier and the reflecting surface. It is found that the peak sound pressures in the barrier's shadow zone, which correspond to the minimum values in the barrier's insertion loss, are largely determined by the resonance frequencies and by the shapes and losses of the trapped modes. These peak pressures usually result in high sound intensity component impinging normal to the barrier surface near the top. The WTB can alter the sound wave diffraction at the top of the barrier if the wavelengths of the sound wave are comparable or smaller than the dimensions of the wedge. In this case, the modified barrier profile is capable of re-organizing the pressure distribution within the bounded domain and altering the acoustic properties near the top of the sound barrier.

Optimization of piezoelectric patch positioning for passive sound radiation control of plates

Abstract: The purpose of this article is the optimization of piezoelectric patch positioning for reducing the radiated sound power of thin plates. To this end, an aluminium plate equipped with a set of piezoelectric patches connected to a passive circuit is considered.The difficulties in designing a smart structure are not only related to the conception of the electric circuit used as controller, but also to the choice of how the circuit itself is coupled with the structure. The selection of the number of transducers to be used, and their positioning, is a crucial step in the designing process. In this work the entire design process of a smart structure is proposed, with the goal of obtaining the best efficiency in terms of reduction of radiated sound power, while using a lightweight optimization procedure in terms of computational costs. To this end, classical instruments of vibrations mechanics are used together with acoustic concepts, such as the modal radiation efficiency. The introduction of this acoustic char...

Aeolian tones generated by a square cylinder with a detached flat plate

Abstract: The case of a rigid flat plate placed in the wake of a square cylinder for passive sound control is investigated numerically at a Reynolds number of 150 and a Mach number of 0.2. Two flow regimes are observed, and these are shown to affect the radiated sound. An overall sound reduction at an observer directly above the cylinder is observed for a small gap distance G between the cylinder and the plate (0≤G≤2.3D, in which D is the cylinder height), where a 2.9 dB reduction in the sound-pressure level is obtained when there is no gap between the two bodies. In contrast, the emitted sound-pressure level increases by at least 8.0 dB for large gap distances (2.4D≤G≤7D). Despite this, a 6.3 dB reduction in the sound-pressure level is obtained due to a sound-cancellation mechanism when the plate length is reduced to 0.26D and placed 5.6D downstream of the cylinder. The maximum sound reduction attainable is shown to be limited by the nonlinear unsteady stall process on the plate.

Reconstruction of the free-field radiation from a vibrating structure based on measurements in a noisy environment.

Abstract: In a noisy environment, the sound field of a source is composed of three parts, which are: The field that would be radiated by the target source into free space, the incoming field from disturbing sources or reflections, and the scattered field that is created by the incoming wave falling on the target source. To accurately identify the sound source with nearfield acoustic holography in that situation, the last two parts must be removed from the mixed field. In a previous study, a method for recovering the free sound field in a noisy environment was proposed based on the equivalent source method and measurements of pressure [J. Acoust. Soc. Am. 131(2), 1260–1270 (2012)]. In the present paper, that method was modified by allowing the input data to be measurements of particle velocity instead of pressure. An experiment was carried out to examine both the pressure- and velocity-based methods, and the performance of the two methods was compared. It was found that both methods are capable of reconstructing the free-field pressure radiated by the target source based on measurements made in a noisy environment, but the velocity-based method shows a large benefit in the reconstruction of the free-field particle velocity.

Middle-ear velocity transfer function, cochlear input immittance, and middle-ear efficiency in chinchilla.

Abstract: The transfer function HV between stapes velocity VS and sound pressure near the tympanic membrane PTM is a descriptor of sound transmission through the middle ear (ME). The ME power transmission efficiency (MEE), the ratio of sound power entering the cochlea to power entering the middle ear, was computed from HV measured in seven chinchilla ears and previously reported measurements of ME input admittance YTM and ME pressure gain GMEP [Ravicz and Rosowski, J. Acoust. Soc. Am. 132, 2437–2454 (2012); J. Acoust. Soc. Am. 133, 2208–2223 (2013)] in the same ears. The ME was open, and a pressure sensor was inserted into the cochlear vestibule for most measurements. The cochlear input admittance YC computed from HV and GMEP is controlled by a combination of mass and resistance and is consistent with a minimum-phase system up to 27 kHz. The real part Re{YC}, which relates cochlear sound power to inner-ear sound pressure, decreased gradually with frequency up to 25 kHz and more rapidly above that. MEE was about 0.5 between 0.1 and 8 kHz, higher than previous estimates in this species, and decreased sharply at higher frequencies.

Experimental verification of enhanced sound transmission from water to air at low frequencies

Abstract: Laboratory measurements of enhanced sound transmission from water to air at low frequencies are presented. The pressure at a monitoring hydrophone is found to decrease for shallow source depths in agreement with the classical theory of a monopole source in proximity to a pressure release interface. On the other hand, for source depths below 1/10 of an acoustic wavelength in water, the radiation pattern in the air measured by two microphones becomes progressively omnidirectional in contrast to the classical geometrical acoustics picture in which sound is contained within a cone of 13.4° half angle. The measured directivities agree with wavenumber integration results for a point source over a range of frequencies and source depths. The wider radiation pattern owes itself to the conversion of evanescent waves in the water into propagating waves in the air that fill the angular space outside the cone. A ratio of pressure measurements made using an on-axis microphone and a near-axis hydrophone are also reported and compared with theory. Collectively, these pressure measurements are consistent with the theory of anomalous transparency of the water-air interface in which a large fraction of acoustic power emitted by a shallow source is radiated into the air.

Effect of Rotor Shielding on Fan-Outlet Guide Vanes Broadband Noise Prediction

Abstract: An analytical model for the rotor–stator broadband noise is improved by accounting for the rotor acoustic shielding. Both the analytical models for the stator broadband noise and for the rotor scattering are strip-theory approaches based on previously published formulations of the three-dimensional unsteady blade loading for a rectilinear cascade. Specific treatment is introduced to address the behavior around the cut on frequency of the duct modes both in the noise generation and in the noise scattering. A simple effect of the swirl between the rotor and the stator is considered using a Doppler shift in frequency. The power spectral density of the acoustic power and the sound pressure level at the duct wall are studied for the NASA source diagnostic test fan rig. The separate effects of the shielding by a cascade of the rotation of the rotor and of the swirl are investigated. Corotating modes dominate in the inlet. The rotor shielding decreases the acoustic power at intermediate frequencies but increases...

Optimization of Noise Transmission Through Sandwich Structures

Abstract: An optimization methodology to increase the noise transmission loss (TL) of damped sandwich structures is presented. The prediction of the TL uses a numerical tool based on a finite element formulation for the sandwich plate coupled to a boundary element method for the acoustic medium. This tool can be used for arbitrarily shaped three-layer sandwich plates with various boundary conditions and it is well adapted to parametric and optimization studies. First, a parametric study was conducted to choose the objective function, the constraints, and the pertinent design variables to use in the optimization problem which consist in reducing the sound power transmitted by a viscoelastically damped sandwich plate. Next, by constraining the acoustical behavior of the sandwich panel, the surface mass of the sandwich structure was minimized. It is shown that a significant reduction in the transmitted sound power can be achieved by selecting the appropriate geometric configuration and damping layer material. [DOI: 10.1115/1.4024216]

Underwater sound radiation from an elastically coated plate with a discontinuity introduced by a signal conditioning plate.

Abstract: Underwater structural sound radiation and reflection can be reduced by adding an elastic coating to a structure. To increase the signal-to-noise ratio of a detecting hydrophone placed in front of the coating, a signal conditioning plate (SCP) of finite size placed between the coating and the hydrophone is proposed to increase the local reflection of incoming sound. This paper studies the effects of a SCP as a distributed discontinuity on the general properties of the sound radiation of a coated plate. The discontinuity introduced by the SCP changes the vibration and radiated sound power of the coated plate by scattering and reflecting structural waves in the coating. The trapped modal response of the structural waves is also observed and found to be responsible for the increase of sound and vibration of the plate at the corresponding resonance frequencies.

Transmission function of collinear acousto-optical interaction occurred by acoustic waves of finite amplitude

Abstract: New physical aspects of collinear acousto-optical interaction, occurred by acoustic waves of finite amplitude, are revealed and analyzed in crystalline materials exhibiting moderate linear acoustic losses. The analysis is performed in the regime of continuous traveling waves allowing a specific mechanism of the acousto-optic nonlinearity. Our consideration has shown that such nonlinearity together with linear acoustic losses is able to affect the transmission function inherent in collinear interaction. In particular, the mere presence of linear acoustic losses by themselves leads to broadening the width of the transmission function beginning already from very low levels of the applied acoustic power. Moreover, the transmission function exhibits a marked and quasi-periodical dependence on the applied acoustic power density, and that periodicity is governed by the linear acoustic losses. As a result, the transmission function can be significantly narrowed near isolated points at the cost of decreasing the interaction efficiency. These novelties related to collinear acousto-optical interaction accompanied by moderate linear acoustic losses have been studied and confirmed experimentally with an advanced acousto-optical cell based on calcium molybdate (CaMoO4) single crystal and controlled by acoustic waves of finite amplitude.

Sound Generated by Boundary-Layer Flow over Small Steps: Effect of Step Noncompactness

Abstract: The effect of acoustically non-compact step height on sound generation in a turbulent boundary layer is studied by computing sound with a boundary-element method and comparing its results with those obtained with a compact-step-height assumption. When the ratio of acoustic wavelength to the step height is large, good agreements between the two methods in predicted sound spectra and directivities are found, thus validating the previous compact Green’s function solutions. Discrepancies between the two results become significant with decreasing �/h. For mildly non-compact step heights, the sound directivity for forward steps exhibits asymmetry, with the upstream side maintaining an approximate dipole shape while the rest significantly distorted. Dips in sound spectra and multiple lobes in directivity are observed along with significantly enhanced sound power when �/h is sufficiently small, indicating that edge-scattering becomes the dominant source mechanism. These results indicate that the compact-step-height assumption is generally applicable for underwater applications, where small Mach numbers give large �/h for most frequencies of interest. For air vehicles with large Mach numbers, its applicability is much more limited.

Measurement and Analysis of Noise in UHV AC Substation

Abstract: In order to obtain the boundary noise level in ultra-high voltage(UHV) AC substation,we measured the noise levels of transformer and reactor in station,as well as their spectral characteristics,attenuation characteristics and relationship to the power load.The measurement was mainly performed in a substation of UHV Test Demonstration Project in China,in its normal operation and load commissioning period,respectively.The measurement results were further analyzed.It is concluded that the noise of transformer is mainly at low and medium frequencies,it is more influenced by cooling fan,and it has an approximate positive correlation with the power load.The energy of reactor noise concentrates on the 1/3 octave band centered at 100 Hz,while has little correlation with the power load.The noise of transformer and reactor slowly decreases with increasing the distance from equipment,so the transformer and reactor should not be placed close to environmentally-sensitive sound areas.In addition,we verified the A-weighted sound power levels of UHV transformers and reactors under normal load to be approximately 103 dB and 97 dB,respectively;we also obtained the parameters of sound power level in 1/3 octave band of the sound source,providing basic data for predictions and evaluations of substation noise.