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The paper presents the problems encountered during calibration of devices equipped with all-weather microphones, which results in frequent omissions in the periodic check-up of monitoring systems. Such a check-up should be regular...
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The paper presents the problems encountered during calibration of devices equipped with all-weather microphones, which results in frequent omissions in the periodic check-up of monitoring systems. Such a check-up should be regularly carried out, as it would result in broadening and unification of the knowledge concerning the measurements and the evaluation of their uncertainty in the environmental conditions.
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Sound transmission through a water-air interface is normally weak because of a strong mass density contrast. We show that the transparency of the interface increases dramatically at low frequencies. Almost all acoustic energy emit...
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Sound transmission through a water-air interface is normally weak because of a strong mass density contrast. We show that the transparency of the interface increases dramatically at low frequencies. Almost all acoustic energy emitted by a sufficiently shallow monopole source under water is predicted to be radiated into air. Increased transparency at lower frequencies is due to the increasing role of inhomoge-neous waves. For sources symmetric with respect to a horizontal plane, transparency is further increased by a destructive interference of direct and surface-reflected waves under water. The phenomenon of anomalous transparency has significant geophysical and biological implications.
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We investigate experimentally the deformation of acoustic wavefronts after crossing of a single, isolated vortex in free space. The incident sound wavelength can be varied in a large domain. We study the wavefronts at variable dis...
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We investigate experimentally the deformation of acoustic wavefronts after crossing of a single, isolated vortex in free space. The incident sound wavelength can be varied in a large domain. We study the wavefronts at variable distance after transmission through the vortex, when the wavelength and the vortex strength are varied. For small wavelength (λ <收起
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We present a study of sound propagation through a single vortex by direct numerical simulations (DNS) compared to experimental measurements. We analyse the amplitude and the phase shift of the sound wave when it interacts with the...
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We present a study of sound propagation through a single vortex by direct numerical simulations (DNS) compared to experimental measurements. We analyse the amplitude and the phase shift of the sound wave when it interacts with the vortical flow and we display the focusing effects produced by the vortex. We show that the turbulent fluctuations have a little effect on the sound phase shift whereas they induce a strong defocusing effect on the sound amplitude.
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We present the experimental realization and characterization of an acoustic metamaterial with negative bulk modulus. The metamaterial consists of a two-dimensional array of cylindrical cavities, and the bulk modulus is controlled ...
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We present the experimental realization and characterization of an acoustic metamaterial with negative bulk modulus. The metamaterial consists of a two-dimensional array of cylindrical cavities, and the bulk modulus is controlled by their radius size and length. Experiments are performed in a two-dimensional waveguide where a slab of seven layers is used to extract the parameters of the metamaterial. A complete characterization of the constructed structure is reported, including the dispersion relation of the acoustic bands and the skin depth effect, which both have been measured, and the data are well supported by semianalytical models and by finite-element simulations.
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This Letter reports the design, fabrication, and experimental characterization of hyperbolic materials showing negative refraction and energy funneling of airborne sound. Negative refraction is demonstrated using a stack of five h...
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This Letter reports the design, fabrication, and experimental characterization of hyperbolic materials showing negative refraction and energy funneling of airborne sound. Negative refraction is demonstrated using a stack of five holey Plexiglas plates where their thicknesses, layer separation, hole diameters, and lattice periodicity have been determined to show hyperbolic dispersion around 40 kHz. The resulting hyperbolic material shows a flat band profile in the equifrequency contour allowing the gathering of acoustic energy in a broad range of incident angles and its funneling through the material. Our demonstrations foresee interesting developments based on both phenomena. Acoustic imaging with subwavelength resolution and spot-size converters that harvest and squeeze sound waves irradiating from many directions into a collimated beam are just two possible applications among many.
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This Letter presents the design, fabrication, and experimental characterization of a directional three-dimensional acoustic cloak for airborne sound. The cloak consists of 60 concentric acoustically rigid tori surrounding the cloa...
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This Letter presents the design, fabrication, and experimental characterization of a directional three-dimensional acoustic cloak for airborne sound. The cloak consists of 60 concentric acoustically rigid tori surrounding the cloaked object, a sphere of radius 4 cm. The major radii and positions of the tori along the symmetry axis are determined using the condition of complete cancellation of the acoustic field scattered from the sphere. They are obtained through an optimization technique that combines genetic algorithm and simulated annealing. The scattering cross section of the sphere with the cloak, which is the magnitude that is minimized, is calculated using the method of fundamental solutions. The low-loss fabricated cloak shows a reduction of the 90% of the sphere scattering cross section at the frequency of 8.55 kHz.
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We study long-range correlation of diffuse acoustic noise fields in an arbitrary inhomogeneous, moving fluid. The flow reversal theorem is used to show that the cross-correlation function of ambient noise provides an estimate of a...
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We study long-range correlation of diffuse acoustic noise fields in an arbitrary inhomogeneous, moving fluid. The flow reversal theorem is used to show that the cross-correlation function of ambient noise provides an estimate of a combination of the Green's functions corresponding to sound propagation in opposite directions between the two receivers. Measurements of the noise cross correlation allow one to quantify flow-induced acoustic nonreciprocity and evaluate both spatially averaged flow velocity and sound speed between the two points.
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This study constitutes a proof of concept of a new feedback-controlled optical microphone for potential use in phased 'beam-forming' arrays utilized in aeroacoustics research. In the new microphone design, a fiber-optic lever sens...
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This study constitutes a proof of concept of a new feedback-controlled optical microphone for potential use in phased 'beam-forming' arrays utilized in aeroacoustics research. In the new microphone design, a fiber-optic lever sensor is employed as a means for measuring the center displacement of a stretched thin membrane caused by incident acoustic pressure. The membrane is constructed from polyvinylidene-fluoride which exhibits piezoelectric properties allowing actuation of the membrane in a feedback system to nullify the optically detected deflection. The feedback provision was used to actively modify sensor parameters, most notably membrane stiffness, resonant frequency and damping. Testing of a prototype microphone was performed using a plane wave tube calibrator. Using feedback control, the fundamental resonant frequency of the prototype capsule was increased from 3.61 to 5.1 kHz, a 41percent increase. Also, feedback was used for dc attenuation, equivalent to 'stiffening' of the microphone membrane. The results demonstrate that feedback control is an effective method for improving the microphone's transient response, as well as for 'self-tuning' and matching of microphone parameters in sensing arrays.
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This article reports the results of an experimental campaign where four different inlet geometries for the compressor of an automotive turbocharger were acoustically characterized. These four geometries (a straight pipe for refere...
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This article reports the results of an experimental campaign where four different inlet geometries for the compressor of an automotive turbocharger were acoustically characterized. These four geometries (a straight pipe for reference, a tapered duct, a 90 degrees elbow and a reservoir) were selected for their potential for deep surge margin enhancement, while being simple enough to be commonly found in production vehicles. A detailed measurement of this surge margin enhancement was performed, together with acoustic measurements of both radiated and orifice noise at design conditions of best isentropic efficiency and also close to the deep surge limit. Results demonstrated that while all the proposed geometries indeed enlarged the usable air mass flow range, changes in the acoustic behavior of the system could be positive, neutral or even negative. It is therefore important to carefully consider accurate noise measurements before implementing these geometric solutions in production vehicles and to further pursue research on the link between the characteristic flow pattern produced by each inlet geometry and the noise emission of the turbocompressor.
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