Search results for "ACOUSTIC"
showing 10 items of 1590 documents
Acoustic transmitters for underwater neutrino telescopes.
2012
In this paper acoustic transmitters that were developed for use in underwater neutrino telescopes are presented. Firstly, an acoustic transceiver has been developed as part of the acoustic positioning system of neutrino telescopes. These infrastructures are not completely rigid and require a positioning system in order to monitor the position of the optical sensors which move due to sea currents. To guarantee a reliable and versatile system, the transceiver has the requirements of reduced cost, low power consumption, high pressure withstanding (up to 500 bars), high intensity for emission, low intrinsic noise, arbitrary signals for emission and the capacity of acquiring and processing recei…
Experimental observations of topologically guided water waves within non-hexagonal structures
2020
International audience; We investigate symmetry-protected topological water waves within a strategically engineered square lattice system. Thus far, symmetry protected topological modes in hexagonal systems have primarily been studied in electromagnetism and acoustics, i.e., dispersionless media. Herein, we show experimentally how crucial geometrical properties of square structures allow for topological transport that is ordinarily forbidden within conventional hexagonal structures. We perform numerical simulations that take into account the inherent dispersion within water waves and devise a topological insulator that supports symmetry-protected transport along the domain walls. Our measur…
Intraluminal Doppler-sonography: a new device for data analysis and its in vitro test
2002
Summary form only given. A special data acquisition unit has been developed which allows to process on-line not only the mean-phasic Doppler shift signal but also to analyze the spectral contents and the received A-mode signal. The probe consists of a 3f-catheter which is activated by a pulsed transmitter-receiver (Millar). The carrier frequency is 20 MHz and the impulse repetition rate is 62.5 kHz. Two modular A/D converters are used. One digitizes directly the Doppler shift frequencies in the range less than 20 kHz (sampling rate 100 kHz, resolution 12 bit); the other digitizes the RF-signal (sampling rate 200 MHz, resolution 8 bit). For further processing, data are transferred via separa…
A perfect Fresnel acoustic reflector implemented by a Fano-resonant metascreen
2018
We propose a perfectly reflecting acoustic metasurface which is designed by replacing the curved segments of the traditional Fresnel reflector by flat Fano-resonant sub-wavelength unit cells. To preserve the original Fresnel focusing mechanism, the unit cell phase follows a specific phase profile which is obtained by applying the generalized Snell's law and Fermat's principle. The reflected curved phase fronts are thus created at the air-metasurface boundary by tailoring the metasurface dispersion as dictated by Huygens' principle. Since the unit cells are implemented by sub-wavelength double slit-shaped cavity resonators, the impinging sound waves are perfectly reflected producing acoustic…
Sound conversion phenomena at the free surface of liquid helium. II. Experimental determination of acoustic coefficients and surface absorption coeff…
1983
The acoustic coefficients of reflection, transmission, and transformation of first-, second-, and gas sound waves incident normally on the free surface of liquid helium have been determined. The experimental results are compared with the theory developed in Part I of this paper. It is shown that equilibrium theories of boundary conditions disagree with the experiments. Nonequilibrium theories, however, which take into account that mass and energy transport processes occur at the liquid-vapor interface of helium II, lead to a very good description of the experimental data below 2 K. Within experimental accuracy the measurements fully confirm several kinetic models of evaporation and energy t…
Theoretical studies of the propagation of sound in narrow channels filled with helium II. I. The dispersion relations of fourth sound and of the fift…
1971
The wave propagation in helium II bounded by two plane-parallel plates forming a narrow channel is considered. The theory is based on the complete linearized set of the Khalatnikov equations. These equations are exactly averaged over the width of the channel taking into account the boundary conditions and symmetry relations. It is shown that in narrow channels three solutions of these equations exist; (a) fourth sound, (b) the so-called fifth-wave mode, and (c) another wave mode, which is very strongly damped. The dispersion relations of these wave modes are calculated with regard to all kinetic coefficients and to the coefficient of thermal expansion. The phase velocities and absorption co…
Sound absorption prediction of linear damped acoustic resonators using a lightweight hybrid model
2019
International audience; A lightweight numerical method is developed to predict the sound absorption coefficient of resonators whose cross-section dimensions are significantly larger compared to the viscous and thermal boundary layer’s thicknesses. This method is based on the boundary layer theory and on the perturbations theory. According to the perturbations theory, in acoustical domains with large dimensions, the fluid viscosity and thermal conductivity only affect the boundary layers. The model proposed in this article combines the lossless Helmholtz wave equation derived from a perfect fluid hypothesis, with viscosity and thermal conductivity values of a real fluid to compute the sound …
Spin caloric effects in antiferromagnets assisted by an external spin current
2018
Searching for novel spin caloric effects in antiferromagnets we study the properties of thermally activated magnons in the presence of an external spin current and temperature gradient. We predict the spin Peltier effect -- generation of a heat flux by spin accumulation -- in an antiferromagnetic insulator with cubic or uniaxial magnetic symmetry. This effect is related with spin-current induced splitting of the relaxation times of the magnons with opposite spin direction. We show that the Peltier effect can trigger antiferromagnetic domain wall motion with a force whose value grows with the temperature of a sample. At a temperature, larger than the energy of the low-frequency magnons, this…
Magnetic skyrmions: from fundamental to applications
2016
In this topical review, we will discuss recent advances in the field of skyrmionics (fundamental and applied aspects) mainly focusing on skyrmions that can be realized in thin film structures where an ultrathin ferromagnetic layer (<1 nm) is coupled to materials with large spin-orbit coupling. We review the basic topological nature of the skyrmion spin structure that can entail a stabilization due to the chiral exchange interaction present in many multilayer systems with structural inversion asymmetry. The static spin structures and the dynamics of the skyrmions are also discussed. In particular, we show that skyrmions can be displaced with high reliability and efficiency as needed for t…
Eulerian models of the rotating flexible wheelset for high frequency railway dynamics
2019
Abstract In this paper three formulations based on an Eulerian approach are presented to obtain the dynamic response of an elastic solid of revolution, which rotates around its main axis at constant angular velocity. The formulations are especially suitable for the study of the interaction of a solid with a non-rotating structure, such as occurs in the coupled dynamics of a railway wheelset with the track. With respect to previous publications that may adopt similar hypotheses, this paper proposes more compact formulations and eliminates certain numerical problems associated with the presence of second-order derivatives with respect to the spatial coordinates. Three different models are dev…