Showing papers on "Acoustic interferometer published in 1997"

A new hybrid passive/active noise absorption system

Abstract: A new hybrid passive–active system is developed for sound absorption over a wide frequency range. The system is comprised of a layer of absorbing material positioned at a distance from an active wall, leaving an air space. The motion of the active wall is based on a new control approach which consists of the minimization of the reflected wave within the airspace which modifies the layer’s back surface impedance so as to match the characteristic impedance of air. This technique is referred here as inducing an impedance-matching condition. Both numerical and experimental results of such a system are presented for normally incident planar waves. The hybrid passive–active system results in a high absorption coefficient of 0.8–1.0 over the frequency range 100–2000 Hz and is insensitive to system parameters such as air space depth and absorbing layer thickness.

Interferometer with air turbulence compensation

Abstract: An improved interferometer measuring system that corrects for errors in the determination of the position of a measurement reflector along a measurement path due to the presence of an atmosphere (e.g. atmospheric turbulence) along the path is disclosed. The system includes a two-wavelength interferometer for measuring the atmosphere and a basic length interferometer for a basic measurement of a change in position of the measurement reflector. A calibration procedure for correcting the basic measurements made by the basic length interferometer uses first and second correction coefficients related to the average refractivity of the atmosphere and the change in the refractivity of the atmosphere, respectively. The coefficients can be determined by interferometric measurements or from a combination of interferometric measurements and data from an atmospheric sensor(s), including a humidity sensor. The two-wavelength interferometer can include a pulsed laser source, a compact reference path element, and path length adjustment element. An algorithm is disclosed for efficiently determining the phase difference between the two beams of the two-wavelength interferometer. Beam combining techniques and apparatus for combining the beams of the two-wavelength interferometer and the basic length measurement interferometer while preserving the polarization of the laser beams are also disclosed. The data obtained from the two-wavelength interferometer can be corrected, based on measurements made by the basic length interferometer, for aging due to the movement of the reflecting mirror subsequent to measurement of the atmosphere by the two-wavelength interferometer.

Reciprocity calibration of acoustic emission transducers in Rayleigh-wave and longitudinal-wave sound fields

Abstract: A new system was developed for the reciprocity calibration of acoustic emission transducers in Rayleigh-wave and longitudinal-wave sound fields. In order to reduce interference from spurious waves due to reflections and mode conversions, a large cylindrical block of forged steel was prepared for the transfer medium, and direct and spurious waves were discriminated between on the basis of their arrival times. Frequency characteristics of velocity sensitivity to both the Rayleigh wave and longitudinal wave were determined in the range of 50 kHz–1 MHz by means of electrical measurements without the use of mechanical sound sources or reference transducers.

On the antenna pattern of an orbiting interferometer

Abstract: We consider the response of an interferometer moving with respect to a fixed reference frame, and derive a simple expression for the beam-pattern factors and the polarization-averaged antenna power pattern...Then, we consider a class of motions where the detector's plane is constrained to move on the surface of a cone of constant aperture; at the same time, the two arms are rotating around a vertical axis...In addition, we consider the case of an Earth-based interferometer...Finally, we discuss LISA's response to circularly polarized sinusoidal waves, coming from a few known binary systems in our Galaxy.

Surface wave formation at an impedance discontinuity

Abstract: The formation of air-coupled acoustic surface waves over an impedance plane has been studied in a series of model experiments Surface waves were generated at an impedance discontinuity on a flat surface, one section of the surface being acoustically hard, and the other composed of an array of small cavities with an effective impedance suitable for surface wave propagation A point source of sound (800 Hz–2 kHz) was located on the hard surface and the vertical sound pressure distribution measured over the impedance plane, at various distances The formation, propagation, and decay of the surface waves were clearly observed The measured pressure distributions are consistent with the theoretical formulation of Rasmussen [J Sound Vib 84, 598–602 (1982)] for propagation of sound over an impedance jump An alternative formulation for the case of plane waves incident at grazing angles to a surface has been described by McAninch and Myers [AIAA 26th Aerospace Sciences Meeting, paper AIAA-88-0179 (1988)] This

Direct experimental investigations of acoustic modes guided by a solid–solid interface using optical interferometry

Abstract: This paper presents direct field measurements of acoustic modes guided by the interface between two transparent solids. The measurement technique is based on the acousto-optical interaction inside the solid between the acoustic field and the probe laser beam of an interferometer. The main advantage of the method is its ability to measure acoustic strain fields in areas of difficult access with the classic detection methods. Moreover, it gives complete information about the dilatation strain field inside the solid, e.g., amplitude and phase. The propagation of a real velocity mode (Stoneley wave) is first illustrated. Then the situation of complex velocity modes is investigated for a Plexiglas–fused quartz slip interface. This material combination supports two possible interface modes theoretically. These modes are simultaneously observed and the differences between their behavior are measured.

An ultrasonic density probe

Abstract: An ultrasonic density probe for liquids using a single transducer is developed, as a first step in the design of an ultrasonic mass flowmeter. The probe is based on the reflection of sound at the probe's interface with the fluid. The amplitude of the echo from that interface indicates the characteristic acoustic impedance of the liquid. The liquid's density is the ratio of its acoustic impedance to the speed of sound through it. The latter is obtained by measuring the time necessary for the transmitted sound pulse to be echoed back from a reflector located at a known distance in the fluid. The probe consists of two buffer rods in series behind which sits a transducer. This arrangement removes any dependence on the amplitude of the initial pulse. The effects of temperature on the buffer rods' characteristics are numerically compensated for by the dependence of the measured speeds of sound on the temperature of the traveled media. Results show an accuracy of better than 1% with a 95% confidence level for water at temperatures ranging from 2 to 40/spl deg/C.

Liquid information measuring device using sound wave

Abstract: PROBLEM TO BE SOLVED: To provide a liquid information measuring device designed to measure the liquid level and the propagation time of a sound wave by use of a single sound wave oscillating element which produces the sound wave. SOLUTION: An ultrasonic sensor S has an ultrasonic oscillating element 20 having a transmitting-receiving part 22 disposed in fuel and has reflectors 33, 34 disposed in the fuel. A reflector 35 reflects ultrasonic waves of mutually proximate frequencies transmitted from the ultrasonic oscillating element 20 and reflects the waves toward the ultrasonic oscillating element 20. The reflector 34 reflects toward the level of the fuel the ultrasonic waves transmitted from the ultrasonic oscillating element 20 and, when sound waves reflected from the level of the fuel impinge thereon, reflects the sound waves toward the transmitting-receiving part 22. A phase measuring apparatus 90 binarizes the received outputs derived from the reflected ultrasonic waves from the ultrasonic oscillating element 20, and calculates the phase difference between the binarized outputs. A microcomputer 40 calculates the propagation time of the sound wave and the level of the fuel according to the phase difference and each of the frequencies.

Speed of Sound and Ideal-Gas Heat Capacity at Constant Pressure of Gaseous Difluoromethane

Abstract: The speed of sound of gaseous difluoromethane (HFC-32) was measured for temperatures from 273.15 K to 333.15 K and pressures from 48 kPa to 390 kPa with a cylindrical, variable-path acoustic interferometer operating at 156.252 kHz. The uncertainty of the speed of sound was less than ±0.1%. The ideal-gas heat capacity at constant pressure and the second acoustic virial coefficients were determined over the temperature range from the speed of sound measurements. The ideal-gas heat capacity at constant pressure results and second virial coefficients calculated from these speed of sound measurements were compared with results from the literature determined from PVT measurements and from speed of sound measurements. The uncertainty of the ideal-gas heat capacity at constant pressure was estimated to be less than ±1%.

Local and selective detection of acoustic waves at the surface of a material

Abstract: A method for detecting selectively elastic waves at the surface of a solid is described. This method is based on the propagation of transversal waves in a tapered rod. The probe is composed of a shear wave transducer coupled to a focusing tip. Selective detection of acoustic waves at the surface of materials is experimentally demonstrated. Conical and parabolic tips are compared. The sensitivity of the probe as well as the local perturbation induced by the contact is investigated with an optical heterodyne interferometer.

Sound damper for ultrasonic waves

Abstract: The invention relates to, according to the first teaching, a sound damper for ultrasonic waves in a stream of gas, which damper has at least one damping member (3) insertable into a line for the stream of gas. According to the invention, the sound damper for ultrasonic waves is characterised in that at least one scatter unit (5, 6, 7) having a plurality of small-area reflection surfaces is arranged in the damping member (3).

A liquid wedge as generating technique for Lamb and Rayleigh waves

Abstract: A model is developed to describe the excitation of Lamb and Rayleigh waves by a Gaussian beam incident on a liquid/air/solid corner. Analytical expressions for the displacement fields of the generated modes are derived by applying a mode theory that accounts for the contribution of the reflected waves. Numerical computations illustrate the optimum conditions under which the strongest stimulation of Lamb and Rayleigh waves may be expected.

Prediction of the vibro-acoustic transmission loss of planar hose-pipe systems

Abstract: Vibro-acoustic energy travels through hose walls as longitudinal waves and flexural waves, apart from the sound waves through the fluid medium inside. Longitudinal waves in the hose wall are coupled to the sound waves inside by means of the hose-wall Poisson’s ratio. Both in turn get coupled to bending or flexural waves because of the energy transfer or interaction at the bends. For any of these three types of waves incident on one end of a hose, waves of all the three types may be transmitted on the other end because of their dynamical coupling with one another. Therefore, in the present paper, expressions have been derived for the 3×3 transmission loss matrix for a two-dimensional or planar piping system in terms of elements of the overall 8×8 transfer matrix of the system. These expressions have then been used in a comprehensive computer program to evaluate the vibro-acoustic performance of hoses, with particular application to the automotive climate control systems with gaseous as well as liquid media. Finally, parametric studies have been made that have led to some general design guidelines.

Microfluidic motion generation with loosely-focused acoustic waves

Abstract: This paper presents a novel approach for producing microfluidic motions by loosely-focused acoustic waves (generated by a piezoelectric ZnO thin film). Results show that the acoustic-waves generated by RF sources, with frequencies corresponding to thickness-mode resonances of the piezoelectric film, are very effective in moving liquid around when the waves are loosely focused. The device operates without any significant temperature increase in the liquid, and will be very attractive for mixing or transporting temperature-sensitive fluids.

Acousto-optical filter for optical spectrometer

Abstract: The filter uses an optical crystal (13) with entry and exit surfaces (15,17) at a given angle to the light direction and an acoustic surface (19) coupled to an acoustic transducer (21) providing acoustic waves which interact with the optical waves within the crystal. The crystal has an extension (25) continuous with the exit surface, so that the acoustic waves reflected from the latter are directed along the extension by successive reflection from its opposing surfaces.

High velocity pseudosurface waves on LiNbO3 with thin overlayers

Abstract: A numerical study is made of high velocity pseudosurface acoustic waves (HVPSAW) on coated LiNbO3. Standard YZ and 41° YX cuts are considered. In contrast to the pure substrate, the presence of typical thin overlayers leads to the appearance of a well-localized low-attenuating HVPSAW mode at certain values of the film thickness. The depth of the localization domain may be smaller than the wavelength.

Loudspeaker with front-mounted acoustic lens e.g. for audio reproduction

Abstract: The loudspeaker has a membrane (12) which is oscillated via a drive system (11) for providing sound waves within a listening room (17), with an acoustic lens (14) between the membrane and the room. The acoustic lens is provided by a layer which is transparent to the sound waves emitted by the loudspeaker, with a sound wave propagation velocity differing from the propagation velocity of the sound waves in the space between the membrane and the acoustic lens. The latter may be made of a foam material with at least one of its surfaces provided with a one-dimensional or two-dimensional curvature.

Laser-generated nonlinear surface acoustic waves and applications for surface cleaning

Abstract: CTuX3 Fig. 1 White-light transmission image of an array of microlenses in Ge,,Se,,. Spaced 400 pm apart, their diameters are 150 2 20 pm. The rings surrounding the lenses are not depressions, but rather the effect of a photoinduced index change in the surrounding material on the transmitted light. This photoinduced change is due to photodarkening occurring in the wings of the 270-pm-diameter write beam. The beam was cw at 800 nm with an incident intensity of 120 W/cmZ.

Some aspects of recording an acoustic wave in a liquid by a fiber-mounted piezoelectric transducer

Abstract: The main characteristics of a piezoelectric transducer mounted on an optical fiber and used to measure the acoustic signals induced by interaction between the radiation and the material are described. Results of an investigation of a laser-induced acoustic signal in a liquid are presented.