Showing papers on "Acoustic interferometer published in 1981"

Precision acoustic measurements with a spherical resonator: Ar and C2H4

Abstract: The spherical acoustic resonator is a remarkably accurate and convenient tool for the measurement of thermophysical properties of gases at low and moderate densities. The speed of sound (c) in a gas of interest can be measured with an accuracy of 0.02% merely by measuring the frequencies of the radial resonances when the resonator is filled with the gas of interest and then repeating the frequency measurements with a reference gas such as argon. The resonance frequencies of the radial modes are easily measured because these modes have very high Q’s, typically 2000–10 000. In this work the precision and accuracy of speed of sound measurements have been substantially improved by including a detailed acoustic model of the resonator in the analysis. Many of the important parameters of the model can be determined from acoustic measurements: Painstaking mechanical measurements are not required. We have used a spherical resonator to measure the speed and attenuation of sound in C2H4 in the temperature range 0–100 °C and the pressure range 0.15–1.0 MPa. Our measured values of c in C2H4 have a precision of 0.003% and agree with those of Gammon within the scatter of Gammon’s data (±0.02%). This agreement is remarkable when one considers that our spherical resonator is operated in the frequency range 4–13 kHz while Gammon has used a more conventional, cylindrical, variable path, acoustic interferometer operating at 500–2500 kHz. To attain this agreement, we did not have to make any highly accurate measurements other than frequency. Our measured values of the bulk relaxation frequency of C2H4 are within the scatter of the more recent values of the literature. In the course of our ’’calibration’’ measurements with argon we have redetermined the leading acoustic virial coefficient of argon. Our values for the virial are in satisfactory agreement with those in the literature. We include several practical suggestions for increasing the accuracy and/or versatility of spherical resonators.

Measurement method and system utilizing ultrasonic wave

Abstract: A measurement method and system for measuring characteristics of attenuation of domains in an object comprises transmitting ultrasonic waves into the object and receiving ultrasonic waves reflected from the object. The measured characteristics of attenuation of reflected waves is determined using signal intensities. A plurality of ultrasonic waves having different frequencies are transmitted either simultaneously or alternately to an object, and the reflected waves are received from the object. The signal intensities corresponding to the transmission frequencies among the reflected waves are stored and the signal intensity ratio is calculated. The signal intensity ratio indicates the attenuation characteristic. The attenuation coefficient can also be obtained using a time interval from the transmission time to the time of the reflected wave is received. The attenuation slope can be obtained from this attenuation coefficient and the frequency difference between transmitted ultrasonic waves.

Refraction and Scattering of Sound by a Shear Layer

Abstract: The angle and amplitude changes for acoustic waves refracted by a circular open jet shear layer were determined. The generalized refraction theory was assessed experimentally for on axis and off axis acoustic source locations as source frequency varied from 1 kHz to 10 kHz and free stream Mach number varied from 0.1 to 0.4. Angle and amplitude changes across the shear layer show good agreement with theory. Experiments confirm that the refraction theory is independent of shear layer thickness, acoustic source frequency, and source type. A generalized theory is, thus, available for correcting far field noise data acquired in open jet test facilities. The effect of discrete tone scattering by the open jet turbulent shear layer was also studied. Scattering effects were investigated over the same Mach number range as frequency varied from 5 kHz to 15 kHz. Attenuation of discrete tone amplitude and tone broadening were measured as a function of acoustic source position and radiation angle. Scattering was found to be stronger at angles close to the open jet axis than at 90 deg, and becomes stronger as the acoustic source position shifts downstream. A scattering analysis provided an estimate of the onset of discrete tone scattering.

Diffraction of light by acoustic waves on a membrane

Abstract: The diffraction of light by acoustic waves on a membrane is reported. A device can be made to perform acousto-optic signal processing at much lower frequencies than can be handled by the standard Bragg acousto-optic cell. Acoustic waveguiding phenomena are demonstrated. The device can handle simultaneous parallel-array data processing.

Acoustic Impedance Measurement by Reflection of an Ultrasonic Impulse on a Specimen Through a Coupling Layer

Abstract: Abstmcr-A new method to measure the acoustic impedance of a solid medium which presents only one plane surface is described. This method is based on the resonance of the liquid coupling layer between the transducer and the medium investigated. First, the resonance of the layer is studied, and from its relative amplitude the acoustic impedance of the medium can be deduced. Second, the influence of the lack of parallelism of the coupling layer is determined, and a model is proposed to explain the experimental results. From the observation of the resonance when the transducer is inclined, it is possible to adjust correctly the parallelism of the layer so the acoustic impedance of the medium can be obtained with an accuracy varying between 4 and 8 percent.

1.6 GHz SH-Type Surface Acoustic Wave Filter

Abstract: Surface skimming bulk waves (SSBW) are considered to be suitable for high frequency applications because of their high phase velocity. We analyzed these waves taking into account electrical and mechanical loading effects of interdigital electrodes. Calculated and experimental results indicate that these waves should be treated as SH-type surface acoustic waves or Love waves. A 1.6 GHz SH-type SAW timing filter was developed for optical fiber transmission systems. The insertion loss was less than 12 dB, loaded Q was more than 800 and good temperature stability was obtained.

Direct Fourier Synthesis of Waves: Application to Acoustic Source Radiation

Abstract: In numerous situations of current interest in aeroacoustics the sound field propagates in a nonhomogeneous medium. This is typically the case of jet noise radiation. A widely used model in that situation consists of a thin vortex sheet separating two uniformly moving fluids. This model may be treated as a layered medium, and acoustic source radiation may be analyzed by Fourier transform techniques. The inversion of the Fourier solution is however difficult and requires far-field approximations. This difficulty is here avoided by constructing the wave field by direct Fourier synthesis. This method was shown, in a previous work to be fast, reliable, and superior to the standard approximations. It is applied here to source radiation in the vicinity of a vortex sheet separating uniformly moving streams. Complete wave field maps are given for subsonic and supersonic flow combinations, providing new insights on jet noise radiation and refraction in open wind tunnels.

SAW Filters at 1 GHz Fabricated by Direct Step on the Wafer

Abstract: Narrow bandwidth low sidelobe SAW filters operating at center frequencies up to 1 GHz are described. Direct step on the wafer lox reduction fabrication techniques were used to achieve the necessary O.52-pm linewidths on ST quartz. Three-halves electrodes (sixthwave) effectively reduce reflection distortion without requiring eighthwave photolithographic resolution.

Dual Differential Interferometer for Measurements of Broadband Surface Acoustic Waves

Abstract: A simple duel interferometer which uses two pairs of orthogonally polarized optical beams to measure both the amplitude and direction of propagation of broadband ultrasonic surface waves is described. Each pair of focused laser probe beams is used in a separate wideband differential interferometer to independently detect the component of surface wave motion along one direction on the surface. By combining the two output signals corresponding to both components, the two dimensional surface profile and its variation as a function of time is determined.

Transmission of High-Frequency Sound Waves Through a Slug Flow Jet

Abstract: An analysis has been performed of sound waves which propagate in a pipe with gas flow. At the pipe exit these waves are partially reflected and the remainder are diffracted. The analysis is carried out by resolving the sound at the exit into its Fourier components and then continuing the solution, which is a combination of elementary plane waves, beyond the exit. These waves are of two types: homogeneous waves which propagate to infinity, and inhomogeneous waves with complex wave numbers which decay. The reflected waves are evaluated from the inhomogeneous waves. At the boundary of the jet, refraction of the elementary plane waves is accounted for and the far field sound is evaluated by the method of stationary phase. Comparisons of the theoretical calculations are made with experimental results and with calculations of other theories.

Interior vs. Exterior Creeping Waves in Acoustic Resonance Scattering

Abstract: In acoustic scattering from elastic objects, resonance features occur in the returned echo at frequencies where the object's eigenfrequencies are located, which are explained by the excitation of interior creeping waves. Correspond- ing resonance terms may be split off from the total scattering amplitude, leaving behind an apparently nonresonant background amplitude. This is demon- strated for spheres, and also for scatterers of arbitrary geometry, using the T-matrix approach. It is shown sub- sequently that the background amplitude can be split further into specularly reflected and geometrically transmitted contributions, plus highly attentuated resonance terms which are explained by the excitation of exterior (Franz-type) creeping waves.