Acoustic interferometer

About: Acoustic interferometer is a research topic. Over the lifetime, 1493 publications have been published within this topic receiving 19355 citations.

Diffuse elastic waves at a free surface

Abstract: For a diffusely vibrating elastic body, the participation of the surface in the general disturbance is evaluated. It is shown that in the vicinity of a free surface a diffuse acoustic field may legitimately be regarded as a sum of incoherent isotropic and homogeneous independent plane waves incident upon the surface together with their respective outgoing reflected consequences. The work contributes to a conceptual basis for the study of acoustic emission signals on time scales large compared to acoustic travel times across the structure.

Refractive acoustic devices for airborne sound

Abstract: It is shown that a system made of periodic distributions of rigid cylinders in air acts as a new material which allows the construction of refractive acoustic devices for airborne sound. This kind of crystal, named sonic crystal, has low impedance and transmits the sound at subsonic velocities. Here, the fabrication and characterization of a convergent lens is presented. Also, the acoustic analog of the Fabry–Perot interferometer based on this crystal is analyzed. It has been concluded that refractive devices based on sonic crystals behave in a manner similar to that of optical systems. [Work supported by CICyT of Spain.]

Liquid-crystal point-diffraction interferometer for wave-front measurements

Abstract: A new instrument, the liquid-crystal point-diffraction interferometer (LCPDI), is developed for the measurement of phase objects. This instrument maintains the compact, robust design of Linnik's point-diffraction interferometer and adds to it a phase-stepping capability for quantitative interferogram analysis. The result is a compact, simple to align, environmentally insensitive interferometer capable of accurately measuring optical wave fronts with very high data density and with automated data reduction. We describe the theory and design of the LCPDI. Afocus shift was measured with the LCPDI, and the results are compared with theoretical results.

Seismic interferometry of scattered surface waves in attenuative media

Abstract: SUMMARY Seismic interferometry can be used to estimate interreceiver surface wave signals by cross-correlation of signals recorded at each receiver that are emitted from a surrounding boundary of impulsive or uncorrelated noise sources. We study seismic interferometry for scattered surface waves using a stationary-phase analysis and surface wave Green's functions for isotropic point scatterers embedded in laterally homogeneous media. Our analysis reveals key differences between the interferometric construction of reflected and point-scattered body or surface waves, since point scatterers radiate energy in all directions but a reflection from a finite flat reflector is specular. In the case of surface waves, we find that additional cancelling terms are introduced in the stationary-phase analysis for scattered waves related to the constraint imposed by the optical theorem for surface waves. The additional terms are of second order even for single-scattered waves, and we show that these can be highly significant in multiple-scattering cases. In attenuative media errors are introduced due to amplitude errors in these additional terms. Further, we find that as the distribution of scatterers in a medium becomes more complex the errors in correlation-type interferometry caused by attenuation in the background medium become larger. Convolution-type interferometry has been shown to be effective when considering electromagnetic wavefields in lossy media, and we show that this is also true for scattered surface waves in attenuating elastic media. By adapting our stationary-phase approach to this case, we reveal why convolution-type interferometry performs well in such media: the second-order cancelling terms that appear in the correlation-type approach do not appear in convolution-type interferometry. Finally, we find that when using both correlation- and convolution-type interferometry with realistic source geometries (illustrative of both industrial seismics and ‘passive noise’ interferometry), we cannot necessarily expect to produce estimates with all dominant scattering events present. This is shown to be especially important if, as proposed previously for electromagnetic applications, the convolution and correlation approaches are compared to help identify errors in the interferometric estimates.

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.