Bulk Acoustic Properties Research Articles

Czochralski-grown Single Crystals with Acentric Symmetry Group32

Single crystals with calcium gallium germanate (Ca3Ga2Ge4O14, CGG) structure such as langasite La3Ga5SiO14 (LGS), its niobium (‘langanite’, La3Ga5.5Nb0.5O14, LGN) and tantalum (‘langataite’, La3Ga5.5Ta0.5O14, LGT) analogs as well as strontium niobium gallium silicate (Sr3NbGa3Si2O14, SNGS) and strontium tantalum gallium silicate (Sr3TaGa3Si2O14, STGS) were grown with high perfection by the Czochralski pulling technique. Specific electric conductivity, optical activity, and dielectric, piezoelectric, bulk and surface acoustic, elastic, and electro-optic properties were measured.

Effects of natural and electrochemical oxidation processes on acoustic waves in porous silicon films

Brillouin scattering has been performed to study the effects of natural and electrochemical oxidation on the surface and bulk acoustic mode properties of porous silicon films. The acoustic mode frequencies are observed to decrease with increasing oxidation time and this is attributed to the progressive transformation of silicon–silicon bonds on the inner pore walls into SiO2. The dependence of the surface and bulk acoustic mode frequencies on the natural oxidation time allows the evaluation of these wave frequencies for a porous-SiO2 glass film, and hence the estimation of its bulk modulus. Both Brillouin and Raman data confirm the expectation that oxidation leads to a reduction in silicon nanoparticle size, and provide a correlation between the redsifted acoustic mode frequency and the oxidation period.

High temperature bulk acoustic wave properties of langasite

High temperature stable piezoelectric materials including langasite (La3Ga5SiO14, LGS) are, in principle, suitable for high temperature bulk acoustic wave applications such as resonant microbalances and gas sensors using specific surface affinity layers. The resonator material LGS has been shown to exhibit bulk oscillations at temperatures of up to at least 935 °C. However, the knowledge of the high temperature bulk acoustic wave properties is required for the evaluation of mass load dependent changes of the resonator characteristics, and, consequently, for the application of these materials as sensors. This paper presents and discusses the bulk acoustic wave resonator behaviour of LGS in the temperature range of up to 935 °C. Test devices are Pt contacted LGS resonators. Based on a one-dimensional description of the resonance behaviour, the material constants are determined as a function of temperature and oxygen partial pressure. The effective viscosity, η, is found to control the resonator quality. At 600 °C and oxygen partial pressures down to ca. 10−20 bar, an environment independent operation of LGS resonators is demonstrated. The mass sensitivities at 800 °C are of the same order of magnitude as for quartz at room temperature.

High-accuracy standard specimens for the line-focus-beam ultrasonic material characterization system.

We prepared standard specimens for the line-focus-beam ultrasonic material characterization system to obtain absolute values of the propagation characteristics (phase velocity and attenuation) of leaky surface acoustic waves (LSAWs). The characterization system is very useful for evaluating and analyzing specimen surfaces. The calibration accuracy of these acoustic parameters depends on the accuracy of acoustical physical constants (elastic constants, piezoelectric constants, dielectric constants, and density) determined for standard specimens. In this paper, we developed substrates of nonpiezoelectric single crystals (viz., gadolinium gallium garnet [GGG], Si, and Ge) and an isotropic solid (synthetic silica [SiO2] glass) as standard specimens. These specimens can cover the phase velocity range of 2600 to 5100 m/s for Rayleigh-type LSAWs. To determine the elastic constants with high accuracy, we measured velocities by the complex-mode measurement method and corrected diffraction effects. Measurements of bulk acoustic properties (bulk wave velocity and density) were conducted around 23 degrees C, and bulk wave velocities were obtained with an accuracy of within +/-0.004%. We clearly detected differences in acoustic properties by comparing the obtained results with the previously published values; the differences were considered to be due to differences of the specimens used. We also detected differences in acoustic properties among four SiO2 substrates produced by different manufacturers.

Measurements of bulk acoustic properties of syntheticα-quartz

A preliminary study of the acoustical physical constants (elastic constant, piezoelectric constant, dielectric constant, and density) of commercially available, synthetic α-quartz has been conducted which involved the accurate measurement of the longitudinal velocities and densities for X-, Y-, and Z-cut specimens. Significant differences in longitudinal velocity exceeding 0.5%, especially along the Z-axis of propagation, were observed between the measured longitudinal velocities and the calculated velocities using the constants published by Mason or Bechmann et al. Research directions into accurately determining all the acoustical physical constants are suggested.

The bulk acoustic properties of a porous medium, derived from a single impedance test

In the majority of engineering applications, porous sound absorbing media may be treated as equivalent fluids, with vibration of the solid skeleton being neglected. Most measurement techniques for the bulk acoustic properties of such media (e.g., complex characteristic impedance and propagation constant) involve tests on large samples of material, rather complex series of tests on smaller single samples, or surface impedance tests on two small samples of differing thicknesses. Recently, however, a least-squares method [C. Braccesi and A. Bracciali, Appl. Acoust. 54, 59–70 (1998)] was employed to find values of the structure factor and flow resistivity, in a two-parameter model, from a single surface impedance curve. Though the technique proved successful, the two-parameter model is less accurate over a wide frequency range that the well-known empirical curve fitting method of Delany and Bazley. In the present paper, a least-squares method is described for determining the eight empirical parameters in the Delany and Bazley method from a single surface impedance curve. Examples of its application are given, and it is demonstrated that it gives a better fit to measured data than the two-parameter model.

Prediction of the bulk acoustic properties of fibrous materials at low frequencies

In many acoustical modelling applications, it is necessary to be able to predict the bulk acoustic properties of sound absorbing porous media at frequencies below the range of the usual measurement methods for these properties. A semi-empirical prediction method for fibrous media is described here, which not only yields physically reasonable predictions for the bulk properties at arbitrarily low frequencies, but also is in good agreement with measured data at higher frequencies. In this it represents an improvement over previously published low frequency models.

A method for calibrating the line-focus-beam acoustic microscopy system

Absolute accuracy of the line-focus-beam (LFB) acoustic microscopy system is investigated for measurements of the leaky surface acoustic wave (LSAW) velocity and attenuation, and a method of system calibration is proposed. In order to discuss the accuracy, it is necessary to introduce a standard specimen whose bulk acoustic properties, (e.g., the independent elastic constants and density) are measured with high accuracy. Single crystal substrates of gadolinium gallium garnet (GGG) are taken as standard specimens. The LSAW propagation characteristics are measured and compared with the calculated results using the measured bulk acoustic properties. Calibration is demonstrated for the system using two LFB acoustic lens devices with a cylindrical concave surface of 1-mm radius in the frequency range 100 to 300 MHz.

VHF/UHF range bioultrasonic spectroscopy system and method

A new system and method for characterizing biological tissues in vitro and liquids in the VHF and UHF ranges is described. Bulk acoustic properties such as the sound velocity, attenuation, acoustic impedance, and density are determined in reflection and transmission modes, with the biological tissue/liquid specimen sandwiched between the parallel surfaces of synthetic silica glass buffer rods having ZnO piezoelectric film transducers on their opposite ends. The method is an ultrasonic transmission line comparison method wherein the reference medium is distilled water, for which all acoustic properties are known. Measurement errors due to diffraction losses in the acoustic media and to mode conversion at the buffer/sample interfaces are corrected. Special techniques for achieving precise parallelism between the two rod surfaces, for movement to adjust the gap distance, and for signal processing are employed in order to obtain high measurement accuracy. Attenuation and reflection coefficients are determined using the gated pulse echo method. The sound velocity is determined with the gated pulse interference method by sweeping the ultrasonic frequency, or by changing the gap distance. Results of measurements on castor oil, cottonseed oil, silicone oil, and bovine liver, in the frequency range from 10 to 500 MHz, are presented and compared with results of earlier reports.

Acoustic Properties Of Reticulated Plastic Foams

Some general aspects of sound propagation in rigid porous media are discussed, particularly with reference to the use of a single—dimensionless—frequency parameter and the role of this, in the light of the possibility of varying gas properties, is examined. Steady flow resistance coefficients of porous media are also considered, and simple scaling relationships between these coefficients and "system parameters" are derived. The results of a series of measurements of the bulk acoustic properties of 12 geometrically similar, fully reticulated, polyurethane foams are presented, and empirical curve-fitting coefficients are found; the curve-fitting formulae are valid within the experimental range of values of the frequency parameter. Comparison is made between the measured data and an alternative, fairly recently published, semi-empirical set of formulae. Measurements of the steady flow-resistive coefficients are also given and both the acoustical and flow-resistive data are shown to be consistent with theoretical ideas. The acoustical and flow-resistive data should be of use in predicting the acoustic bulk properties of open-celled foams of types similar to those used in the experimental tests.

Impedance tube measurements on porous media: The effects of air-gaps around the sample

The effects, on the measured impedance, of air-gaps around the edge of a sound-absorbing test sample in an impedance tube are examined both theoretically and experimentally, for a rigid-framed anisotropic porous medium. The results show that media of high flow resistivity tend to be more susceptible to measurement errors incurred by air-gaps than those with low resistivities. Experimental and predicted data concerning the effects of air-gaps on the impedances of both isotropic and anisotropic materials are presented, and are in reasonable agreement. Errors in the bulk acoustic properties, infered from normal surface impedance data on two different thicknesses of an isotropic material against a rigid backing, are also examined in an illustrative numerical study.

The attentuation of dissipative flow duct mufflers with internal mean fluid flow in the absorbent

In dissipative flow duct mufflers incorporating layers of porous bulk sound absorbent, mean flow within the pores of the material is inevitable because the porous medium is exposed to pressure gradients in the duct flow. The acoustical effects of internal mean flow can be considerable, particularly in the case of internal cumbustion engine exhaust mufflers of the dissipative type. Theoretical treatments are presented here, both to describe the effect of internal flow on the bulk acoustic properties of the porous medium and to find the effect of the absorbent in situ, in the form of the sound transmission loss of the silencer. The measured transmission loss of an experimental silencer is compared to predicted data, and good agreement between the two is obtained. The effects of mean fluid flow in the central passage and internal flow in the absorbent are separately demonstrated.

Bottom classification using backscattering at vertical incidence

A technique has been developed for measurement of bottom geoacoustic properties using backscattering at vertical incidence. The parameters of a scattering model are determined by a maximum‐likelihood fit to received intensity as a function of time. The procedure provides measurements or bounds for the following sediment bulk acoustic properties; impedance, absorption coefficient, and volume scattering strength. In addition, a measure of interface roughness can be obtained if the coherent component of recieved intensity is negligible. The method has been tested using high‐frequency data from shallow silty and clayey sites. [Work supported by ONT and NORDA.]