Frequency dependence of ultrasonic backscatter from human trabecular bone: theory and experiment

A model describing the frequency dependence of backscatter coefficient from trabecular bone is presented. Scattering is assumed to originate from the surfaces of trabeculae, which are modeled as long thin cylinders with radii small compared with the ultrasonic wavelength. Experimental ultrasonic measurements at 500 kHz, 1 MHz, and 2.25 MHz from a wire target and from trabecular bone samples from human calcaneus in vitro are reported. In both cases, measurements are in good agreement with theory. For mediolateral insonification of calcaneus at low frequencies, including the typical diagnostic range (near 500 kHz), backscatter coefficient is proportional to frequency cubed. At higher frequencies, the frequency response flattens out. The data also suggest that at diagnostic frequencies, multiple scattering effects on the average are relatively small for the samples investigated. Finally, at diagnostic frequencies, the data suggest that absorption is likely to be a larger component of attenuation than scattering.     

Field dependence of ultrasonic attenuation in semiconducting ZnO

The variation with resistivity and electric field of the attenuation constant for 40 Mc/s shear waves in semiconducting ZnO has been measured. Good agreement was found with the theory of Hutson and White except for the high field range where the coherent signal is attenuated by the generated acoustic flux.     

Time dependence of absorption lineshapes

We study the dependence of the absorption lineshape on the time of interaction with the electromagnetic field, taking into account the finite lifetime of the excited state. We find important deviations from the lorentzian lineshape (broadening of the central peak and oscillatory structures) when the interaction time is comparable with the natural lifetime. Experimental implications are discussed.     

Laser ultrasonic absorption measurement in fatigue-damaged materials

Changes in the materials microstructure caused by fatigue processes affect the ultrasonic absorption. Thus, quantitative measurement of the ultrasonic absorption should provide an indirect measure of fatigue damage. In this paper we present a study of the ultrasonic absorption in fatigue-damaged metals using the reverberation technique in combination with laser-based ultrasound. The reverberation technique allows one to measure absorption independently of scattering.     

Size dependent ultrasonic absorption in superfluid helium

Ultrasonic waves in liquid He II at temperatures below 0.6°K can be absorbed by thermal phonons in 3- or 4-phonon processes. For the case that the mean free path of thermal phonons is sufficiently long 3-phonon processes are generally excluded because energy and momentum cannot be conserved. The conservation requirements cannot be fullfilled because the group velocity of thermal phonons is lower than the ultrasonic velocity due to dispersion. — The purpose of this note is to point out that insmall samples of liquid the uncertainty in phonon energy prevents a violation of the conservation laws for 3-phonon processes. In small volumes of liquid, ultrasonic waves are therefore strongly absorbed in 3-phonon processes, while in larger volumes of liquid this contribution to the absorption is absent. — A similiar size dependent absorption is to be expected for longitudinal waves at low temperatures in ideal crystals.     

Frequency scanning ultrasonic pulse echo reflectometer

The frequency scanning ultrasonic pulse echo reflectometer (FSUPER) is a device which can be used to measure the ultrasonic velocity, attenuation coefficient and specific acoustic impedance of liquid samples as a function of frequency (0.3–6 MHz).     

Frequency dependence of the electron-lattice interaction

The polarisability of weakly bound states should be strongly frequency dependent, modifying the conduction electron-lattice interaction. In many cases the interaction determined from low frequency measurements will not be appropriate for calculations of high frequency phenomena.     

Frequency dependence of EPR signal-to-noise.

Direct measurements of electron spin-echo signal and noise in well-characterized X-band and S-band spectrometers agree with predictions of frequency dependence based on first principles. For the particular spectrometers compared, the echo at 9.52 GHz was 9.5 times larger than the echo at 2.68 GHz, after scaling for differences in spectrometer gain. The calculated ratio was 7.6. This result contrasts with prior predictions that the frequency dependence would be much greater.     

Frequency dependence of third harmonic generation in polyacetylene

The frequency dependence of the third harmonic generation in trans-polyacetylene is quantitatively interpreted by considering the bands of finite chain and the damping of excitation. Our theory shows that there are two distinct peaks in the spectrum of the third harmonic generation. The first one around 0.6 eV comes from the three-photon resonant transition between the edges of the valence band and conduction band; the second one around 0.9 eV is produced by the two-photon interband transition. The excitation-dependent damping is essential to determine the position of the second peak. The theoretical results agree well with the experimental data.     

Frequency dependence of equivalent dielectric constant in finlines

This paper develops a simple but accurate LSE model for accurate prediction of phase velocity in fin-lines. The model is general and equally applicable to unilateral, bilateral and insulated fin-lines. Closed-form expressions, derived from the model, give the phase velocity within to ±0.8 percent of sophisticated numerical techniques.     

Frequency dependence of electroluminescence processes in ZnS crystals

The frequency dependence of the integral electroluminescence brightness and the electrical conductivity of ZnS-Cu, Cl and ZnS-Mn single crystals was examined. It was established that the correlation between the integral electroluminescence brightness and the electrical conductivity in ZuS-Cu, Cl crystals improves slightly at high frequencies of the exciting field, whereas no such correlation exists for the frequency range 30 Hz-18 Hz in ZnS-Mn crystals. Conclusions regarding the various excitation mechanisms of the electroluminescence in ZnS-Cu, Cl and ZnS-Mn crystals are drawn.In conclusion, the authors express their sincere gratitude to P. E. Ramazanov for his valuable advice.     

Frequency dependence of electron lifetime in one-dimensional metals

It is shown that the same approximations which give a nearly frequency independent electron lifetime in 3D-metals lead to a strong ω-dependence of τ for 1D-metals. τ(ω) is discussed in a simple parametrized form and the optical properties derived from it are compared with experimental data for KCP.     

Pressure dependence of the absorption edge in ZnTe

Pressure dependence of the absorption edge in ZnTe has been studied at room temperature. In contrast with the previous result by Drickamer, a monotonous high-energy shift was observed up to about 90 kbar. Small anomalies observed at 30 and 50 kbar may correspond to the resistance decrease of ZnTe.     

Temperature dependence of multiphonon absorption in ionic insulators

We present a calculation for a simple model of the temperature dependence of the absorption in the high frequency wing of the reststrahlen absorption band in NaCl and NaF. The theoretical results are in qualitative agreement with the experimental results.