Biomedical subjects
Publications and source records attributed to O I Lobkis.
We find that the Vollhardt and Wolfle self-consistent theory of Anderson localization makes simple predictions for transport dynamics in unbounded one- and two-dimensional media. These predictions are derived and explored and compared with direct numerical simulations.
Noise generated in an ultrasonic receiver circuit consisting of transducer and amplifier is usually ignored, or treated as a nuisance. Here it is argued that acoustic thermal fluctuations, with displacement amplitudes of 3 fm, contain substantial ultrasonic information. It is shown that the noise autocorrelation function is the waveform that would be obtained in a direct pulse/echo measurement. That thesis is demonstrated in experiments in which direct measurements are compared to correlation functions. The thermal nature of the elastodynamic noise that generates these correlations is confirmed by an absolute measurement of their strength, essentially a measurement of the sample temperature.
Measurements of the ultrasonic modal density of a disordered elastic frame, a 20 pore-per-inch open-celled aluminum foam, are reported. While the material is dissipative, with a Q only around 700, sufficiently careful signal processing has allowed reliable counts of the modes up through a few hundred, corresponding to wavelengths comparable to the strut lengths. The modal density is found to be essentially constant over this range, and to bear no resemblance to theoretical estimates based on long-wavelength effective moduli.
The statistics of the ultrasonic resonance peaks of a finite elastic body are investigated. The distribution of peak phases, and the normalized variance of peak amplitudes, are shown to be consistent with a hypothesis that the modes themselves are complex Gaussian random numbers. A value q = 0.33 for the ratio of the standard deviations of the imaginary and real parts of the modes is found to fit the data, and to bring recent theory of power variances into better accord with measurements.