Acoustical Physics最新文献

Physical and mathematical numerical models have been developed to predict the effective acoustic properties of sound-absorbing honeycomb structures at sound pressure levels of 100 and 130 dB with normal sound wave incidence. The sound absorption coefficients and patterns of acoustic interactions of cells installed at the end of a cylindrical duct with normal sound wave incidence on them were studied by numerical mathematical and physical modeling. The sound absorption efficiency of single and groups of resonators of various shapes and sizes is estimated, and unique combinations of cells in groups are identified, taking into account their acoustic interactions. Representative samples of fragments of sound-absorbing structures were 3D-printed; laboratory tests of the samples were carried out with an interferometer with a normal sound wave incidence on the cells at a sound pressure level of 130 dB.

Motors driven by surface acoustic waves (SAWs) have been extensively investigated due to their potential applications in various environments. One of the problems we are confronted with is the lifetime of the substrate which supports SAWs since it becomes fragile when large-amplitude SAWs are excited on the substrate. In order to prolong the lifetime of the substrate, we propose to apply modulated driving voltage with a duty ratio to the interdigital transducers. With the implementation of such a proposal, we found an intriguing phenomenon which is out of expectation, i.e., when the driving voltage is high, the slider speed under modulated driving with a suitable duty ratio might be larger than that with the continuous driving.

In physics, in particular, acoustics, time is traditionally considered as a continuous coordinate. Some exception is signal processing, where sampling is necessary for calculations on computers. But all acoustic problems are formulated and solved using time-continuous models described by differential equations and their solutions in the form of continuous time functions. Meanwhile, these problems can be formulated and solved in an equivalent way using discrete-time models described by finite-difference equations and their solutions in the form of time series. As the experience of some other fields of science, for example, control theory, shows, the discrete approach has a number of advantages over the continuous approach, the use of which greatly facilitates the solution of many problems. This paper aims to partially fill the gap in acoustics that exists here and is aimed at creating the theoretical foundations of a discrete-time approach to solving acoustic problems. The paper is limited to the consideration of one oscillatory system widely used in acoustics—a linear structure with N degrees of freedom consisting of lumped inertial, elastic and dissipative elements, to which, in particular, the finite element method leads. For several continuous models of this system, equivalent discrete-time models are constructed in the paper, finite-difference equations are derived and their solutions are obtained. The criterion of equivalence of continuous and discrete models in the paper is the mathematically exact equality of the corresponding solutions at all discrete points in time. Based on this criterion, analytical relations have been established between the parameters of continuous and discrete models and their equations, which make it possible to build its discrete-time model based on a continuous model of the system and, conversely, to build its continuous model based on a known discrete model. Special attention is paid in the paper to the forced vibrations of the system under the action of kinematic excitation, which is important in many acoustic problems, whereas in the literature only force excitation is considered. The paper also discusses one of the most useful properties of discrete modeling—the simplicity of constructing discrete models based on experimentally measured signals. A corresponding example is given. Note that the term “ARMA-model” is an abbreviation for “autoregressive and moving average model”, generally accepted in control theory, systems theory and other fields of science.

Laser ultrasonic detection of rail defects has become a new method of rail nondestructive testing. Obtaining accurate rail defect signal is a prerequisite to judge the size of defects and avoid train accidents and ensure driving safety. In order to effectively improve the SNR of defect echo, a denoising algorithm combining CEEMD and wavelet soft threshold was proposed. First, CEEMD decomposition was performed on the signal to determine the demarcation point k of IMF components by autocorrelation function. The signal after k + 1 component was reconstructed. Then, the reconstructed signals were decomposed by wavelet transform. The high frequency coefficients after soft threshold processing and the low frequency coefficients of wavelet transform were reconstructed to complete the denoising of rail surface defect signals. The rail with defect of a depth of 0.5 mm and a width of 0.5 mm was tested and verified by laser ultrasonic experiment. By experiment the denoising method combining CEEMD and wavelet soft threshold suppressed effectively the noise. It retained the detailed characteristics of the defective reflected waves. It achieved the good denoising characteristics. It improves the signal-to-noise ratio by 7.12 and 0.77 dB, respectively, over the EMD denoising algorithm and CEEMD denoising algorithm at 1 dB noise intensity and improves the signal-to-noise ratio by 3.37 and 1.23 dB, respectively, over the EMD denoising algorithm and CEEMD denoising algorithm at 20 dB noise intensity.

The rate of nuclear spin-lattice relaxation is determined by the efficiency of interaction between thermal phonons and nuclear spins. The results on reducing the efficiency of spin–phonon coupling by suppressing the contribution from paramagnetic centers to quadrupole nucleus relaxation are presented. The suppression has been performed by continuous magnetic action at the Larmor frequency. It is shown that, as in the presence of an acoustic field, the rate of spin-lattice relaxation of ²³Na nuclei in a sodium fluoride crystal at magnetic saturation of the NMR signal does not change in the region of a negative average spin temperature. In the region of positive spin temperature, the rate of relaxation of ²³Na spins significantly decreases and nuclear magnetization recovery with time is described by the sum of two exponentials. The contribution from nuclear spins with a lower efficiency of spin–phonon coupling, corresponding to the exponential with a long relaxation time, increases with increasing saturating field intensity. It is demonstrated that the efficiency of spin–phonon coupling for ¹⁹F nuclei, which do not have the quadrupole moment, does not change under the saturation conditions. The results obtained can be used for analyzing the structure of real crystals.

A laser scanning vibrometer was used to measure the amplitudes and phases of the vibrational velocity of shear waves excited by a one-dimensional source in the form of a narrow rectangular bar in a gel-like medium. The vibrations of 26 plates reflecting the laser beam and located inside an optically transparent phantom along a segment with a length of 84.5 mm at a distance of 20 mm from the source were measured. The angular distributions of the amplitude and phase of shear waves at discrete frequencies from 59 to 500 Hz were measured in continuous mode. In pulsed mode, the vibrator excited a pulse in the medium with a duration of 1.5 periods of the 300 Hz frequency. The amplitudes and phases of shear waves were calculated by fast Fourier transform of the time profile of the vibration velocity of the plates with a duration of 50 ms. The angular amplitude distributions measured in the pulsed and continuous modes are qualitatively the same. At all frequencies, the distributions are symmetrical with respect to the vertical axis. The maximum oscillation amplitude is observed at angles close to ±45°. The velocity of shear waves, calculated from the measured phase distributions, increases from 2 to 2.5 m/s with a change in frequency from 50 to 500 Hz. It is shown that this velocity behavior is well described by a relaxation model of the medium with one relaxation time equal to 0.3 ms. Shear wave attenuation depends on frequency and exceeds 1 cm^–1 for waves with frequencies above 250 Hz. The maximum attenuation per wavelength is observed near the relaxation frequency of the medium in the 300–400 Hz range. The results can be used to optimize devices for measuring the elasticity of soft tissues.

A new method for separating acoustic and pseudosound pressure fluctuations is proposed, based on analysis of signals at a pair of closely located points, so that the total size of the measurement zone is much smaller than the correlation scale of pseudosound perturbations. It is suggested that hydrodynamic fluctuations propagate at a velocity significantly lower than the sound velocity and obey the “frozen-in” perturbation model, which makes it possible, in real time or during data postprocessing, to convert the spatial derivative of the signal into a temporal derivative, which, after time integration, yields an estimate for pseudosound perturbations at the measurement point. A theoretical model of the approach and test results using model examples and numerical simulation data are presented.

A methodology is proposed for high-precision local measurement of the group velocity of longitudinal waves in solid samples with millimeter thickness. Achievement of the required accuracy involves laser thermo-optical excitation of submicrosecond ultrasonic video pulses and ultrawideband piezoelectric recording of acoustic signals reflected from the test sample. Plane-parallel samples made of duralumin, quartz, and steel with a thickness of 2–6 mm are studied. To achieve the required accuracy in measuring the group velocity of ultrasound, the signal shape is mathematically processed with compensation for diffraction of the ultrasonic beam as it propagates through the sample. The possibility of ensuring the uncertainty in measuring the group velocity of ultrasound in the 1–15 MHz frequency range at a level of 0.1% in samples with millimeter thickness is demonstrated.