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The paper reviews the publications on low-frequency vibration in power plants. It is noted that the regulatory literature poorly defines the issues of rating and diagnosing self-excited and subharmonic vibrations. Analysis of the literature and ISO standards shows that despite the considerable experience gained by specialists in low-frequency vibration control, a number of issues, such as subharmonic resonances and nonlinear properties of complicated multi-seated rotor systems supported by journal bearings, are still understudied and require further calculations and experiments. The paper presents some results obtained in experiments of low-frequency vibrations for a rotor supported by journal bearings and having a residual deflection. The experimental results confirmed the classical hysteresis of self-excited vibrations – the difference in the boundaries of self-excited vibrations during ascent and descent is about 4 Hz. The arears of appearance and disappearance of subharmonic vibrations are shown using the spectral characteristics of vibrations and cascade spectra.

Spontaneous otoacoustic emissions (SOAEs) have become a hallmark feature in modern theories of an ‘active’ inner ear, given their numerous correlations to auditory function (e.g., threshold microstructure, neurophysiological tuning curves), near universality across tetrapod classes, and physiological correlates at the single hair cell level. However, while several different classes of nonlinear models exist that describe the mechanisms underlying SOAE generation (e.g., coupled limit-cycle oscillators, global standing waves), there is still disagreement as to precisely which biophysical concepts are at work. Such is further compounded by the idiosyncratic nature of SOAEs: Not all ears emit, and when present, SOAE activity can occur at seemingly arbitrary frequencies (though always within the most sensitive range of the audiogram) and in several forms (e.g., peaks, broad ‘baseline’ plateaus). The goal of the present study was to develop new signal processing and stimulation techniques that would allow for novel features of SOAE activity to be revealed. To this end, we analyzed data from a variety of different species: human, lizard, and owl. First, we explored several strategies for examining SOAE waveforms in the absence of external stimuli to further ascertain what constitutes ‘self-sustained sinusoids’ versus ‘filtered noise’. We found that seemingly similar peaks in the spectral domain could exhibit key differences in the time domain, which we interpret as providing critical information about the underlying oscillators and their coupling. Second, we introduced dynamic stimuli (swept-tones, tone bursts) at a range of levels, whose interaction with SOAEs could be visualized in the time-frequency domain. Aside from offering a readily accessible way to visualize many previously reported effects (e.g., entrainment, facilitation), we observed several new features such as subharmonic distortion generation and competing pulling/pushing effects when multiple tones were present. Furthermore, the tone burst data provide quantitative bounds on the dynamics of the relaxation oscillations. These data should provide new insights into how precisely how SOAE generators are related to (the more commonly measured) OAEs evoked via external stimuli and presumably speak to the robustness of the hair cell as the underlying basis for SOAE activity.

Experiments have been performed to investigate the effects of interstitial air on the impact bifurcations in vertically vibrated granular beds. The impact of particles on the container bottom commonly undergoes a series of subharmonic bifurcations in the sequence of period-2, period-4, chaos, period-3, period-6, chaos, period-4, period-8, and so on. In the container with an air impermeable bottom, air flows are induced and air drag on the particles causes the bifurcations to be dependent on the particle size; the bifurcation point increases with the decreasing of particle size. This makes the higher-order bifurcations become unobservable when the particle size is sufficiently small. Meanwhile, such bifurcations are shown to be controlled by both the normalized vibration acceleration and the vibration frequency. However, in the container with an air permeable bottom the size dependence of bifurcations is cancelled, and the bifurcations turn to be controlled solely by the normalized vibration acceleration. The observed results are explained in terms of an inelastic bouncing ball model with air dragging terms involved.

As the first step of the improvement of developed FEM code to extend the subharmonic wave generation at nano scale gap crack, we investigated the basic crack model for accurate simulator of subharmonic behavior using the voxel mesh FEM code.

To ensure the safety and reliability of atomic power plants and airplanes, the technique of monitoring closed fatigue cracks is requisite. Here we monitored the distribution of the crack depths and closure behavior in the length direction after 48000 and 87000 fatigue cycles using subharmonic phased array for crack evaluation (SPACE). The crack depths in the subharmonic images were larger than those in the fundamental images. Specifically, the difference was larger at near the side surface than at the center. The percentage of the closed part varied with the crack growth in the specimen. In addition, we fabricated shoe for SPACE to facilitate mechanical scanning. Thus, it was demonstrated that SPACE is useful in monitoring closed fatigue crack growth.

To enhance the selectivity of subharmonic phased array for crack evaluation (SPACE) we propose long burst SPACE with narrowband spectrum. But image of a scatterer is elongated towards direction connecting the object and the center of receiving array (RA). However, since the inclination angle is changed by changing the position of the RA, the invariant part of the images obtained with different RA positions gives true image of the scatterer, with an improved temporal resolution. In this study, we show a simulation of this method using product of image intensity with different RA positions.

The role of cavitation in bulk ultrasound ablation has been evaluated in a series of in vitro experiments. Fresh bovine liver tissue was ablated with a 3.1 MHz ultrasound image‐ablate probe at 31 W/cm2 for 20 minutes under normal and elevated ambient pressures. A 1 MHz passive cavitation detector recorded acoustic emission signals which were quantified by computation of average subharmonic, broadband, and low‐frequency emission levels. After ablation, tissue was sliced and stained with 2% TTC to evaluate thermal damage. Emission levels were quantified and correlated with tissue ablation histology. The results indicate that bubble activity significantly affects heat deposition in ultrasound bulk ablation, in a manner different from high‐intensity focused ultrasound (HIFU) ablation.

The results of experimental research of specific features of acoustical subharmonics excitation in granular medium are considered. It is founded that signal subharmonics registered at the scale of single granular extremely high varies in time and spectrum of this frequency component lies in some band. Stimulated parametric excitation of subharmonics component is discussed in the paper as well.

Intravenously injected encapsulated microbubbles improve the contrast of an ultrasound image. Their destruction is used in measuring blood flow, stimulating arteriogenesis, and drug delivery. We measure attenuation and scattering of ultrasound through solution of commercial contrast agents such as Sonazoid and Definity. We have developed a number of different interfacial rheology models for the encapsulation of such microbubbles. By matching with experimentally measured attenuation, we obtain the characteristic rheological parameters. We compare model predictions with measured subharmonic responses. We also investigate microbubble destruction under acoustic excitation by measuring time‐varying attenuation data.

We have conducted numerical precise simulations using an event-driven method to investigate the pattern formations in two-dimensional, shallow, granular materials on a vertically vibrating plate. A detailed analysis of the profiles of the vertical and the horizontal velocities inside a granular system during the formation of subharmonic waves has been done for the first time. Kinetic information inside the granular system over time explains the diffusion mechanism of the energy input from the bottom plate through continuous collisions and the mechanism of subharmonic wave formation.

In this paper we report on the first observation of microwave subharmonic generation in ferromagnetic metals. It has been accomplished in iron at room temperature through a new transmission resonance experiment which will be described here. As in all transmission resonance experiments, the sample forms the common wall of two microwave cavities, one of which is the excitation and the other the detection cavity. The subharmonic has been found in the geometry in which both the static and the microwave fields in the excitation cavity are mutually parallel and also parallel to the sample surface. The detected subharmonic signal strength is about 10−17 watts for iron samples a few microns thick. Of course thinner samples will give stronger signals. The samples used were 99.9% pure. The nonlinear phenomena described here, as opposed to the linear transmission resonance phenomena, are sensitive to the exchange stiffness constant and could be used for its determination as a function of temperature.