In this paper, we propose a technique to detect the surface oscillation of an attached bubble with radius ranging from 20 to 200 μm in an acoustic standing wave using a laser Doppler vibrometer (LDV) and a high-speed camera. The threshold condition, where the surface oscillation mode of the bubble was excited, was investigated for three different driving frequencies of 28, 39, and 81 kHz. Frequency spectrum analyses of bubble oscillation measured by the LDV and the images of the bubble simultaneously obtained by the high-speed camera experimentally demonstrated that the surface oscillation was excited when the power-law dependence Δ Rf_th=AR0B was satisfied. Here, R0 is the initial bubble radius and Δ Rf_th is the oscillation displacement of the bubble for the fundamental frequency of the incident ultrasound under the threshold condition where the displacement for the subharmonic component abruptly increased. Interestingly, the coefficient B was independent of the driving frequency. This result suggests that the proposed system can be used to check the validity of current models of surface instability on an oscillating bubble.
Ultrasound imaging of deep parts in a living body with high resolution and high signal-to-noise ratio is strongly required. The pulse compression technique is efficient for achieving a high signal-to-noise ratio, and wide-band imaging is important for realizing high image resolution. Hence, a wide-band transducer is intensively studied. However, high-frequency components in a wide-band transmission tend to be affected by frequency-dependent attenuation. This decreases not only the signal-to-noise ratio but also the image resolution, since the distortion of echo signals makes exact pulse compression impossible, i.e., the pulse width of the compressed echo signal becomes broad. In this study, we examine the compensation of frequency-dependent attenuation in pulse compression imaging using FM chirp signals.
The goal of the present study is to clarify the formation and behavior of sound pressure fields from a statistical point of view when the individual transducers constituting an array source have random performances or, alternatively, conversion efficiencies from electric to acoustic power that vary with the individual transducer. Linear and nonlinear fields are considered herein. Based on experimental data, we assume that the amplitudes and phases of pressure signals emitted from the transducers are random variables that obey Gaussian distributions. The phase changes are, however, not taken into consideration in our theory subject to their small effects on the field formation. Spatial variation in pressure fields attributed to the random performance of transducers is large near the source, and fades with propagation in the farfield. Linear theory predicts that the mean value of the pressure amplitudes is the same as the value when the pressure on the array source is distributed uniformly. Interestingly, the standard deviation around the mean pressure is independent of the radial distance in the plane perpendicular to the beam axis, being inversely proportional to the square root of the number of transducers. For the second-harmonic components, both the mean value and standard deviation are dependent on the radial distance. The validity of these theoretical findings is verified by Monte Carlo simulation and experimental data.
The generation of high-intensity aerial ultrasonic waves is achieved by focusing the acoustic waves radiated from a vibration plate into a point. We developed a new point-converging source of high-intensity aerial ultrasonic waves that comprises a striped-mode rectangular vibrating plate to radiate an aerial ultrasonic wave with high efficiency and paraboloid reflectors. The roughness corresponds to the stripe interval arranged on the surface of the paraboloid reflector. The roughness difference depends on the wavelength of the sound wave. The new convergence method that we propose can converge all ultrasonic waves radiated by the vibrating plate. In addition, the reflectors used to converge the radiated ultrasonic waves are characterized by a much simpler construction than the conventional reflectors. The ultrasonic source fabricated on an experimental basis produced the maximum ultrasonic pressure of about 15,000 Pa (178 dB) at the supplied electric power of 50 W.
The authors are developing a completely new direct-radiator loudspeaker as an alternative to the conventional electrodynamic loudspeaker. It is driven by the continuous revolution of piezoelectric ultrasonic motors, and is useful for the radiation of very low-frequency signals because it shows almost flat phase-frequency characteristics in the low-frequency region. A preliminary model, named the dual-motor, de-spin (DMDS) model, includes two coaxial ultrasonic motors. The stator of one motor is fixed to the base and that of the other is connected to the cone radiator. Velocity modulation of either motor induces driving force to the cone radiator. The low-frequency sound (for example, 30–120 Hz) output by this model was excellent because it has no significant resonance in this frequency region. However, the output sound was occasionally poor. In this paper, a highly improved model, named the quad-motor, de-spin (QMDS) model, is presented. It is based in two coaxial DMDS mechanisms. The experimental model has a cone radiator of 46 cm diameter and an enclosure volume of 268 l. Its working frequency range is the same as that of the DMDS model. Harmonic distortions included in the output signal are improved to be less than 10 percent of DMDS. Its sound quality is excellent.
A linear vibration locus is conventionally used in ultrasonic metal welding. We have previously proposed the use of a nondirectional planar vibration locus as a means of improving the overall and orientation-dependent weld strength compared with the case of using a linear vibration locus. In this study, experiments in which a copper plate and an aluminum plate were used as welding targets were conducted and the weld strength under various conditions was measured to assess the welding characteristics. We found that for a short welding time, the strength of a weld produced using a planar vibration locus was 1.7-fold that produced using a linear vibration locus. We attribute this result to the aluminum plate vibrating more easily, and thus creating stronger vibrations at the interface between the copper and aluminum plates, when we used the nondirectional planar locus.
In the past, much research has centered on dispersing liquids into gases, and dispersing solids and liquids into liquids. However, there has been almost no research on dissolving gases into liquids by dispersion, possibly because of concerns about the effect of deaeration due to cavitation of a liquid by ultrasound. Here, we consider a method using ultrasound to finely disperse and dissolve a supplied gas. The method entails placing the gas supply outlet close to the tip of an ultrasonic longitudinal vibration source and dissolving the gas into the liquid by finely dispersing the gas by means of the vibrations. In this method, the occurrence of cavitation is reduced as much as possible and deaeration effects are reduced. Here, air is used as the gas to be dissolved and water is used as the solvent. The unsaturated dissolved oxygen concentration in water is used as an indicator for evaluating the proportion of dissolved air. The results show that ultrasonic vibration increases the concentration of oxygen dissolved in the liquid and that almost no deaeration by cavitation occurs.
In order to estimate the water stress of a plant, the natural frequency of the leaf-stalk system was investigated. As a means of vibrating the leaf, acoustic radiation force was utilized, and the successive measurement of the natural frequency of ``komatsuna'', which was cultivated in a pot of soil, was performed for a week until wilting after stopping irrigation. As a result, it was found that the natural frequency is decreased drastically by the wilting of the leaf before the drooping occurs. In addition, daily variation was also observed in the early days, but it was gradually suppressed as the day went on. These behaviors were discussed referring to a simple cantilever beam model. In conclusion, it was ascertained that the acoustic radiation force is efficient for vibrating a leaf-stalk system. Furthermore, it was confirmed that measuring the natural frequency of the leaf-stalk system is effective for the early detection of water stress of a plant.
The coding of a transmitted signal by a maximum-length sequence (M-sequence) is employed in the pulse-echo method with pulse compression for ultrasonic distance measurement. To estimate the distance, the time of flight (TOF) of an echo is determined from the cross-correlation function between the received signal and the reference signal that corresponds to the transmitted signal. When codes of the received signal and the reference signal match, a high correlation value is obtained in the cross-correlation function. On the other hand, when they do not match, a low correlation value is obtained. In the case of the finite-length M-sequence code, however, truncation noise is generated at the beginning and end of the cross-correlation function. In this study, truncation noise in autocorrelation functions of all patterns of M-sequence codes is quantitatively evaluated from the 4th order to the 15th order. The peak amplitude and intensity of truncation noise vary depending on the patterns of M-sequence codes. Therefore, they can be reduced by selecting suitable M-sequence codes.
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