Constant Pressure Amplitude Research Articles

Holographic generation of arbitrary ultrasonic fields by simultaneous modulation of amplitude and phase

Acoustic holograms have been used widely to generate desired acoustic fields. Following the rapid development of 3D printing technology, the use of holographic lenses has become an efficient method to produce acoustic fields with high resolution and low cost. In this paper, we demonstrate a technique to modulate the amplitude and phase of ultrasonic waves simultaneously using a holographic method with high transmission efficiency and high accuracy. On this basis, we generate an Airy beam with high propagation invariance. We then discuss the advantages and disadvantages of the proposed method when compared with the conventional acoustic holographic method. Finally, we design a sinusoidal curve with a phase gradient and a constant pressure amplitude and realize transport of a particle on a water surface along a curve.

Quantification of numerical damping in the acoustic boundary element method for the example of a traveling wave in a duct

The boundary element method (BEM) is a popular numerical method for solving linear time-harmonic acoustic problems. Using the BEM, modeling is limited to the boundary of the fluid domain, which is particularly advantageous for exterior problems with unbounded domains. A widely unknown drawback of the acoustic BEM is numerical damping. This work is concerned with numerical damping encountered in the benchmark problem of an air-filled duct with rigid walls. A traveling wave, induced by a particle velocity at the inlet, is fully absorbed at the outlet of the duct by imposing an impedance boundary condition. The exact solution gives a constant pressure amplitude over the entire frequency range. However, the numerical solution exhibits decay of the pressure amplitude, which is clearly an indication for numerical damping. This phenomenon is studied for different frequencies and elements-per-wavelength ratios. The extent of numerical damping is quantified by relating the decay to the pressure distribution obtained for a fluid model considering damping. The gained knowledge enables more accurate estimations of real damping phenomena in the future.

Transient and Stationary Characteristics of Thermally Induced Ultrasonic Emission from Nanocrystalline Porous Silicon

Due to complete carrier depletion and strong phonon confinement effects, the thermal conductivity and the heat capacity per unit volume of nanocrystalline porous silicon (nc-PS) are extremely low in comparison to those of single-crystalline silicon (c-Si). Utilizing this great difference in the thermal property between nc-PS and c-Si, it is possible to fabricate an efficient ultrasonic emitter based on thermoacoustic conversion with no mechanical vibration. In this study, the fundamental characteristics of this new ultrasonic generator are investigated in terms of the dynamic behavior. It is shown that acoustic pressure with little harmonic distortion can be produced very quickly in response to the electrical input, and that due to flat frequency characteristics, the device emits an acoustic impulse signal. The stable acoustic emission at a constant pressure amplitude with low noise is observed for a long-time continuous operation under a fixed frequency. The technological significance of such performances is also discussed.

Heating as a mechanism for ultrasonically-induced petechial hemorrhages in mouse intestine

Anesthetized hairless mice were exposed to 1.035 MHz unfocused ultrasound in a temperaturecontrolled water bath. Visible petechial hemorrhages and hyperemia were produced in the intestines. Histologically, the petechiae appeared to be localized mostly to the lamina propria of the mucosa. For 2 min exposure, the number of petechiae and length of hyperemic tissue both increased with increasing spatial peak pressure amplitude above a threshold of about 0.4–0.57 MPa. The magnitude of these effects increased, and the threshold decreased, with increasing exposure duration from 1 min to 4 min. The effects also increased for increasing bath temperature from 32°C to 42°C. The effects decreased markedly for 1 ms burst-mode exposure with 2 ms and 4 ms repetition periods with constant pressure amplitude, but were essentially constant for constant temporal-average intensity. Both effects were also produced by heating from radio-frequency diathermy at 10.35 MHz. The effects were associated with abdominal temperatures above about 42–43°C, which were estimated with a thermocouple at the end of the exposure. The hyperemia is a clearly thermal effect, while the petechiae have been previously associated with the cavitation, mechanism. However, the ultrasonically-induced petechiae observed in this study do not represent an unequivocal marker for cavitation, but rather appear to be attributable to heating. These findings help to reduce the expectation of ultrasonic cavitation during medical therapy treatments.