Acoustical Physics最新文献

The acoustic and electrical properties of a 128-element ultrasonic transducer designed to generate high-intensity focused ultrasound in air in the low-frequency ultrasonic range are investigated. To reduce parasitic grating maxima of the acoustic field, a spiral arrangement of piezoelectric elements on a spherical base was used. The operating frequency of the transducer was 35.5 kHz, and the diameter of the source and focal length were approximately 50 cm, significantly exceeding the wavelength (approximately 1 cm). This selection of parameters allowed for effective focusing, with localization of wave energy in a small focal region, thereby achieving extremely high levels of ultrasonic intensity. The parameters of the ultrasonic field were studied using a combined approach that included microphone recording of the acoustic pressure and measuring the acoustic radiation force acting on a conical reflector. Acoustic source parameters were determined from the two-dimensional spatial distribution of the acoustic pressure waveform, which was measured by scanning the microphone in a transverse plane in front of the source. Numerical modeling of nonlinear wave propagation was also used based on the Westervelt equation to simulate the behavior of intense waves. The acoustic pressure level reached 173 dB, with a focal spot size comparable to the wavelength.

The self-contained vertical acoustic–hydrophysical measuring systems Mollyusk-19 and Mollyusk-21 were developed to study spatiotemporal inhomogeneities in the sound field velocity and in modal structures of low-frequency sound fields and internal waves. This paper describes the circuit, structural, and software solutions for the main problem posed when developing new systems to improve their performance characteristics. The possibilities of applying these systems to solve the formulated problems was illustrated by the results of field measurements on the Posiet Bay shelf in the Sea of Japan.

The study derives an equation and solves for the first time the problem on the formation and radiation dynamics of a quasi-empty pulsating spherical cavity in a cavitating liquid under the influence of variable sound velocity in a cavitation and cavitation nuclei concentration zone. The data on the cavity dynamics, radiation, and collapsing velocity for a spectrum of initial internal pressures show that, at a maximum gas phase concentration, pulsations are different in the degree of their compression. They have almost identical character: after the first collapse, only a single half-cycle is completed to attain different constant equilibrium radii. The condition of equality between the pressures in a cavitation zone and inside a spherical cavity at its boundary makes it possible to establish a dynamic relation between the volumetric concentration (sound speed) in the cavitation zone and the radius of this spherical cavity for the first time. When calculating and constructing the solution, the condition that the initial cavity size takes a value corresponding to the initial pressure is changed. The dependences of radiation amplitudes over the entire range of applied pressures are plotted. It turns out that the radiation amplitude increases by five orders of magnitude, when the initial pressure inside a cavity changes by three orders of magnitude from 10^–2 to 10^–5 atm.

The propagation of flexural elastic waves in notched metal bars with a rectangular cross section with the depth of notches increasing by a power law has been studied by numerical modeling and experimental laser scanning vibrometry. Three types of notch arrangement have been considered: uniform and more frequent and sparse towards the end of a bar. Such structures exhibit the characteristics of an acoustic black hole. For all the studied samples, in the 10–100 kHz frequency range, an increase in amplitude and decrease in length of the flexural wave have been experimentally found as a wave approaches the end of a bar. It has been shown that there is a critical frequency, above which the modes exhibit a section with highly reduced amplitude of oscillations.

The pulse width of multibubble sonoluminescence flashes in an aqueous NaCl solution was measured by a correlation method for the spectral range of 300–800 nm. The flash pulse width had a maximum value of 21 ns in the spectral region adjacent immediately to the Na D-line peak (589 nm) and decreased to 2 ns with distance from the line peak. The measured dependence of the flash pulse width on the wavelength agreed with the dynamic Na line shape model proposed by us earlier, where the spectral line width and shift were governed by a fast change in the emitting medium density during bubble collapse. Using the correlation method, the sequence of metal and continuum flashes was determined to measure the relative delay between them. The results showed that Na emission takes a longer time as compared to continuum emission and occurred almost symmetrically in time around a continuum flash with a vanishingly small delay of 0.21 ns after the continuum flash. Using the same method for a CeCl₃ solution, a cerium line flash (350 nm) was revealed to occur after a continuum flash with a delay of 31 ns close to the Ce emission lifetime of 33 ns to be indicative of essential distinction between the mechanisms of Na and Ce emission under multibubble sonoluminescence.

We present the results of numerical simulation and experimental studies of the propagation of fle-xural elastic waves in a notched metal bar with a rectangular cross section that approximates the acoustic black hole effect. The sample is a notched bar; the depth of notches increases according to a power law with an exponent of 4/3. The experimental results and the results of numerical simulation confirm that such bars slow the propagation velocity of an elastic wave towards the end of the bar. It is demonstrated that flexural waves in such structures exhibit dispersion and their amplitude at the end of the bar for some eigenfrequencies is higher than that in a solid bar. The eigenmode shapes of a solid and notched bar are compared together with the distribution of the flexural wave amplitude along the bars. The frequency dependence of the flexural wave length is studied during wave propagation towards the end of the notched bar.