摘要:

The sound field of different focusing piezoelectric transducers designed for lithotripsy was investigated. The shock wave parameters according to proposed standards, e.g., FDA Draft 1991, were determined. The parameters achieved are based on measurements using a fiber‐optical probe hydrophone. In contrast to PVDF‐based hydrophones, this hydrophone is able to give a correct representation also of the tensile components of a complete signal and provides a higher spatial resolution. Thus some shock wave parameters like beam energy can be calculated more precisely. The different lithotripter pulses were applied to model stonesinvitro, recording the amounts of removed material. Fragmentation results were compared with the different parameters. Attempts were made to arrange these parameters according to their relevance to disintegration. The evaluation of these experiments revealed up to now that there is no significant relation between some of the shock wave parameters and fragmentation efficiency. It is concluded that most of the proposed parameters do not describe well the efficacy of different lithotripter sources on concrements.

ISSN:0001-4966    

DOI:10.1121/1.422867

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The Langevin vibrator is made of the longitudinal and torsional piezoelectric plates, which can be used for an ultrasonic motor and an ultrasonic welding transducer. Because of the difference between the longitudinal and torsional sound velocity, their resonant frequencies are not equal. To make resonant frequencies of two vibration modes in degeneration, a matching block with a thin neck was connected to the end of the vibrator. Two calculation methods, a dimension network which transmits matrixes and a finite element method have been used. The feasibility of frequencies tuning through a matching block has been analyzed and how to effect the resonant frequency by the radius and thickness of the thin neck and matching block has been summarized. The resonant frequencies of several steel cylinders with variant thin necks and matching blocks were calculated. Some experimental results of measuring the resonant frequencies have been compared with the calculation. According to the calculation and experimental results, a prototype of hybrid longitudinal‐torsional ultrasonic motor has been manufactured, and it can be operated very well [Tomikawaetal., IEEE Trans. Ultrason. Ferroelectr. Freq. Control39(1992)].

ISSN:0001-4966    

DOI:10.1121/1.422870

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The transfer function (or two‐microphone) technique is a tool for measuring materials absorption related parameters as impedance or absorption coefficients. But this method is laboratory bound in its original form due to free field assumption and practical issues. It is shown that after proper modifications the transfer function technique can be used successfully in noisy and reverberating fields and work well at low frequency even in small measurement rooms, becoming therefore a reallyinsitumeasurement technique. A wide frequency range implementation is presented, based on a two‐channel MLS measurement system, ‘‘energy ratio invariant’’ time windowing, a basic specular reflection propagation model with spherical decoupling, and a particular geometrical configuration. The measurement method is tested on a wide range of absorbers. For impedance measurements using infinite surface propagation models the uniqueness of the inversion procedure is investigated using the conformal mapping technique. In addition, faster inversion and model computation routines are proposed. Measurement results obtained under realisticinsituconditions are provided and compared with reference methods.

ISSN:0001-4966    

DOI:10.1121/1.422874

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The phenomenon of the generation of acoustic waves in liquids based on interaction of alternating ion current and constant magnetic field is analyzed. The properties of such a magneto‐electric transducer having no mechanical vibrating elements is examined theoretically (with the use of an electric equivalent circuit) and experimentally. The following measurements were carried out: (1) amplitude of conductivity current as a function of frequency, (2) amplitude of acoustic pressure at the distance of 0.5 m without constant magnetic field and (3) with magnetic field. For water from a municipal water‐pipe network, the upper limit frequency of the current characteristic is 4 to 5 kHz. The same frequency properties exhibit a pressure characteristic. The directional pattern of the source depends on the shape of the electrodes and can be optimized for special aims. The energetic efficiency of the transducer is not great because the majority of the electric power is converted into heat and causes an increase in temperature of the liquid. That process is relative to a big value of resistance of weak electrolytes. That type of transducer can be applied in liquids without ion conductivity by submerging a container with very thin rubber walls and with mounted electrodes and filling with an electrolyte.

ISSN:0001-4966    

DOI:10.1121/1.422877

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Studies of sonoluminescence (SL) in magnetic fields up to 20 T have revealed a striking magnetic field dependence. The intensity of emitted light is suppressed under increasing magnetic fields and vanishes above threshold magnetic field that depends on the applied sound pressure. Further increase in the magnetic field leads to the destruction of the bubble through dissolution. At a constant magnetic field, the light intensity is found to increase roughly linearly with increasing sound pressure until the bubble disappears above a cut‐off pressure which depends on the strength of the magnetic field. The cut‐off pressure is found to approximately double between 0 and 20 T. Further study of the SL bubble through the measurement of the phase of the emitted radiation relative to acoustic drive shows a large increase in phase under magnetic fields. These results suggest that the bubble grows significantly in size and that there may be a substantial modification of bubble dynamics under magnetic field. The origin of the observed effects, possibly due to magnetic field‐induced anisotropies in the SL bubble, will be discussed in light of our recent anisotropy studies under magnetic fields.

ISSN:0001-4966    

DOI:10.1121/1.422880

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Single‐bubble sonoluminescence (SBSL) in water occurs when a bubble undergoes highly nonlinear volume oscillations in a sound field. In the presence of gravity, the SBSL bubble experiences a buoyancy force described by ρgV(t). There is also a Bjerknes force from the sound field. The time average of these two forces balances at the equilibrium location. If the SBSL apparatus is in normal gravity the bubble is displaced from the pressure antinode of the acoustic standing wave [Matulaetal., J. Acoust. Soc. Am.102, 1522–1524 (1997)]. Because of this displacement, the bubble has a vertical oscillation. One method of exploring the effects of the translational oscillations is to compare the light output of SBSL in microgravity to the light output of SBSL in normal or hypergravity. These were examined by an experiment performed aboard NASA’s KC‐135A. The airplane performs parabolic maneuvers, giving short periods of microgravity and hypergravity (approximately 0 and 2 g, respectively). The results suggest a correlation of higher light output in microgravity than in hypergravity. [Work supported by Flags Up Feed, Inc., Space Grant Consortium, NASA, and W.S.U.]

ISSN:0001-4966    

DOI:10.1121/1.422887

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Whether a shock is formed or not in a sonoluminescing gas bubble is a fundamental problem in the sonoluminescence study. The problem is numerically studied by solving Euler equations of gasdynamics in conjunction with an approximate equation of Keller type for the motion of the bubble radius. A high‐resolution TVD upwind finite‐difference scheme is used for Euler equations and a fourth‐order Runge–Kutta method for a Keller‐type equation. The result shows that, owing to the strong nonlinearity of the bubble motion, a slight increase of the driving pressure over a critical value makes the maximum collapse speed of a bubble wall supersonic, and hence the gas behavior changes from a nearly incompressible and adiabatic flow to a compressible one including a shock wave. The relation between the critical driving pressure and other parameters (the driving frequency, the initial bubble radius, etc.) is analyzed. The maximum temperature and pressure attained in the bubble are compared with those in earlier theoretical and numerical works.

ISSN:0001-4966    

DOI:10.1121/1.422889

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Biological effects of ultrasound involve nonlinear phenomena (1) in the propagation of sound itself, (2) in the generation of acoustic cavitation, and (3) in the biochemistry, physiology, and pathology of the biological system. Most nonthermal bioeffects of interest to users of diagnostic ultrasound require acoustic pressures great enough that the wave can become distorted by nonlinear propagation. Under limiting conditions this process can increase the absorption parameter of weakly absorbing media by orders of magnitude and make the absorption parameter of a material such as water as great as the linear absorption coefficient of liver tissue. At low amplitudes the response of the bubbles is dominated by the acoustic pressure. At a critical acoustic pressure, however, the inertia of the surrounding medium becomes controlling. At this threshold of acoustic pressure a 10% or 20% increase in acoustic pressure leads to an increase in the collapse pressure in the bubble by orders of magnitude. The rates of biochemical processes, including denaturation of biological macromolecules, are exponential functions of the temperature. Whether the physical process of heating by ultrasound is linear or nonlinear, this leads to a very strong nonlinear dependence of thermal tissue damage and teratological effects upon the levels of ultrasound.

ISSN:0001-4966    

DOI:10.1121/1.422894

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Numerical modeling of high‐intensity focused sound beams is important for medical applications of ultrasound. The KZK equation, which may be modified to include relaxation in the time domain or arbitrary absorption and dispersion in the frequency domain, is widely used to model ultrasound beams in tissue. Numerical solutions are available in either the time or frequency domain, but they become time consuming at high intensities when thin shocks are developed. To speed up calculations, a known high‐frequency asymptotic result for shocks can be incorporated in the frequency domain algorithm. In comparison with exclusively numerical schemes, the present approach substantially reduces the number of harmonics and thus the computation time [Yu. A. Pishchal’nikov, O. A. Sapozhnikov, and V. A. Khokhlova, Acoust. Phys.42, 362–367 (1996)]. Focused cw beams radiated from Gaussian and piston sources are simulated in water and biological tissue. The accuracy of the asymptotic approach is verified via conventional numerical methods of solution. It is shown that the present asymptotic method provides accurate results with a considerable reduction in computation time. In combination with conventional numerical schemes, the developed asymptotic method provides an effective tool for investigating focused sound beams for a wide range of parameters. [Work supported by NIH PO1‐DK43881, Fogarty Research Program, and CRDF.]

ISSN:0001-4966    

DOI:10.1121/1.422897

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

For the large distance traversed by the sonic beam the exact analytical and physically realistic solutions of the Khokhlov–Zabolotskaya (KZ) equation were found for the first time. The procedure of finding these solutions as the self‐similar traveling waves identifies their asymptotical generality with respect to the initial arbitrary wave profiles. The resulting solutions consist of the periodically repeating parts of the second‐order curvers such as hyperbolas, ellipses, and, in particular, parabolas. The boundaries between these parts are defined with the use of the preserved integrals of the KZ equation. The influence of nonlinearity, diffraction, which is seen in many‐dimensional cases, and initial curvature of the phase front of the wave beam on the forming of the typical form of a specific asymptotical wave profile (N shaped or U shaped) was discussed. A comparison was made between analytical solutions with the numerical results for the KZ equation and the experimental data and a full agreement was discovered. These solutions play the same role among all the variations of solutions of the KZ equation, such as the well‐known sawtoothlike wave for the equation of the simple waves (Riemann’s equation). [Work supported by CRDF.]

ISSN:0001-4966    

DOI:10.1121/1.422903

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP