Ultrasonic cavitation monitoring by acoustic noise power measurement

In this paper, a new tool is proposed to carry out acoustic cavitation monitoring and to have an overview of its effects in applications. After a brief review of the cavitation characterization techniques, it is shown that cavitation noise is a suitable and accurate indicator of the cavitation activity induced in a liquid. In the first part of this study, the origin of the first spectral component of the cavitation noise is discussed. The f/2 and 2 f component evolution measurement at a driving frequency around 1 MHz confirms Neppiras' ones and gives an indicator of the cavitation inception. In the second part, the cavitation noise spectrum distortion is considered as a function of the acoustic power transmitted to the liquid in order to obtain an indicator of cavitation activity. In the last part, this new tool is used to bring to the fore the hysteresis effect associated with the cavitation. An experimental correlation between cavitation noise power measurement and the sonochemical activity in an oxidization process is also presented.

A unified formalism using effective surface permittivity to study acoustic waves in various anisotropic and piezoelectric multilayers.

A unified formalism is presented that uses the effective surface permittivity (ESP) to study surface acoustic waves (SAW) in layered substrates and guided waves in layered plates. Based on known mathematical tools, such as ordinary differential equation and transfer matrix, a generalized surface impedance (GSI) concept is developed and exploited to investigate the acoustic propagation in various anisotropic and piezoelectric layered structures. The ESP function, originally defined for the surface of a homogeneous and semi-infinite piezoelectric substrate, is extended to both the top surface of and an interface in a layered half space, as well as to either surface of a finite-thickness plate. General ESP expressions for all mentioned configurations are derived in terms of an equivalent GSI matrix. It is shown that, when using the appropriate GSI matrices, the same form of the ESP expressions applies no matter whether the structure is a homogeneous half space alone or coated with a layered plate or a layered plate alone. GSI matrices are explicitly given in terms of the bulk partial mode solutions for a substrate and via the transfer matrix for a plate. Modified GSI matrices for structures consisting of both a plate and a substrate are also specified. Analytical development is fully detailed to suit program implementation. To illustrate its versatility, the formalism is also applied to two-substrate configurations, allowing one to analyze guided waves in a plate sandwiched between and interfacial waves existing along the boundary of two different media. Numerical examples are given to illustrate the spectrum features that the ESP shows for various structures. Deduced ESP expressions allow one to locate directly all piezoelectrically active waves in any structure including at least one piezoelectric layer. Acoustic modes that are not piezoelectrically active and those in non-piezoelectric materials can be also obtained by using the intermediate results, such as derived GSI matrices.

Noncontact measurement of vibration using airborne ultrasound.

A noncontact ultrasonic method for measuring the surface normal vibration of objects was studied. The instrument consists of a pair of 420 kHz ultrasonic air transducers. One is used to emit ultrasounds toward the moving surface, and the other receives the ultrasound reflected from the object under test. Two effects induce a phase modulation on the received signal. The first effect results from the variation of the round trip time interval tau required for the wavefront to go from the emitter to the moving surface and back to the receiver. This is the Doppler effect directly proportional to the surface displacement. The second effect results from the nonlinear parametric interactions of the ultrasonic beams (forward and backward) with the low frequency sound field emitted in the air by the vibrating surface. This latter phenomenon, which is a volume effect, is proportional to the velocity of the vibrating surface and increases with the distance between the transducers and the surface under test. The relative contribution of the Doppler and parametric effects are evaluated, and both have to be taken into account for ultrasonic interferometry in air.

Acousto-optic low-frequency shifter.

Here a large deflection angle, low optical frequency-shift acousto-optic device is presented. This is realized by two successive acousto-optic interactions in the same cell. The relevant parameters of operation are analyzed in detail. A practical case with paratellurite material is then considered. Results from numerical computations leading to practical design parameters are given and compared with experimental ones.

Acousto-optic multifrequency modulators: reduction of the phase-grating, intermodulation products.

The multifrequency acousto-optic modulator efficiency is limited mainly by the two-tone, third-order intermodulation products. We show here that a suitable anisotropic interaction can greatly reduce this undesirable effect. Numerical computations have been drawn for a paratellurite acousto-optic cell, and it is shown that a reduction of ~ 16 dB can be reached, limited by the acoustic nonlinearity intermodulation products. A specific method for experimental validation, based on optical heterodyning on a photodetector, is presented. The experimental results agree well with the theoretical ones.