Acoustic probes

There are many different kinds of acoustical probes including microphones [57-62], hydrophones, radiometers, and piezoelectric devices (most often small barium titanate transducers) [63-68], and the hot wire microphone (based on acousto-resistive effect) [63], Their resonance frequency is generally very different from that of the ultrasonic field under study. 

These probes can be used to measure the pressure amplitude in the system [19]. In principle the local acoustic power can be obtained by measuring the pressure amplitude P, the velocity v of an imaginary particle submitted to the field, and their 

This is a good method for local measurements, but rather tedious for overall power. Furthermore, to calculate the ultrasonic power one needs to measure the particle velocity, and this is not a trivial task. Indeed one might assume that the particle velocity is the same in the liquid and at the tip of the probe (see Section 4.2). This is almost never exactly true, but this assumption can lead to a reasonable estimate of the dissipated ultrasonic power. 

High pressures generated by the oscillations of the bubbles and (or) their collapse cannot be directly measured, but an indication can be obtained by using very small microphones as probes [59-61]. 

Although these acoustical probes can be made very small they will always slightly disturb the ultrasonic field. Just as in the case of coated thermal probes, the response signal depends on the nature and size of the probe, thus it is important that the microphones are carefully calibrated. They are however widely used, especially to calibrate medical ultrasonic equipment. Recently, very small and sensitive devices using PVDF membranes [68,69] or fiber optics [70] have been described. PVDF has piezoelectric properties and miniature membrane hydrophones (about 0.5 mm in diameter) are available. Fiber optic probes can even be smaller and a spatial resolution of 0.1 mm has been claimed [70], 

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Figure 4 Acoustically probed photoactkm spectrum of 1.1 fun thick HR-100 photoresist (left axis) rale of change of APM velocity R(Ao), normalized to spectral density /( )> indicates rale of film cross-linking as a function of optical wavelength Ao. Optical absorption spectrum of identical Him on quartz flat (right axis) spectrum peaks at 3SS nm due to absorption by photoinitiator, while strong absorption below 300 nm is due to the polymer backbone. (Reprinied wiUi pennission. See Ref. [195].)...

Class 3-Methods Based on Direct Mechanical Effects. These include the use of acoustical probes [57-71], acoustic impedance measurements [72—75], acoustic fluxmeter [76], the measurement of radiation forces [17,21,77—112], the distortion of liquid surface [ 113-115], surface cleaning, dispersive effects, emulsification [ 116-118], erosion [ 19,22,119-125], mass transfer measurements (electrochemical probe) [26,129], absorption methods [93,132], particle velocity measurements [132], and optical methods [133-141],... 

A further interest in the use of acoustical probes is that the Fourier transform of the P = f(t) signal gives a P = f(w) signal, that is the noise emitted in the sonicated medium. Noise measurements will be further described in Section 6 below. 

The response of hydrophones or microphones can be calibrated in terms of either sound level (dB) or acoustic pressure either of which are related to ultrasound intensity. The Fourier transform of these give a frequency-dependent signal, which is also related to sound intensity. It is possible therefore to obtain a plot of sound level (and thus theoretically ultrasound intensity) against frequency. However, as already mentioned, ultrasound power measurements using acoustic probes are not straightforward and require preliminary calibration of the probes with another... 

As has already mentioned, acoustic probes can be made very small and very sensitive however their use requires somewhat sophisticated equipment together with accurate calibration which is not easy to achieve. 

Small or large areas of laminates or honeycombs can be tested by using single or multiple acoustic probes mounted upon appropriate scanning mechanics and feeding data process and presentation equipment. 

Methods based on direct mechanical effects, including measurements using acoustic probes, optical methods and acoustic impedance, radiation forces, liquid surface distortion or particle velocity measurements. 

Today, microelectrodes can be prepared and used as electrical, magnetic, or acoustic probes or sensors. In a future process the membranes, spacers or modules could be equipped with a pattern of microelectrodes for different types of measurements and the collected data would be used for optimizing the process conditions within the membrane process. It could also be possible that, in an industrial process, there would for instance be one test module equipped in this way that could be placed somewhere in the process design. 

Of course the spectrum of quantities, which need to be measured in a fluidized bed, is much wider. These include, for example, local solids volume concentrations, solids velocities and solids mass flows, the vertical and the horizontal distribution of solids inside the system or the lateral distribution of the fluidizing gas. In response to these needs a number of more sophisticated measurement techniques were proposed. For example, suction probes were developed to measure local solids and mass flow, heat transfer probes were proposed for detection of de-fluidized zones and solids flow inside fluidized-bed reactors. Other techniques include capacitance probes, optical probes, or y-ray densitometry - a detailed review was given recently by Werther [1]. Cody et al. 2 reported the use of an acoustic probe to measure particle velocity at the wall of fluidized beds. 

Then, the weld depths penetration are controlled in a pulse-echo configuration because the weld bead (of width 2 mm) disturbs the detection when the pump and the probe beams are shifted of 2.2 mm. The results are presented in figure 8 (identical experimental parameters as in figure 7). The slow propagation velocities for gold-nickel alloy involve that the thermal component does not overlap the ultrasonic components, in particular for the echo due to the interaction with a lack of weld penetration. The acoustic response (V shape) is still well observed both for the slot of height 1.7 mm and for a weld depth penetration of 0.8 mm (lack of weld penetration of 1.7 mm), even with the weld bead. This is hopeful with regard to the difficulties encountered by conventional ultrasound in the case of the weld depths penetration. 

The properties of the piezocomposite material mentioned above offer special benefits when the transducer is coupled to a material of low acoustic impedance. This especially applies to probes having plastic delay lines or wedges and to immersion and medical probes. These probes with piezocomposite elements can be designed to have not only a high sensitivity but also at the same time an excellent resolution and, in addition, the effort required for the probe s mechanical damping can be reduced. 

Shear Horizontal (SH) waves generated by Electromagnetic Acoustic Transducer (EMAT) have been used for sizing fatigue cracks and machined notches in steels by Time-of-Flight Diffraction (TOED) method. The used EMATs have been Phased Array-Probes and have been operated by State-of-the-art PC based phased array systems. Test and system parameters have been optimised to maximise defect detection and signal processing methods have been applied to improve accuracy in the transit time measurements. 

Up to now it was demonstrated, that the probe design enables a fast positioning and that the acoustical parameters of the probe ensure a reliable crack detection implying even a coupling control. Now, the final customer s requirement was the inspection of the blades without demounting them from the engine. 

ISONIC - Complete Ultrasonic Weld Inspection Documentation by Continuous Recording of Manual Probe Manipulations, Acoustic Coupling and Echoes. 

This paper intends to give, through different examples, guide-lines for characterization of array probes. We discuss, particularly, beam pattern measurement methods and raise the question whether it is useful to achieve a full characterization of all beams steered by the probe or to limit the characterization to a minimum set of acoustic configurations. An automatic bench for full characterization of tube inspection probes is described. 

Insofar as Ultrasonic Array probes have come onto the market from several years and are now moving from prototype stages into industrial tools for on-site inspections, methods and tools for acoustic characterization is becoming a real concern. Furthermore, the lack of standards, either national or European, enhances the needs for guidelines proposal. 

For conventional probes, acoustic verification aims at characterizing the beam pattern, beam crossing, beam angle, sensitivity, etc., which are key characteristics in the acoustic interaction between acoustic beam and defect. For array transducers, obviously, it is also a meaning to check the acoustic capabilities of the probe. That is to valid a domain (angle beam, focus, etc.) in which the probe can operate satisfactorily. 

As any conventional probe, acoustic beam pattern of ultrasound array probes can be characterized either in water tank with reflector tip, hydrophone receiver, or using steel blocks with side-drilled holes or spherical holes, etc. Nevertheless, in case of longitudinal waves probes, we prefer acoustic beam evaluation in water tank because of the great versatility of equipment. Also, the use of an hydrophone receiver, when it is possible, yields a great sensitivity and a large signal to noise ratio. 

The encircling probe was characterised with its mirror in water. As we did not own very tiny hydrophone, we used a reflector with hemispherical tip with a radius of curvature of 2 mm (see figure 3c). As a result, it was possible to monitor the beam at the tube entrance and to measure the position of the beam at the desired angle relatively to the angular 0° position. A few acoustic apertures were verified. They were selected on an homogeneous criteria a good one with less than 2 dB of relative sensitivity variations, medium one would be 4 dB and a bad one with more than 6 dB. 

The required acoustic verifications depend on what the probe is made for. If the probe is used as an angular scamiing system with a fix set of elements, then we think it is only needed to characterize the array behavior with a few selected time delay laws to isolate the angular steering capability and the foeusing capability as explained before. 

However, if the probe is used as linear scanning system, the acoustic beam depends on the element characteristics which are liable to change from one element to an other. Therefore, the only two alternative proposals are to characterise the aeoustie behaviour of all active sub-set of elements or to proeeed to a statistical characterization. 

In the case of the Superphenix probes we were asked to provide a 100% characterization of the probes, that meant to verify all acoustics characteristics over the 160 groups of element multiplexed around the probe. This task has required the development of an automatic acquisition and analysis system which is described below. 

The automatic acquisition and analysis system we developed within the scope of the Super-Phenix steam generator tube inspection by ultrasonic arrays is a remarkable example of an exhaustive acoustic verification system. It works for every type of probe for tube inspection. 

The principle of the acquisition system is to translate the probe into a tube (including hemispherical drilled holes) step by step, every 0.04 mm, after a forwards and backwards 360 rotation of the tube trigging every 0.2° angular step a 360° electronic scanning of tube with the 160 acoustic apertures. During the electronic scanning the tube is assumed to stay at the same place. The acquisition lasts about 30 minutes for a C-scan acquisition with a 14 kHz recurrence frequency. 

Figure 4 Acoustic beam characterization set-up for the immersion SW array probe a/ Angle beam characterization principle b/ General view of the automatic bench...

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