Broadband Transducers

Fig. 5, also an A-scan, shows the possibility of the echo-technique for concrete. The interface and backwall-echo of a 20 cm thick concrete specimen are displayed (RF-display). A HILL-SCAN 3041NF board and a broadband transducer (40mm element 0) are used which enable optimal pulse parameters in a range of 50 to 150 kHz. Remarkable for concrete inspections is the high signal-to-noise ratio of about 18 dB. 

Fig. 5.5. (top) Example of frequency response functions of a resonant and a so-called broadband transducer. The frequency response function was measured using a laser-vibrometer (middle) and a face-to-face method (bottom). Amplitudes are on a linear scale. 

There are significant differences between resonant and broadband transducers. The resonant sensor for which the response is shown in the top part of Fig. 5.5 is sensitive in a small range around 40 kHz and the multiresonance sensor shown below is sensitive to a broader range betw een 50 and 450 kHz with resonances at 50, 130, 160, 215,270, 300, 350, 370,420 and 450 kHz. The graphs in the middle and bottom of Fig. 5.5 are very similar, and appear to be independent of the particular calibration techniques, with regard to the position of the natural frequencies. 

Whittingham TA (1999a) Broadband transducers. Eur Radiol 9 [Suppl 3] S298-303... 

Composite transducers will replace conventional transducers in applications where the improvement of test sensitivity, signal to noise ratio and axial resolution are mandatory. It must nevertheless also be noted in connection with the broadband feature that though composite probes have a specified nominal frequency, the echo signals allow no echo amplitude... 

The data acquired using broadband data is limited to a value that represents the total energy that is being generated by the machine-train at the measurement point location and in the direction opposite the transducer. Most programs trend and compare the recorded value at a single point and disregard the other measurement points on the common-shaft. 

Figure 6. Schematic drawing of the spectral and polarization imaging experiment using one of the two-transducer AOTFs with three cameras to image the two diffracted and one of the undiffracted beams. The two cameras on the side, image orthogonally polarized spectral images from the two diffracted beams and the middle camera with a linear polarization filter in front, acquires a broadband image from one of the zero order...

B. Belikov, V. B. Voloshinov, A. B. Kasyanov, et. al., Broadband matching of an Acousto-optical-cell transducer using Youla s complex normalization theory, Radioelectronics and Communication Systems 31, p. 28-33 (1988). 

Two approaches are proposed to resolve this difficulty. The first is based on generation of a high-purity acoustic field in a spherical, unharmonic resonator, and the second is based on comparison of a set of transducers with limited but overlapping dynamic ranges. In the second approach, simultaneous measurement of a broadband signal produces an accurate characterization over the entire range. Candidates for sensor fabrication include solid dielectric capacitive sensors, piezoelectric transducers, and high-temperature lithium niobate or quartz transducers. 

Increasingly sophisticated, broadband or multiple focusing transducers also play an important role. Contrast-enhanced sonography can be performed not only with 3.5-MHz transducers usually used in abdominal ultrasound, but also with 5- to 12-MHz transducers which offer a higher spatial resolution. 

A second kind of transducer is the broadband sensor. In construction it is similar to a resonance sensor except that it has mechanical damping. These respond over range of several hundred kilohertz. 

However, most sensors used currently in AE applications for concrete are manufactured in a more traditional way, showing either a resonant behavior or several particular resonances. These sensors, which are called multi-resonance transducers, have a higher sensitivity than sensors with a backward mass used outside of their resonance fiequency. Such sensors should not, however, be considered as (true) broadband and it is essential to know their frequency response function. Otherwise, signal characteristics from the source are not distinguishable from artifacts introduced by incorrect knowledge of the frequency response. A calibration of the sensors frequency response, as well as understanding of the direction sensitivity, is important for many applications of AET. 

The zone location method is frequently used to monitor large structures such as buildings, pipes, vessels, etc. Sensors are distributed over a wide area on the surface of the structure and/or concentrated at the most critical locations. To enhance the detection radius, resonant transducers, which are more sensitive than broadband sensors, are the first choice. Fig. 5.7 shows an example of a sensor configuration used to monitor leaks in pipes. The leak or failure must be within the detection range of the sensor, to allow the AE to be detected. If AE is recorded by a particular sensor, the technician should inspect the vicinity next to this sensor for leaks or cracks. 

Cannata, J. M., Ritter, T. A., Chen, W.-H., et al. (2003) Design of efficient, broadband singleelement (20-80 MHz) ultrasonic Transducers for medical imaging applications, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 50, 1548-57. 

In most cases a Wheatstone bridge, or a compensation circuit is used when small changes in Ohmic resistance are to be measured. In this case a new design was used, in which a reference resistance is not needed (Fig.16). A constant electric current of 1 mA flows through the transducer, which corresponds to a certain voltage drop. The difference with an externally adjustable reference voltage is amplified by a broadband instrumentation amplifier (50 x). 

In these studies, we used unfocused ultrasound with a frequency of 0.88 MHz and an intensity of 2 W/cm. Cavitation properties of gels containing impurities and without them were comparatively estimated using an IS-3 MS cavitation indicator developed at the Belarusian State University of Informatics and Radioelectronics. The device s operating principle is based on the measurement of the level of scattered noise in a frequency band from 2 to 10 MHz, received by a broadband hydrophone. An emitter 2 cm in diameter was in direct contact with the face of the cylindrical gel sample (2 cm high and 2.5 cm in diameter), that is, measurements were performed in the transducer s near field. The hydrophone with a sensitive element 2 mm in diameter was placed in the immediate vicinity from the side surface of the sample. 

The applications of PVDF and other fluorinated analogues as piezo- and pyroelectric materials is a growth area. Broadband ultrasonic transducers have been constructed from PVDF which respond to a very wide frequency band They... 

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