00:44 Sep-28-1996 Re: To all: Is there any disadvantage for composite transducers? s acoustically loaded by a low impedance medium even when the material or the piece to be inspected has a high impedance : for instance in the case of immersion probes and probes incorporating (or associated with) a shoe, a wedge or a delay line. made of polymer materials.In these numerous cases transducers with an active part made of 1-3 piezocomposite material can provide better sensitivity in a wider freqency band than transducers produced with other presently known technologies. Such transducers can bring an important improvement in inspection quality ( see for instance results given in workshop articles from G.Splitt and G.Fleury ) in applications where signal to noise ratio and bandwidth are critical parameters . Of course composite transducers have some limitations and yet Bill Grandia noticed some of them . The case of applications where the transducer is used in direct and intimate contact with a high impedance material is one of them . Due to their relatively low acoustic impedance, piezocomposite materials may be not appropriate to build such transducers. The case of applications" requiring High Q transducers" should be discussed into more details : Is the High Q required to create a narrow band or is it only a way to design an efficient transducer (at a given frequency) and /or to transmit high intensity ultrasound ? If the application imposes a narrow band system ,this narrow band can be obtained by different ways : . Narrow band transducer is one way to do it and then composite technology may be not appropriate to get it .However in such case the frequency will be fixed by transducer design,dependant from manufacturing tolerances and may be will vary with temperature or transmitted power ( which creates heating effects inside the transducer) . Narrow band of the system can be obtained by electronic filtering associated with a medium or wide band transducer and then the frequencycan be more easily choosen and controlled. If the application requires an efficient transducer,then composite transducer can be extremely useful with a high transmit/receive efficiency quite close to what can produce narrow band transducers and again with the possibility to adjust and/or control the frequency. Finally, if the application requires power generation then specially designed composite transducers can produce very high level of intensities in CW, high duty cycle burst or impulse mode . Are composite transducers more expensive than conventional ones ? Such question requires special attention about the basis of comparisons : .Is the comparison to be done on standardised probes manufactured with the same general specifications excepted acoustical properties? Then, as far as I know, differences in prices are quite low between composite transducers and conventional transducers and I am not able to say if the difference is due to the composite aspect or is due to other aspects as for instance general quality or associated services . .On special transducers dedicated to special applications the analysis is still more difficult to do but again as far as I know differences in price are not important in comparison with the gain on global functional performances. Limitations in frequency : Piezocomposite technology based on 1-3 active materials is physically limited in frequency due to the microstructure of materials .As far as I know the upper limit is a center frequency of 15 Mhz for standard transducers and 18 Mhz for special fabrications . There are many other things ( array applications ,low frequency transducers,aspherical focusing,high temperature, ...) to discuss about composite transducers but excepted the above comments I do not see many restrictions for the use of such transducers in NDE applications.Of course these transducers are presently a small part of the total market of transducers but they bring so much advantages in many applications that I am sure they will play a much more important role in the future . Gerard Fleury
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