Terminology concerning the performance of an acoustic emission transducer is varied and sometimes downright confusing. We hear the expressions; "broad band," "wide band," "flat with frequency," "high fidelity," and "resonant" applied to a particular transducer performance. Broad band and wide band imply high sensitivity over a large frequency range. Resonant implies high sensitivity over a narrow frequency range. Flat with frequency and high fidelity implies that there are no resonances over the frequency band of interest. The confusion comes about when the term "broad band" is used in the context of being high fidelity, and "resonant" is inferred as a transducer having sensitivity over a narrow range of frequencies. An AE transducer can have resonances but still be responsive to signals over a broad frequency range. A high fidelity transducer can have flat response with frequency, but be limited to a narrow range of frequency over which it exhibits the flat response. Traditionally, if one wished high sensitivity, a resonant type transducer was selected, if one wished high fidelity a high frequency, damped "ultrasonic" type transducer was selectred and operated well below its resonant frequency with a resulting loss in sensitivity of approximately 20dB compared to the resonant transducer.
To add to the confusion the response of a particular transducer will be dependent on its aperture size and the way it is used. The calibration curve one generates to describe the sensitivity verus frequency is also influenced by the aperture size. If you would like more information on these topics let me know and I will send you a technical report.
HIGH SENSITIVITY HIGH FIDELITY TRANSDUCER
It is no longer necessary to use a high frequency damped ultrasonic type transducer to achieve high fidelity performance for AE testing. Nor is it necessary to sacrifice 20dB of sensitivity one normally encounters when using such a transducer. Over the past several years we have been actively pursuing design of a high fidelity transducer based on concepts developed by Proctor at NIST many years ago. We have now achieved equal sensitivity to a traditional 150Khz resonant transducer in use for over 25 years. This same transducer design which was first developed by the author at Lawrence Livermore Labs has been designated by many different model names over the years dependent on which manufacturer produces the device (S140, S9204, R15, SE150). Figure 1 is a displacement calibration curve traceable to NIST, showing the sensitivity versus frequency for this resonant type transducer (SE150-M) in comparison with the DECI SE1000-H and SE1000-HI up to a frequency of 400Khz. The new high fidelity transducer is a "broad band" transducer, having frequency response to a 1Mhz or more but loses its high fidelity properties above 400 Khz.
Note that the frequency response of the SE150-M differs from the ultrasonic calibration that is normally provided for this transducer. The reason for the difference is the small 1.5 mm aperture used by the driving transducer (SE100-H) in performing the calibration. The ultrasonic calibration uses a driving transducer with a large aperture that uniformly excites the transducer aperture over its total surface. This results in a calibration curve showing higher sensitivity to the higher frequencies than shown by figure 1. If one is using the transducer for small specimen testing involving a diffuse field effect due to many reflections, the ultrasonic calibration curve is more likely to agree with measured results. On the other hand if one is using the transducer on the surface of a structure for measuring transient plate waves the displacement type calibration curve in figure 1 is more likely to represent the performance of the transducer for this situation.