Figure of merit of piezoelectret transducers for pulse-echo or transmit-receive systems for airborne ultrasound

Abstract

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DC-Biased Piezoelectret Film Transducers for Airborne Ultrasound

“…These results suggest that the internal polarization due to the electret charges corresponds to a value of about +800 V. Data for r = 30 mm and r = 90 mm, not shown in the figure, indicate the dependence of the sound pressure on 1/r, as expected from theory. The theory and also the relatively high sound pressures achieved with single-layer piezoelectret transducers are discussed in more detail and compared with results obtained with other transducers in an earlier paper [13].…”

“…In piezoelectric as well as in piezoelectret ultrasonic senders and emitters often one or more of the three effects are utilized for significantly improving their performance. It can be shown [13] that the performance of piezoelectret ultrasonic transducers can be simply characterized by indicating the d 33 -and g 33 -coefficients of the employed piezoelectret films, where g 33 is the piezoelectric voltage constant. Extending the definition of d 33 and g 33 to "effective" values allows thus the comparison of transducers independent of their specific operating principle.…”

“…Using the definition given by eq. (1) the on-axis sound pressure p(r) generated by a circular piston with radius R and area π R 2 , mounted in an infinite baffle, at a distance r is given by [13]:…”

“…In particular, their piezoelectric d 33 -coefficients are significantly higher than those of most other materials, they therefore possess a high figure of merit [12,13], they can be easily handled, and their acoustic impedance is smaller than that of the ceramics and thus closer to the values for air. As opposed to the ceramics, they show a relatively small Q-factor and can therefore be used as broadband devices below resonance.…”

Energy harvesting from vibration with cross-linked polypropylene piezoelectrets

“…This equation shows that d 33 g 33 = d 33 2 /ε 0 ε r which is known to be a figure of merit for piezoelectret transmit-receive systems, 35 is also a FOM for piezoelectric energy harvesting. 27 Larger thickness and smaller area also results in an increase of the harvested energy.…”