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Journal of the Acoustical Society of America

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Jun 2011

Volume 129, Issue 6, pp. EL217-4101

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Electret accelerometers: Physics and dynamic characterization

J. Hillenbrand, S. Haberzettl, T. Motz, and G. M. Sessler

J. Acoust. Soc. Am. Volume 129, Issue 6, pp. 3682-3689 (2011); (8 pages) | Cited 1 time

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Electret microphones are produced in numbers that significantly exceed those for all other microphone types. This is due to the fact that air-borne electret sensors are of simple and low-cost design but have very good acoustical properties. In contrast, most of the discrete structure-borne sound sensors (or accelerometers) are based on the piezoelectric effect. In the present work, capacitive accelerometers utilizing the electret principle were constructed, built, and characterized. These electret accelerometers comprise a metallic seismic mass, covered by an electret film, a ring of a soft cellular polymer supplying the restoring force, and a metallic backplate. These components replace membrane, spacer, and back electrode, respectively, of the electret microphone. An adjustable static pressure to the seismic mass is generated by two metal springs. The dynamic characterization of the accelerometers was carried out by using an electrodynamic shaker and an external charge or voltage amplifier. Sensors with various seismic masses, air gap distances, and electret voltages were investigated. Charge sensitivities from 10 to 40 pC/g, voltage sensitivities from 600 to 2000 mV/g, and resonance frequencies from 3 to 1.5 kHz were measured. A model describing both the charge and the voltage sensitivity is presented. Good agreement of experimental and calculated values is found. The experimental results show that sensitive, lightweight, and inexpensive electret accelerometers can be built.
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43.38.Bs Electrostatic transducers
43.40.Yq Instrumentation and techniques for tests and measurement relating to shock and vibration, including vibration pickups, indicators, and generators, mechanical impedance
43.38.Ar Transducing principles, materials, and structures: general

Reflection and transmission of plane waves from a fluid-porous piezoelectric solid interface

Anil K. Vashishth and Vishakha Gupta

J. Acoust. Soc. Am. Volume 129, Issue 6, pp. 3690-3701 (2011); (12 pages) | Cited 2 times

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The reflection and transmission of plane waves from a fluid-porous piezoelectric solid interface is studied. The porous piezoelectric solid, having 6 mm symmetry, is supposed to be filled with viscous fluid. The expressions for amplitude ratios and energy ratios corresponding to reflected wave and transmitted waves are derived analytically. The Christoffel equation of a leaky wave propagating along the surface of a porous piezoelectric solid is derived. The effects of the angle of incidence, frequency, porosity, piezoelectric interaction, and anisotropy on the reflected and transmitted energy ratios are studied numerically for a particular model BaTiO3. The porous piezoelectric solid half space is assumed to be loaded with water. The effects of porosity and frequency on the leaky wave velocity are also studied.
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43.38.Fx Piezoelectric and ferroelectric transducers
43.20.Gp Reflection, refraction, diffraction, interference, and scattering of elastic and poroelastic waves
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