Stephen Horowitz

A MEMS acoustic energy harvester

This paper presents the development of a micromachined acoustic energy harvester for aeroacoustic applications. The acoustic energy harvester employs a silicon-micromachined circular, piezoelectric composite diaphragm for electroacoustic transduction. Lumped element modeling, design, fabrication and characterization of a micromachined acoustic energy harvester prototype are presented. Experimental results indicate a maximum output power density of 0.34 µW cm−2 at 149 dB (ref. 20 µPa) and suggest a potential output power density, for this design, of 250 µW cm−2 with an improved fabrication process.

Acoustic energy harvesting using an electromechanical Helmholtz resonator.

This paper presents the development of an acoustic energy harvester using an electromechanical Helmholtz resonator (EMHR). The EMHR consists of an orifice, cavity, and a piezoelectric diaphragm. Acoustic energy is converted to mechanical energy when sound incident on the orifice generates an oscillatory pressure in the cavity, which in turns causes the vibration of the diaphragm. The conversion of acoustic energy to electrical energy is achieved via piezoelectric transduction in the diaphragm of the EMHR. Moreover, the diaphragm is coupled with energy reclamation circuitry to increase the efficiency of the energy conversion. Lumped element modeling of the EMHR is used to provide physical insight into the coupled energy domain dynamics governing the energy reclamation process. The feasibility of acoustic energy reclamation using an EMHR is demonstrated in a plane wave tube for two power converter topologies. The first is comprised of only a rectifier, and the second uses a rectifier connected to a flyback converter to improve load matching. Experimental results indicate that approximately 30 mW of output power is harvested for an incident sound pressure level of 160 dB with a flyback converter. Such power level is sufficient to power a variety of low power electronic devices.

Analytical Electroacoustic Model of a Piezoelectric Composite Circular Plate

This paper presents an analytical two-port, lumped-element model of a piezoelectric composite circular plate. In particular, the individual components of a piezoelectric unimorph transducer are modeled as lumped elements of an equivalent electrical circuit using conjugate power variables. The transverse static deflection field as a function of pressure and voltage loading is determined to synthesize the two-port dynamic model. Classical laminated plate theory is used to derive the equations of equilibrium for clamped circular laminated plates containing one or more piezoelectric layers. A closed-form solution is obtained for a unimorph device in which the diameter of the piezoelectric layer is less than that of the shim. Methods to estimate the model parameters are discussed, and model verification via finite-element analyses and experiments is presented. The results indicate that the resulting lumped-element model provides a reasonable prediction (within 3%) of the measured response to voltage loading and the natural frequency, thus enabling design optimization of unimorph piezoelectric transducers.

Development of a micromachined piezoelectric microphone for aeroacoustics applications

This paper describes the design, fabrication, and characterization of a bulk-micromachined piezoelectric microphone for aeroacoustic applications. Microphone design was accomplished through a combination of piezoelectric composite plate theory and lumped element modeling. The device consists of a 1.80-mm-diam, 3-microm-thick, silicon diaphragm with a 267-nm-thick ring of piezoelectric material placed near the boundary of the diaphragm to maximize sensitivity. The microphone was fabricated by combining a sol-gel lead zirconate-titanate deposition process on a silicon-on-insulator wafer with deep-reactive ion etching for the diaphragm release. Experimental characterization indicates a sensitivity of 1.66 microVPa, dynamic range greater than six orders of magnitude (35.7-169 dB, re 20 microPa), a capacitance of 10.8 nF, and a resonant frequency of 59.0 kHz.

A multiple degree of freedom electromechanical Helmholtz resonator.

The development of a tunable, multiple degree of freedom (MDOF) electromechanical Helmholtz resonator (EMHR) is presented. An EMHR consists of an orifice, backing cavity, and a compliant piezoelectric composite diaphragm. Electromechanical tuning of the acoustic impedance is achieved via passive electrical networks shunted across the piezoceramic. For resistive and capacitive loads, the EMHR is a 2DOF system possessing one acoustic and one mechanical DOF. When inductive ladder networks are employed, multiple electrical DOF are added. The dynamics of the multi-energy domain system are modeled using lumped elements and are represented in an equivalent electrical circuit, which is used to analyze the tunable acoustic input impedance of the EMHR. The two-microphone method is used to measure the acoustic impedance of two EMHR designs with a variety of resistive, capacitive, and inductive shunts. For the first design, the data demonstrate that the tuning range of the second resonant frequency for an EMHR with non-inductive shunts is limited by short- and open-circuit conditions, while an inductive shunt results in a 3DOF system possessing an enhanced tuning range. The second design achieves stronger coupling between the Helmholtz resonator and the piezoelectric backplate, and both resonant frequencies can be tuned with different non-inductive loads.

Characterization of a Compliant-Backplate Helmholtz Resonator for An Electromechanical Acoustic Liner

Passive acoustic liners are currently used to reduce the noise radiated from aircraft engine nacelles. This study is the first phase in the development of an actively-tuned electromechanical acoustic liner that potentially offers improved noise suppression over conventional multi-layer liners. The underlying technical concept is based on the idea that the fundamental frequency of a Helmholtz resonator may be adjusted by adding degrees of freedom (DOF) via substitution of a rigid wall with a piezoelectric composite diaphragm coupled to a passive electrical shunt network. In this paper, a Helmholtz resonator containing a compliant aluminum diaphragm is investigated to provide a fundamental understanding of this two DOF system, before adding complexity via the piezoelectric composite material. Using lumped elements, an equivalent circuit model is derived, from which the transfer function and acoustic impedance are obtained. Additionally, a mass ratio is introduced that quantifies the amount of coupling between the elements of the system. The theory is then compared to experiment in a normal-incidence impedance tube. The experimental results confirm the additional DOF and overall acoustic behavior but also suggest the need for a more comprehensive analytical model to accurately predict the acoustic impedance. Nevertheless, the experiments demonstrate the potential benefits of this approach for the reduction of aircraft engine noise.

A MICROMACHINED GEOMETRIC MOIRÉ INTERFEROMETRIC FLOATING-ELEMENT SHEAR STRESS SENSOR

This paper presents the development of a floating-element shear stress sensor that permits the direct measurement of skin friction based on geometric Moire interferometry. The sensor was fabricated using an aligned waferbond/thin-back process producing optical gratings on the backside of a floating element and on the top surface of the support wafer. Experimental characterization indicates a static sensitivity of 0.26 µm/Pa, a resonant frequency of 1.7 kHz, and a noise floor of 6.2 mPa/√Hz.

Design and Characterization of a Micromachined Piezoelectric Microphone

*† ‡ ‡ This paper presents the development of a micromachined piezoelectric microphone for aeroacoustic measurement applications. The microphone consists of a circular diaphragm of silicon possessing a thin annular ring of piezoelectric material deposited at the edge of the diaphragm. The microphone was designed by combining a linear piezoelectric composite plate model with a lumped-element, electroacoustic model. Experimental characterization indicates a

Compliant-Backplate Helmholtz Resonators for Active Noise Control Applications

The results of a theoretical and experimental investigation into compliant-backplate Helmholtz resonators are presented. The motivation for this study is to develop an actively tuned noise control system, based upon a Helmholtz resonator that uses a compliant piezoelectric composite backplate. To establish a baseline case, rigid backplate Helmholtz resonators were designed using a lumped element model and tested in a plane-wave tube. After this initial testing, the rigid backplate of the resonator was replaced by several compliant aluminum diaphragms and characterized. The experimental results are compared to lumped element models of the Helmholtz resonator with a compliant diaphragm.

A Self -Powered Wireless Active Acoustic Liner

This paper presents the proof of concept demonstration of a self -powered wirelessly -controlled active acoustic liner. The system consists of a tunable Helmholtz resonator for acoustic impedance boundary condition modification and an acoustic energy reclamation module as a system power supply for the wireless receiver and analog switches. The common electroacoustic element for both of these components is an electromechanical Helmholtz resonator (EMHR) with the standard rigid backplate replaced by a compliant piez oelectric composite diaphragm. The acoustic impedance of the resonator is adjusted by coupling the EMHR to a pas sive electrical shunt network. Different impedance boundary conditions for the same resonator are realized by switching from o ne passive network to another. Acoustic energy harvesting is achieved by connecting an EMHR with unit cell area of 4.83 cm 2 to an energy reclamation circuit that converts the acoustically generated ac voltage signal across the piezoceramic to a condition ed dc signal. This harvested energy is used to power the wireless receiver and a nalog switches, which requires 6 mW of power when operated at 3.5 V to permit the wireless tuning of the liner. The commands to modify the acoustic impedance of the active li ner are sent by a 300 MHz transmitter external to the self -powered wirelessly tuned active liner.