Electroacoustic Conversion Efficiency Research Articles

Sensorless control of a harmonically driven linear reciprocating electrodynamic machine for a thermoacoustic refrigerator

In electrically driven thermoacoustic refrigerators, the acoustic pressure and actuator displacement are typically monitored while a nominally harmonic drive frequency is adjusted to achieve a given operating point (e.g., maximum electroacoustic conversion efficiency, acoustic resonance, maximum power, etc.) Most often, pressure and displacement are measured directly, using sensors. Using a model of the electrical and mechanical dynamics of a linear electrodynamic machine and resonant assembly, the actuator displacement and pressure at the piston can be accurately estimated via direct measurement of the voltage and current at the machine terminals. This provides a means for sensorless control of the machine. The estimation method and system parameter extraction method will be described, as well as results of comparisons to sensors. Results from tests on a thermoacoustic refrigerator operating at 3-kW acoustic input power using a sensorless control system implemented to maintain an acoustic resonance will be shown, wherein the refrigerator cold temperature was varied over a practical operating range. The control system tracked the acoustic resonance frequency and achieved the desired phase relationship between pressure and displacement to within 3 deg based on the estimation, thus maintaining essentially optimal electroacoustic conversion efficiency during the test. [Work sponsored by ONR.]

Influence of nonparallelism of the surfaces of a piezoelectric layer on the frequency band of an electroacoustic piezoelectric transducer

The influence of nonparallelism of the surfaces of a piezoelectric transducer on its impedance and frequency band is analyzed theoretically. The model of a wedge-shaped piezoelectric layer taken for the analysis consists of a set of n plane-parallel elements arranged in succession along the length of the transducer with a gradual increase in the thickness of the piezoelectric layer. The analysis performed demonstrates the possibility of significantly expanding the band of working frequencies of a piezoelectric transducer when its electroacoustic conversion efficiency is reduced. In some cases, however, a decrease in conversion efficiency can be employed to optimize an acoustic device, for example, in creating a filter-type piezoelectric transducer, where electromagnetic energy is converted into acoustic energy successively from one cell of the filter to another. The proposed method for expanding the frequency band can be useful, for example, in creating high-frequency acousto-optic Bragg cells.

High-efficiency 2-kW thermoacoustic driver

A moving-magnet electrodynamic driver will be described which is intended to maintain an acoustic resonance within a thermoacoustic air conditioner containing an inert gas mixture of helium and argon at 30 bar. It is energized by a linear motor with a Bl-product of 41 N/A and a coil resistance of 0.24 Ω. An auxiliary spring system augments the magnetic stiffness (154 kN/m) to provide a driver mechanical resonance frequency near 60 Hz. An Inconel bellows forms a flexure seal which provides an effective piston area that maximizes electroacoustic conversion efficiency by making the Joule heating losses in the coil equal to the mechanical dissipation in the magnetic suspension. The driver’s electroacoustic conversion efficiency is predicted to be in excess of 83%. [Work supported by the Office of Naval Research.]

Hydrodynamic siren as a broadband underwater sound projector

The siren mechanism is adapted to underwater uses for sound production. Such a hydrodynamic siren works as a powerful, broadband mixed sound projector as well as a less powerful, more noiselike sound projector. The broadband mixed sound consists of the harmonics due to the rotor rotation rate and the hydrodynamic cavitation noise. The cavitation noise level at rotor rotation is almost the same as that when the rotor is not rotated. The maximum acoustic pressure of a model siren is 189 dB re: 1 μPa at 1 m when integrating between 0 and 10 kHz, and 179 dB re: 1 μPa at 1 m as to one peak line spectrum. The siren gain attained by the rotation rate harmonics is approximated at about 20 dB. The maximum overall electro-acoustic conversion efficiency is estimated as 1.2% for the rotor–stator clearance of 0.15 mm and the applied water pressure of 8.2 kg/cm2, that is, 0.80 MPa. Since the siren has a dual hole configuration, a third set of harmonics is virtually derived from the greatest common divisor obtained from the numbers of holes in the two sets of holes (i.e., 8 holes and 20 holes give an effective virtual 4 holes). The radiation directionality and depth dependence of the model siren are also examined. The experimental results on the hydrodynamic cavitation noise at nonrotation of the rotor almost match those revealed in previous literature on the noise from cavitating water jets and hydrofoils. However, the results show a strong dependence of the spectrum level on the receiving position. More detailed knowledge on the cavitating jet length, the bubble concentration along it, the vibration of the surviving bubbles, etc., are needed to understand the hydrodynamic cavitation noise more intensively. Although the siren cannot avoid the harmonic generation due to the rotation rate, a siren as a noise maker is proposed by considering a random configuration of many holes and random occurrence of hole opening in one rotation period.

Sandwich-layer transducer—a versatile design for ultrasonic transducers operating in air

Abstract This paper reports the results of our work concerning PZT/polymer 2-2 composites for airborne ultrasonic applications in industrial automation. The shape of rectangular piezoceramic plates is important for efficiency and radiation characteristics. Three distinct geometries are found interesting, having width-to-length ratios of less than 0.3, equal to 0.42 and between 1.0 and 1.2, respectively. The electroacoustic conversion efficiency increases with decreasing acoustic impedance of the sandwich. Contour extensional resonances can be controlled for a PZT-volume fraction as small as 2.5%, producing a gain in efficiency of +30 dB compared to the solid piezoelement. The efficiency is increased by additional polymer layers fixed on the radiating surface. Extreme broadband transducers have been developed for applications in the field of object recognition and acoustic holography by the use of rubber-like material for inactive layers.

A model for evaluating the efficiency of an interdigital transducer for bulk wave excitation into liquid

An equivalent circuit model analysis of a liquid-loaded interdigital transducer is described. The electric loss related to the complex relative dielectric constant and the electroacoustic conversion efficiency are represented in the form of a function of the specific acoustic impedance (ρv) of the loaded liquid. The conversion efficiency for ultrasound into the liquid is estimated from only the acoustic impedance and the relaxation time of the substrate material, which are obtained relatively easily from the water measurement. The estimated theoretical efficiency is in good agreement with the experimental results for various liquids. These results also coincide with the results from the wave equation for the leaky surface acoustic wave propagation at the solid–liquid boundary.

Electrostrictive Materials for Ultrasonic Probes in the Pb(Mg1/3Nb2/3)O3–PbTiO3 System

Electrostrictive ceramics in the Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) solid solution system are investigated as the transducer material for ultrasonic probes whose electro-acoustic conversion efficiency is controlled by an applied DC bias voltage. Considering the electromechanical coupling factors induced under practical DC bias voltage and the normal using temperature, it is concluded that the most promising composition is 0.09 mole fraction of PbTiO3. A basic pulse-echo measurement confirmed that electro-acoustic conversion efficiency of an ultrasonic probe made of electrostrictive ceramics is controlled by an applied DC bias field.

Equivalent circuit analysis of performance of acousto—optic deflector using liquid—loaded interdigital transducer

AbstractAn acoustic wave can be generated in a liquid by operating an interdigital transducer (IDT) at a solid‐liquid interface. This paper describes the operation characteristics of an optical deflector in which the acoustic wave thus generated is used as a grating. By representing the admittance characteristics of the IDT as a function of the relative acoustic impedance of the liquid used as a load, the evaluation of the electroacoustic conversion efficiency and the characteristics as an AO device is simplified. By means of a broadband IDT with chirped electrodes, an acoustic diffraction grating is formed in water. Investigations of the characteristics as an acousto‐optic deflector reveal excellent results in diffraction efficiency, deflection bandwidth and resolution number.

Enhanced acoustic levitation using horn loaded and focusing radiators

An improved piezoelectric driven, focusing acoustic radiator for acoustic levitation has been designed, tested, and evaluated. This device displays a significant improvement in focusing to produce better specimen stability and energy well shaping as well as increased transducer power bandwidth and electroacoustic conversion efficiency. Corresponding effects resulting from the horn loading of this acoustic radiator are considered as well as the benefits of using the radiator in conjunction with hot wall high‐temperature furnaces. In order to aid in the evaluation of these acoustic transducer improvements, a simple and useful technique for acoustic field mapping will also be discussed. [Work supported by NASA.]