Index of content:
Volume 129, Issue 5, May 2011
- TRANSDUCTION 
129(2011); http://dx.doi.org/10.1121/1.3562558View Description Hide Description
The theory of orthogonal polynomial (Zernike) expansions of functions on a disk, as used in the diffraction theory of optical aberrations, is applied to obtain (semi-) analytical expressions for the spatial impulse responses arising from a non-uniformly moving, baffled, circular piston. These expressions are in terms of the expansion coefficients of the non-uniformity and the responses of the orthogonal expansion functions. The latter impulse responses have a closed form as finite series involving the Legendre functions and the sinc function. The method is compared with a similar method, proposed by P. R.Stepanishen [J. Acoust. Soc. Am.70, 1176–1181 (1981)] where zeroth order orthogonal Bessel functions, rather than Zernike polynomials, are used as expansion functions.
129(2011); http://dx.doi.org/10.1121/1.3533736View Description Hide Description
The electromechanical properties of thin tangentially polarized, stripe-electroded piezoceramic elements are considered. Theoretical analysis is made based on a piecewise linear approximation of the actual nonuniform electric field in the volume of the stripe-electroded cylinder design. Analytical expressions are derived for the electromechanical properties (including the effective coupling coefficient) of the tangentially polarized elements as a function of the geometry and electrode configuration (i.e., electrode width and spacing). The properties of stripe-electroded cylinders are calculated and compared with the ideal case of a uniform circumferential electric field, which may be realized in practice with segmented cylinders comprised of 33-polarized bar elements. The reasons for the reduction in the effective coupling coefficient of tangentially polarized stripe-electroded cylinders compared to the ideal coupling coefficient k33 achievable with the segmented cylinder designs are discussed. The results of calculations of the electromechanical properties are in good agreement with experimentally obtained data on a variety of samples.
129(2011); http://dx.doi.org/10.1121/1.3569707View Description Hide Description
The acoustic impedance at the diaphragm of an electroacoustic transducer can be varied using a range of basic electrical control strategies, amongst which are electrical shunt circuits. These passive shunt techniques are compared to active acoustic feedback techniques for controlling the acoustic impedance of an electroacoustic transducer. The formulation of feedback-based acoustic impedance control reveals formal analogies with shunt strategies, and highlights an original method for synthesizing electric networks (“shunts”) with positive or negative components, bridging the gap between passive and active acoustic impedance control. This paper describes the theory unifying all these passive and active acoustic impedance control strategies, introducing the concept of electroacoustic absorbers. The equivalence between shunts and active control is first formalized through the introduction of a one-degree-of-freedom acoustic resonator accounting for both electric shunts and acoustic feedbacks. Conversely, electric networks mimicking the performances of active feedback techniques are introduced, identifying shunts with active impedance control. Simulated acoustic performances are presented, with an emphasis on formal analogies between the different control techniques. Examples of electric shunts are proposed for active sound absorption. Experimental assessments are then presented, and the paper concludes with a general discussion on the concept and potential improvements.