Piezoelectric ceramic headphone development

An examination of the pertinent characteristics of headphones is followed by a discussion of the influence of the electromechanical parameters of mounted piezoelectric elements. A comparison of these parameters for Rochelle salt and PZT
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ceramic elements reveals a two to one difference in the energy conversion capabilities of the two materials. The increased activity of Rochelle salt is present for all modes of operation but the difference is very slight for longitudinal and shear expander modes. This is not so for flexure type elements for which the difference is as quoted above«, Despite this advantage, there are some environmental limitations associated with this material. Thus, for some applications, the use of piezoelectric ceramics as the active element in such transducers is recommended. The net result of the difference in electromechanical coupling (energy conversion capability) in the two materials is a compromise in the characteristics of the modified headphone. The important performance characteristics of any headphone include (a) sensitivity, (b) frequency response, and (c) impedance level. Piezoelectric audio transducers in general are characterized as high impedance devices. A significant reduction in impedance does little to change this classification. Consequently, the design objective was to limit the compromise to a change in impedance only. Selection of a PZT ceramic element of suitable dimensions to yield the required sensitivity is facilitated by the use of equivalent circuit techniques. At low frequencies, the electroacoustical circuit representation of the piezoelectric headphone takes on a simple form. Most of the components of this circuit are known, but the dimensions of the mounted element are taken as variable. A solution of the pressure sensitivity of the headphone as a function of element dimensions is readily obtained. Use of the resultant equation is facilitated by plotting design curves. Pressure response measurements in accordance with existing techniques
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yield data in good agreement with results predicted by the aforementioned analysis. The important characteristics include a sensitivity of 8 μbars/volt flat within ±3 db from 50 cps to 8 kc. The only significant change in performance between the crystal and ceramic phones is as anticipated; namely, a reduction in impedance from 90, 000 ohms to approximately 15,000 ohms at 1 kc.