Controlled Response of a Ceramic Microphone

Several alternative designs are under consideration as transducer elements for the electronic telephone.1 One of the leading candidates is a bilamellar piezoelectric ceramic plate consisting of two thin circular ceramic wafers that are electroded on both surfaces and cemented together. 2 The disks are joined with opposing polarity so that the flexural response of the assembly to applied acoustic loading results in additive voltage output. The design objectives for the transducer response as a function of driving frequency are shown in Fig. I. 3 The cross-hatched areas indicate the template within which the response should fall. Important characteristics of this template are: (i) the electrical output rolls off below 300 Hz in order to exclude low-frequency room noise; (ii) output is * It has since been decided to eliminate rubber from the design and to replace the bilamellar transducer by a metal/ceramic combination. 543 544 T H E BELL SYSTEM TECHNICAL JOURNAL, FEBRUARY 1972 + 5dB I K H Z LEVEL