The history of the contact microphone is related to the history of piezoelectricity, since most contact mikes have made with piezoelectric materials.1 The discovery of piezoelectricity is attributed to Pierre and Jacques Curie, who published in 1880 the first experimental demonstration of the connection between piezoelectric phenomena and crystallographic structure in the article titled Développement par pression de l’électricite polaire dans les cristaux hémièdres à faces inclinées. They experimented with measurement of the surface charges of crystals of tourmaline, quartz, topaz, cane sugar and Rochelle salt, when subjected to mechanical strain. This phenomenon was soon named “piezoelectricity” (from the Greek word πιέζειν piezein = press, squeeze) [See Appendix 1]. In order to distinguish it from other scientific phenomenological experience such as "pyroelectricity" (electricity generated from crystals by heating), or "contact electricity" (friction generated static electricity). The following year Gabriel Lippman deduced mathematically from fundamental thermodynamic principles the inverse piezoelectric effect (mechanical strain results from the injection of an electrical field). Based on experimental observations the Curie brothers confirmed the converse effect in 1882. In the following years, the European scientific community worked interactively to establish the core of piezoelectricity: the identification of 20 natural classes of piezoelectric crystals on the basis of asymmetric crystal structure, the reversible exchange of electrical and mechanical energy, and the usefulness of thermodynamics in quantifying complex relationships among mechanical, thermal and electrical variables. The first monograph on piezoelectricity and the relevant crystallography was Lehrbuch der Kristallphysik, published in 1910 by Woldemar Voigt's, the book became the standard reference offering the understanding which had been reached. Nevertheless, it took a while to develop from the scientific theory concrete technological applications, also because the mathematics required to understand the phenomenon of piezoelectricity was still quite obscure.
The first practical applications of piezoelectric principles appeared during World War I, as ultrasonic submarine detectors, most famously sonar, based on research done between 1916 and 1917, by the French physicist Paul Langevin (previously a doctoral student of Pierre Curie) and the British/Canadian Robert William Boyle. An electric pulse was sent to a piezoelectric crystal, which produced high-frequency mechanical vibrations that were transmitted through the water. Upon encountering an object, these signals reflected back. A second piezoelectric sensor detected this reflected energy and converted it back into an electrical signal. The distance from the ultrasonic source and the reflecting object was determined by the elapsed time between transmission and reception. This technology was of strategic importance in both world wars. Years later musicians and sound-artists began using underwater microphones (hydrophones) with far more peaceful intentions. The trickle-down of sonar technology stimulated the development of many other kinds of piezoelectric devices. After World War I, more familiar piezoelectric applications – such as microphones, accelerometers, ultrasonic transducers, bender element actuators, phonograph pick-ups and signal filters – were invented and put into practice. During World War II, isolated research groups in the U.S., Japan and the Soviet Union replaced naturally-occurring crystals with ferroelectrics – new discovered artificial materials, that exhibited stronger piezoelectric properties; these were incorporated into more powerful sonars, ceramic phono cartridges, piezo ignition systems, the sonobuoy (sensitive hydrophone listening and transmitting buoys for monitoring ocean vessel movement), miniature sensitive microphones, and ceramic audio tone transducers.
Intense development of materials and devices proceeded, dominated by industrial groups in the U.S. who secured an early lead with strong patents. In U.S post-war companies maintained strict policies and secrecy habits resulting from the development of this field during the war. Consequently, the attempts to develop other applications and build a market for piezoelectric devices were not very fruitful. In contrast, the open-policy atmosphere in Japan encouraged several companies and universities to collaborate, providing a context for the creation of new knowledge, new applications, new processes, and new commercial market areas in a coherent and profitable way. After World War II, Japan dominated the international market for piezo materials, manufacturing several types of piezoceramic signal filters that addressed needs arising in television, radio and communications equipment, as well as piezoceramic igniters for natural gas/butane appliances. The market for piezoelectric applications continued to grow, with the emergence of audio buzzers (such as those in appliances and smoke alarms) and ultrasonic transducers (used in motion detecting intrusion alarms and early television remote controls). More recently, piezoelectric technology has been applied in the automotive domain (wheel balancing, seatbelt buzzers, tread wear indicators, keyless door entry, and airbag sensors); computers (microactuators for hard disks, piezoelectric transformers); a wide range of other commercial and consumer devices (inkjet printing heads, strain gauges, ultrasonic welders, smoke detectors); and medical, biomedical and bioengineering applications, including insulin pumps, ultrasound imaging and therapeutics, piezoelectric and biomedical implants with associated energy harvesting.
>> go to 1.2 Musical Application