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The World Of AcousticsThe Evolving World of Acoustics and Vibration TechnologiesIn a report of 1921, Rochelle salt was identified as a ferroelectric material—a material electrically polarized even in the absence of an external electric field, whose polarization can be reversed by applying an electrical field. Incorporated since then as a piezoelectric element in earphones and microphones, the substance is a double salt grown and produced in the past by the Kobayasi Institute of Physical Research. Once an essential material in sound transmission, it has since been replaced by other materials.Interview made possible by the cooperation of the Museum for the Science of Sound, Kobayasi Institute of Physical ResearchPhoto by Yuki AkabaRochelle SaltOwned by the Museum for the Science of Sound, Kobayasi Institute of Physical Research