Christian Gottlieb Kratzenstein

Christian Gottlieb Kratzenstein (30 January 1723, Wernigerode – 6 July 1795, Frederiksberg) was a German-born doctor, physicist and engineer. From 1753 to his death he was a professor in Copenhagen, and became the rector of University of Copenhagen four times.

In 1769 Kratzenstein constructed resonant cavities which, when he excited them by a vibrating reed, produced the sounds of the five vowels: a, e, i, o, and u.

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