Adriaan Daniël Fokker was a prominent Dutch theoretical physicist. He is known in theoretical physics for the Fokker-Planck equation and his work on the theory of relativity. In the years around 1930, however, Fokker also became interested in room acoustics and the tuning of musical instruments. He designed several sound reflectors, which amplify the sound reaching the audience, especially in churches, but his work on musical instrument tuning is what made him more widely known. He ardently advocated dividing the octave into thirty-one parts, as proposed by Huygens, and commissioned a pipe organ using this tuning. For most of the period between 1928 and 1955, Fokker was the director of research at Teylers Museum in Haarlem and a professor at Leiden University. He served as the first chair of the Dutch Sound Foundation, and in 1949 was elected to the Royal Netherlands Academy of Arts and Sciences.