Passage: The theory of mechanical filters was first applied to improving the mechanical parts of phonographs in the 1920s.  By the 1950s mechanical filters were being manufactured as self-contained components for applications in radio transmitters and high-end receivers.  The high "quality factor", Q, that mechanical resonators can attain, far higher than that of an all-electrical LC circuit, made possible the construction of mechanical filters with excellent selectivity. Good selectivity, being important in radio receivers, made such filters highly attractive.  Contemporary researchers are working on microelectromechanical filters, the mechanical devices corresponding to electronic integrated circuits.

Question: What device was this used for initially?
phonographs