His doctoral thesis is a study of unpublished writings on composition, acoustics, and playing the recorder by the French musical theorist Étienne Loulié (d ca 1707). Semmens joined the staff of the Faculty of Music of the University of Western Ontario in 1979 and was acting chairman of the faculty's history department. His areas of specialization are the theory and practice of French baroque music; the history of woodwind instruments; music and science; and 17th- and 18th-century dance and dance music. He was also an editor of Studies in Music from the University of Western Ontario.
- 'An early eighteenth century discussion of musical acoustics by Étienne Loulié,' CUMR, 2, 1981
- 'Method for learning how to play the recorder by Étienne Loulié,' The American Recorder, vol 24, Nov 1983
- 'The bassoons in Mersenne's Harmonie universelle,' J of the American Musical Instrument Society, vol 10, 1984
- 'Étienne Loulié and the new harmonic counterpoint,' J of Music Theory, vol 28, Spring 1984
- Sauveur's'Treatise on the Theory of Music:' A Study, Diplomatic Transcription and Annotated Translation (London, Ont 1988)
- 'Music and poetry in a chanson by Gilles Binchois,' Beyond the Moon: Festschrift Luther Dittmer, eds Bryan Gillingham and Paul Merkley (Ottawa 1990)