The drone (bourdon) effect reﬂects the presence of a constant layer of stable pitch in a sound ensemble with no noticeable variation in intensity. Linked to music in its designation (the drone is a permanent bass note over which other elements are laid), the drone effect can also be observed in urban and industrial soundscapes. Many technical systems generate constant sounds that are close to a drone, even if the frequencies concerned are not limited to the bass range that originally characterized it.
Sonic Experience; Ed Jean-François Augoyard and Henry Torgue
McGill-Queen’s University Press (2005) (p.40-46)