Where Does the Sound Come From? – Alan Palmer (p.29)

Although the pioneer of auditory experiment and theory, von Helmholtz, didn’t have much to say about binaural processing of sound, the study of binaural hearing has a long and distinguished history. One of the earliest and most influential investigators was Lord Rayleigh who, with the help of Lady Rayleigh, an assistant, several tuning forks and various lengths of tube, formulated the duplex theory of binaural hearing, (Rayleigh 1907). The essence of the theory is as follows: the effect of head shadowing, in reducing the sound level at the ear furthest from a sound source, is used to localize sources above about 1500 Hz, while the location of lower-frequency sources is computed from the difference between the phases of the sound at the two ears resulting from the longer path length to the remote ear.