Monaural Hearing and Sound Localization :: Biology Biological Hearing Essays

Monaural Hearing and Sound Localization Human hearing and the ability to perceive the location of a sound source has long been accepted as a process requiring the use of two ears (Kistler, 1997; Butler & Humanski, 1992; Carlile, 1990). This process is referred to as binaural hearing. The subjective experience of binaural hearing during the location of a sound source was thought at first to be the result of an interactive process of evaluating two auditory cues (Kistler, 1997; Butler & Humanski, 1992; Carlile, 1990; Middlebrooks & Green, 1991). A man by the name of Lord Raleigh developed a "duplex theory" (Strutt, cited by Carlile, 1990) which stated that sound localization arises out of the fact that the ears are separated by both space and an acoustically opaque mass (the head) that creates two distinctive properties to incoming sounds. First, a sound originating outside the medial vertical plane will reach one ear before it reaches the other creating a time-of-arrival difference that can be detected and used in localizat ion. This process is referred to by Fuzessery, Wenstrup, and Pollak (1990) as an interaural time difference (ITD). Second, the mass of the head causes the incoming sound to lose intensity as it passes from one side of the head to the ear on the opposite side. Fuzessery, Wenstrup, and Pollak (1990) call this process an interaural intensity difference (IID), because the head acts as a muffler. The duplex theory survived until neuroanatomists and neurophysiologists began to search for the biological mechanisms of which the theory attempted to predict (Butler & Humanski, 1992). The duplex theory did prove to be, at least in part, accurate. In 1936 Stevens and Newman (cited by Butler & Humanski, 1992) proved empirically the existence of IIDs and ITDs in locating a sound source. However, they neglected to consider the possibility of other auditory cues that may provide additional localization information. The duplex theory assumed there were no other ways in which the perceptual location of a sound source could be obtained. It was not until much later that the role of the external structures of the ear, namely the pinnae, were considered. According to Butler and Humanski (1992), the role of the pinnae in localizing sound was only taken seriously when scientists began to study sound localization in situations where binaural differences were nonexistent. Some experiments were eventually performed using sound sources which lay directly on the medial vertical plane (referred to as elevation) and did not stray to either horizontal side (Butler & Humanski, 1992; Carlile, 1990; Wightman & Kistler, 1997).