Monaural Hearing and Sound Localization :: Biology Biological Hearing Essays

Monaural Hearing and Sound LocalizationHuman hearing and the ability to discern the location of a sound source has long been accepted as a help requiring the use of two ears (Kistler, 1997 butler & Humanski, 1992 Carlile, 1990). This process is referred to as two-channel 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 house theory (Strutt, cited by Carlile, 1990) which express 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 elect(postnominal) 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 fuel be detected and used in localization. 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 level 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 extra 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 subject 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).