The duplex theory cannot account for the ability of subjects to localize sounds on the vertical median plane (directly in front of the listener), where interaural cues are minimal. Similarly, when subjects listen to stimuli over headphones, the sounds are perceived as being lateralized inside the head even though interaural temporal and intensity differences appropriate to an external source location are present. The results of many studies now suggest that these deficiencies of the duplex theory reflect the important contribution to localization of the direction-dependent filtering that occurs when incoming sound waves interact with the outer ears or pinnae and other body structures such as the shoulders and torso. The main cue the human auditory system uses to determine the elevation of a sound source is the monaural spectrum determined by the interaction of the sound with the pinnae 55Wightman, F. L., & Kistler, D. J. (1997). Monaural sound localization revisited. Journal of the Acoustical Society of America, 101(2), 1050–1063.. However, small head asymmetries may provide a weak binaural elevation cue. Specifically, there is a spectral notch that moves in frequency from approximately 5 kHz to 10 kHz as the source moves from 0° (directly ahead of listener) to 90° (above the listener’s head) that is considered to be the main elevation cue 66Musicant, A. D., & Butler, R. A. (1985). Influence of monaural spectral cues on binaural localization. Journal of the Acoustical Society of America, 77(1), 202–208. , 77Moore, B. C. J., Oldfield, S. R., & Dooley, G. (1989). Detection and discrimination of spectral peaks and notches at 1 and 8 kHz. Journal of the Acoustical Society of America, 85, 820–836.. As sound propagates from a source to a listener’s ears, reflection and refraction effects tend to alter the sound in subtle ways and the effect depends on frequency. For example, for a particular location, a group of high frequency components centered at 8 kHz may be attenuated more than a different band of components centered at 6 kHz. Such frequency dependent effects or filtering also vary greatly with the direction of the sound source. Thus, for a different source location, the band at 6 kHz may be more attenuated than the higher frequency band at 8 kHz. It is clear that listeners use these kinds of frequency-dependent effects to discriminate one location from another. Experiments have shown that spectral shaping by the pinnae is highly direction-dependent, that the absence of pinna cues degrades localization accuracy, and that pinna cues are partially responsible for externalization or the “outside-the-head” sensation 88Gardner, M. B., & Gardner, R. S. (1973). Problem of localization in the median plane: Effect of pinnae cavity occlusion. Journal of the Acoustical Society of America, 53, 400–408. , 99Oldfield, S. R., & Parker, S. P. (1984a). Acuity of sound localization: A topography of auditory space: I: Normal hearing conditions. Perception, 13, 581–600. , 1010Plenge, G. (1974). On the differences between localization and lateralization. Journal of the Acoustical Society of America, 56, 944–951. , 1111Shaw, E. A. G. (1974). The external ear. In W. D. Keidel & W. D. Neff (eds.), Handbook of Sensory Physiology, Vol. 5/1, Auditory System (pp. 455–490). New York: SpringerVerlag..
Other monaural cues are provided by the ratio of direct-to-reverberant energy that expresses the amount of sound energy that reaches our ears directly from the source versus the amount that is reflected off the walls in enclosed spaces 1212Larsen, E., Iyer, N., Lansing, C. R., & Feng, A. S. (2008). On the minimum audible difference in direct-to- reverberant energy ratio. Journal of the Acoustical Society of America, 124(1), 450–461.. In general, monaural cues are more ambiguous spatial cues than binaural cues because the auditory system must make a priori assumptions about the acoustic features of the original sound in order to estimate the filtering effects corresponding to the monaural spatial cues.