Pinna reflections 

Batteau first proposed the localization theory based on pinna reflections. Batteau suggested that the structure of the pinna caused multiple reflections of sound and the delay betwee the direct and the reflected sound varies with the direction of the sound source.

[ batteau-1967 ]  D. W. Batteau, The Role of the Pinna in Human Localization Proceedings of the Royal Society of London. Series B, Biological Sciences, Vol. 168, No. 1011. (Aug. 15, 1967), pp. 158-180.

 "The role of the pinna in localization is to introduce, by means of delay paths, a transformation of the incoming signal which is mentally inverted to provide attention, and that the inverse transform required defines the localization of the sound source."

One of the earliest papers to give  model for the pinna in terms of multiple reflections. There are multiple paths sound should travel before reaching the pinna. Because of the different path-length s there are multiple delays involved. The delays inroduced by the pinna were easliy seen to depend on the orientation of the sound source with respect to the pinna.

Hebrank and wright argue that the pinna notch is domiant cue for localization in the median vertical plane. The case of the pinna spectral notches is attributed to the reflection of the sound from the posterior concha wall. 

[ wright_jasa1974 ] Wright D, Hebrank JH, and Wilson B. Pinna reflections as cues for localization. J Acoust Soc Am 56: 957-962, 1974.

This paper presents experiments to determine whether delays caused due to pinna reflections are detectable by humans. The results show that delay times of of 20 usec are easily recognizable when the amplitude ratio of the dealyed  delayed signal to the leading signal is greater than 0.67. Just noticeable results agreed with the measurements of the minimum audible angle for monaural localization.

[ hebrank_jasa1974b ] Hebrank, J., Wright, D., Spectral Cues Used in the Location of Sound Sources on the Median Plane, Pages 1829 - 1834, J. Acoust. Soc. Amer., 56(6), 1974.

Sound spectra from 4 to 16 khz are necessary for localization. Specifically identified three cues by experiments. Frontal cues are a 1-octave notch betwee 4 to 10 kHz, above cue is a one-octave peak between 7-9 khz and behind cue is a peak between 10 to 12 kHz. Increases in frontal elevation are signalled by the increase in the lower cutoff frequency of the 1-octave notch. The notch appears to be generated by time-dealyed reflections off the posterior concha wall interfering with sound directly entering the external auditory canal. Alsp speculate that the association between auditory and spatial highness results from the frontal elevation cue.

This idea was further refined by Lopez Poveda who incorporated diffractionin the model and the predicted spectral notches agreed closely with the measured ones.

[ lopez_jasa1996 ] Lopez-Poveda, EA, and Meddis, R. (1996). "A physical model of sound diffraction and reflections in the human concha," J. Acoust. Soc. Am. 100, 3248-3259.

This paper proposes a physical model for the human concha based on diffraction and reflection. The sound wave is scattered within the concha cavity and therefore significant reflections must occur on an infinite number of points along the posterior wall for all source locations. The model predicts the elevation-dependent spectral features related to the transverse dimensions of the concha. Specifially nulls N1 and N3 agreed with their prediction. However the null N2 did not show up. This could be due to the crus helias. One thing to be noted from this paper is that a simple dealy model may not accrately predict the location of the nulls (due to diffraction).  Can explain the notches in the contrlateral side if we assume that the sound creeps around the head entering the contralateral concha at approximately the same elevation angle as if the source was in the ipsilateral hemisphere