the eardrum, relating to the acoustic sources. Then auditory nerves localize the sources using this frequency spectrum. Mathematical model These spectrum clues generated by the pinna filtering effect can be presented as a head-related transfer function (HRTF). The corresponding time domain expressions are called the Head-Related Impulse Response (HRIR). The HRTF is also described as the transfer function from the free field to a specific point in the ear canal. We usually recognize HRTFs as LTI systems: where L and R represent the left ear and right ear respectively, and represent the amplitude of the sound pressure at the entrances to the left and right ear canals, and is the amplitude of sound pressure at the center of the head coordinate when listener does not exist. In general, an HRTF's and are functions of source angular position , elevation angle , the distance between the source and the center of the head , the angular velocity and the equivalent dimension of the head . HRTF database At present, the main institutes that work on measuring HRTF database include CIPIC International Lab, MIT Media Lab, the Graduate School in Psychoacoustics at the University of Oldenburg, the Neurophysiology Lab at the University of Wisconsin–Madison and Ames Lab of NASA. Databases of HRIRs from humans with normal and impaired hearing and from animals are publicly available. Other cues for 3D space localization Monaural cues The human outer ear, i.e. the structures of the pinna and the external ear canal, form direction-selective filters. Depending on the sound input direction in the median plane, different filter resonances become active. These resonances implant direction-specific patterns into the frequency responses of the ears, which can be evaluated by the auditory system for vertical sound localization. Together with other direction-selective reflections at the head, shoulders and torso, they form the outer ear transfer functions. These patterns in the ear's frequency responses are highly individual, depending on the shape and size of the outer ear. If sound is presented through headphones, and has been recorded via another head with different-shaped outer ear surfaces, the directional patterns differ from the listener's own, and problems will appear when trying to evaluate directions in the median plane with these foreign ears. As a consequence, front–back permutations or inside-the-head-localization can appear when listening to dummy head recordings, or otherwise referred to as binaural recordings. It has been shown that human subjects can monaurally