filtering effect theory. the shape of human pinna is concave with complex folds and asymmetrical both horizontally and vertically. reflected and direct waves generate a frequency spectrum on the eardrum, relating to the acoustic sources. then auditory nerves localize the sources using this frequency spectrum. hrir 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: h l = h l ( r , θ , φ , ω , α ) = p l ( r , θ , φ , ω , α ) / p 0 ( r , ω ) =h_(r,\theta ,\varphi ,\omega ,\alpha )=p_(r,\theta ,\varphi ,\omega ,\alpha )/p_(r,\omega )} h r = h r ( r , θ , φ , ω , α ) = p r ( r , θ , φ , ω , α ) / p 0 ( r , ω ) , =h_(r,\theta ,\varphi ,\omega ,\alpha )=p_(r,\theta ,\varphi ,\omega ,\alpha )/p_(r,\omega ),} where l and r represent the left ear and right ear respectively, p l and p r represent the amplitude of the sound pressure at the entrances to the left and right ear canals, and p 0 is the amplitude of sound pressure at the center of the head coordinate when listener does not exist. in general, an hrtf's h l and h r are functions of source angular position θ , elevation angle φ , the distance between the source and the center of the head r , the angular velocity ω and the equivalent dimension of the head α . 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 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 localize high frequency sound but not low frequency sound. binaural localization, however, was possible with lower frequencies. this is likely due to the pinna being small enough to only interact with sound waves of high frequency. it seems that people can only accurately localize the elevation of sounds that are complex and include frequencies above 7,000 hz, and a pinna must be present. when the head is stationary, the binaural cues for lateral sound localization (interaural time difference and interaural level difference) do not give information about the location of a sound in the median plane. identical itds and ilds can be produced by sounds at eye level or at any elevation, as long as the lateral direction is constant. however, if the head is rotated, the itd and ild change dynamically, and those changes are different for sounds at different elevations. for example, if an eye-level sound source is straight ahead and the head turns to the left, the sound becomes louder (and arrives sooner) at the right ear than at the left. but if the sound source is directly overhead, there will be no change in the itd and ild as the head turns. intermediate elevations will produce intermediate degrees of change, and if the presentation of binaural cues to the two ears during head movement is reversed, the sound will be heard behind the listener. hans wallach artificially altered a sound's binaural cues during movements of the head. although the sound was objectively placed at eye level, the dynamic changes to itd and ild as the head rotated were those that would be produced if the sound source had been elevated. in this situation, the sound was heard at the synthesized elevation. the fact that the sound sources objectively remained at eye level prevented monaural cues from specifying the elevation, showing that it was the dynamic change in the binaural cues during head movement that allowed the sound to be correctly localized in the vertical dimension. the head movements need not be actively produced; accurate vertical localization occurred in a similar setup when the head rotation was produced passively, by seating the blindfolded subject in a rotating chair. as long as the dynamic changes in binaural cues accompanied a perceived head rotation, the synthesized elevation was perceived. distance of the sound source the human auditory system has only limited possibilities to determine the distance of a sound source. in the close-up-range there are some indications for distance determination, such as extreme level differences (e.g. when whispering into one ear) or specific pinna (the visible part of the ear) resonances in the close-up range. the auditory system uses these clues to estimate the distance to a sound source: direct/ reflection ratio: in enclosed rooms, two types of sound are arriving at a listener: the direct sound arrives at the listener's ears without being reflected at a wall. reflected sound has been reflected at least one time at a wall before arriving at the listener. the ratio between direct sound and reflected sound can give an indication about the distance of the sound source. loudness: distant sound sources have a lower loudness than close ones. this aspect can be evaluated especially for well-known sound sources. sound spectrum: high frequencies are more quickly damped by the air than low frequencies. therefore, a distant sound source sounds more muffled than a close one, because the high frequencies are attenuated. for sound with a known spectrum (e.g. speech) the distance can be estimated roughly with the help of the perceived sound. itdg: the initial time delay gap describes the time difference between arrival of the direct wave and first strong reflection at the listener. nearby sources create a relatively large itdg, with the first reflections having a longer path to take, possibly many times longer. when the source is far away, the direct and the reflected sound waves have similar path lengths. movement: similar to the visual system there is also the phenomenon of motion parallax in acoustical perception. for a moving listener nearby sound sources are passing faster than distant sound sources. level difference: very close sound sources cause a different level between the ears. signal processing sound processing of the human auditory system is performed in so-called critical bands. the hearing range is segmented into 24 critical bands, each with a width of 1 bark or 100 mel. for a directional analysis the signals inside the critical band are analyzed together. the auditory system can extract the sound of a desired sound source out of interfering noise. this allows the listener to concentrate on only one speaker if other speakers are also talking (the cocktail party effect). with the help of the cocktail party effect sound from interfering directions is perceived attenuated compared to the sound from the desired direction. the auditory system can increase the signal-to-noise ratio by up to 15 db, which means that interfering sound is perceived to be attenuated to half (or less) of its actual loudness. in enclosed rooms not only the direct sound from a sound source is arriving at the listener's ears, but also sound which has been reflected at the walls. the auditory system analyses only the direct sound, which is arriving first, for sound localization, but not the reflected sound, which is arriving later (law of the first wave front). so sound localization remains possible even in an echoic environment. this echo cancellation occurs in the dorsal nucleus of the lateral lemniscus (dnll). in order to determine the time periods, where the direct sound prevails and which can be used for directional evaluation, the auditory system analyzes loudness changes in different critical bands and also the stability of the perceived direction. if there is a strong attack of the loudness in several critical bands and if the perceived direction is stable, this attack is in all probability caused by the direct sound of a sound source, which is entering newly or which is changing its signal characteristics. this short time period is used by the auditory system for directional and loudness analysis of this sound. when reflections arrive a little bit later, they do not enhance the loudness inside the critical bands in such a strong way, but the directional cues become unstable, because there is a mix of sound of several reflection directions. as a result, no new directional analysis is triggered by the auditory system. this first detected direction from the direct sound is taken as the found sound source direction, until other strong loudness attacks, combined with stable directional information, indicate that a new directional analysis is possible. (see franssen effect) specific techniques with applications auditory transmission stereo system this kind of sound localization technique provides us the real virtual stereo system. it utilizes "smart" manikins, such as kemar, to glean signals or use dsp methods to simulate the transmission process from sources to ears. after amplifying, recording and transmitting, the two channels of received signals will be reproduced through earphones or speakers. this localization approach uses electroacoustic methods to obtain the spatial information of the original sound field by transferring the listener's auditory apparatus to the original sound field. the most considerable advantages of it would be that its acoustic images are lively and natural. also, it only needs two independent transmitted signals to reproduce the acoustic image of a 3d system. sound localization with manikin 3d para-virtualization stereo system the representatives of this kind of system are srs audio sandbox, spatializer audio lab and qsound qxpander. they use hrtf to simulate the received acoustic signals at the ears from different directions with common binary-channel stereo reproduction. therefore, they can simulate reflected sound waves and improve subjective sense of space and envelopment. since they are para-virtualization stereo systems, the major goal of them is to simulate stereo sound information. traditional stereo systems use sensors that are quite different from human ears. although those sensors can receive the acoustic information from different directions, they do not have the same frequency response of human auditory system. therefore, when binary-channel mode is applied, human auditory systems still cannot feel the 3d sound effect field. however, the 3d para-virtualization stereo system overcome such disadvantages. it uses hrtf principles to glean acoustic information from the original sound field then produce a lively 3d sound field through common earphones or speakers. multichannel stereo virtual reproduction since the multichannel stereo systems require many reproduction channels, some researchers adopted the hrtf simulation technologies to reduce the number of reproduction channels. they use only two speakers to simulate multiple speakers in a multichannel system. this process is called as virtual reproduction. essentially, such approach uses both interaural difference principle and pinna filtering effect theory. unfortunately, this kind of approach cannot perfectly substitute the traditional multichannel stereo system, such as 5.1/7.1 surround sound system. that is because when the listening zone is relatively larger, simulation reproduction through hrtfs may cause invert acoustic images at symmetric positions. animals since most animals have two ears, many of the effects of the human auditory system can also be found in other animals. therefore, interaural time differences (interaural