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. 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 phase differences) and interaural level differences play a role for the hearing of many animals. But the influences on localization of these effects are dependent on head sizes, ear distances, the ear positions and the orientation of the ears. Smaller animals like insects use different techniques as the separation of the ears are too small. Lateral information (left, ahead, right) If the ears are located at the side of the head, similar lateral localization cues as for the human auditory