## 7 Conclusions  
Distance perception cues include level, spectral, curved wavefront, HRTF, and direct to reverberation energy ratio (reflections) cues. The level and spectral cues are relative, and the spectral cues are weak. The curve wavefront and HRTF cues are absolute cues for proximal source distances of less than 1.0 m in the lateral direction in a free field. The direct to reverberation energy ratio is an absolute cue and the most effective cue at a distance near the reverberation radius. Various distance perception cues depend on frequency, source distance and direction, and the acoustic environment in a complex manner. The interactions and redundancies of various cues are also complicated. These complexities have been under-researched in terms of the mechanism of distance perception in comparison with directional localization. To control the perceived auditory distance, ideal spatial audio reproduction is required to accurately create all major distance perception cues. However, practical spatial audio techniques are usually only able to recreate parts of the distance perception cues. Creating level and spectral cues in reproduction is relatively easy; accurately creating curve wavefront and HRTF cues, as well as the reflection cue, is relatively complicated and difficult, depending on the spatial audio principles and methods used. Moreover, reflections in the listening room may influence the accurate control of reflection-related distance perception cues. The three basic categories of principles and methods for spatial audio reproduction include the sound-field-based methods, sound field approximation and psychoacoustic-based methods, and binaural-based methods. Sound-field-based methods, such as Ambisonics and WFS, are theoretically able to reconstruct various target sound fields and thereby to recreate various spatial auditory perception cues, including distance perception cues. However, limited by the Shannon-Nyquist spatial sampling theorem, in practice, sound-field-based methods are only able to reconstruct a target sound field up to certain frequency limit, usually in the order of 2-3 kHz, or 4 kHz, at most. Psychoacoustic principles should be incorporated to recreate different distance perceptions. Moreover, sound-field-based methods may be able to compensate for the reflections in a listening room. Sound field approximation and psychoacoustic-based methods, including conventional stereophonic and multichannel sound, are unable to accurately create all spatial perception cues. They rely completely on the psychoacoustic principle to recreate various spatial auditory events and perceptions, including directional localization and distance perception. In contrast to the control of directional localization, the means for perceived distance control in stereophonic and various multichannel sounds are limited. They usually rely on the control of the direct to reverberation energy ratio in the reproduced signals. However, the reflections in a listening room may hinder such control. Binaural-based methods are theoretically able to create various spatial auditory cues. Schemes of perceived distance control in binaural-based methods are relatively mature. As a result, a VAD often serves as the experimental tool in psychoacoustic research related to spatial auditory perception. However, the in-head-localization in the headphone presentation must be eliminated and accordingly a dynamic VAD is needed. Near-field HRTFs are required to synthesize free-field virtual sources at proximal distances. Accurate acquisition of near-field HRTFs is relatively complicated; only a few near-field HRTF databases are available. The various approximate near-field HRTFs datasets are mainly valid for consumer applications. Simplifying signal processing in VADs may result in the loss of some distance perception information. Overall, in comparison with the relatively mature means for perceived direction control, perceived distance control in spatial audio remains a challenging task.