## 3 General Consideration of Perceived Distance Control in Spatial Audio Reproduction  
Existing spatial audio techniques can be classified into the following three types according to their principles and methods [1]:  
(1) Sound field-based methods  
Sound field-based methods aim to reconstruct an original or target sound field as exactly as possible in an appropriate area. Listeners acquire the target spatial information and corresponding auditory perceptions as they enter the reproduced sound fields. High-order ambisonics and wave field synthesis are two examples of these methods [31]. (2) Sound field approximation and psychoacoustic-based methods  
Under certain conditions, two physically different sound fields may produce similar spatial auditory perceptions. Psychoacoustic-based methods aim to recreate target auditory perceptions. The sound field reproduced by this method is physically different from the original or target sound field. It is, at most, a rough approximation of the target sound field. With an appropriate number of channels and loudspeaker configuration, this method uses appropriate psychoacoustic principles to recreate auditory perceptions similar to those of the target sound fields. In other words, these methods aim to deceive our hearing [29]. Examples of this method include amplitude or time panning in stereophonic and multichannel sound [32], and the first order Ambsonic signal mixing with psychoacoustic optimization [27]. (3) Binaural-based methods  
Binaural pressures or signals embody the auditory information in any complex sound field. Binaural-based methods aim to exactly duplicate and render binaural signals, then to create the desired auditory perceptions. Examples of binaural-based method include binaural recording and reproduction, and virtual auditory displays [7,33]. The first and second types of methods are generally intended for loudspeaker reproduction, and the third type of method is generally intended for headphone presentation. However, after appropriate signal processing, such as HRTF-based virtual loudspeaker synthesis or transaural processing [33,34], the spatial audio signal for loudspeaker reproduction can be converted to signals for headphone presentation, and vice versa. As stated, the major distance perception cues are the level cue, spectral cue, HRTF cue, and direct to reverberation energy ratio (reflection cue). An intuitive method to control perceived distance is to recreate all desired distance perception cues and information in spatial audio reproduction. However, this method is often infeasible. The level and spectral cues can be easily controlled and recreated using various reproduction methods. The former is formed by controlling the overall gain in the signals, and the latter can be implemented by an appropriate low-pass filtering of the signals. However, both level and spectral cues are relative rather than absolute cues of auditory distance perception. The practical effects of controlling these two cues only are limited. Effective methods for perceived distance control in spatial audio strongly rely on effectively controlling HRTF and reflection cues. However, practical spatial audio reproduction is often able to recreate parts of these two cues to some extent, depending on the spatial audio system and the technique in use. In this situation, the knowledge of the interaction, cooperation, and redundancy of different distance cues is important, by which the perceived distance can be controlled by manipulating the major cue being reproduced. However, such knowledge is not complete due to the multiple-factor dependence and complicated interaction between different distance perception cues, which creates challenges for the perceived distance control in practical spatial audio reproduction. Therefore, perceived distance control is more complicated than perceived directional control in spatial audio. Reflections in the listening room may also spoil the distance perception information in spatial sound reproduction.