## 6 Perceived Distance Control in Binaural-Based Methods  
Binaural-based methods create binaural signals in a target sound field by binaural synthesis processing or artificial head recording, and reproduce the resultant signalsby headphone [7]. The former is termed the virtual auditory display (VAD) or virtual auditory environment (VAE) technique and the latter is termed the binaural recording and reproduction technique. In a static VAD, the binaural signals of a free-field virtual source are synthesized by filtering the input stimulus with a pair of HRTFs at the target source positions or, equally, by convolving with a pair of head-related impulse responses (HRIRs), which are the time-domain counterparts of HRTFs and related to HRTFs by inverse Fourier transform. Moreover, according to the temporary position and orientation of a listener's head detected by a head tracker, a dynamic VAD updates the signal processing of binaural synthesis in real time and provides dynamic information for spatial auditory perception. The binaural signals in a reflective environment can also be synthesized by convoluting the input stimulus with a pair of binaural room impulse responses (BRIRs), which are the acoustic impulse responses from a source to two ears in a reflective environment. By careful equalization, the non-ideal transmission in the signal and reproduction chain (such as the headphone to ear canal transfer functions), the binaural-based methods are theoretically able to reconstruct binaural pressures in an arbitrary sound field and, therefore, to accurately recreate various spatial information, including directional localization and distance perception information. Moreover, it is free from disturbances caused by listening room reflections by using headphone presentation. Therefore, the schemes for perceived distance control in binaural-based methods, especially in VAD, are relatively mature. VAD also allows various perceived cues (including distance cues) in binaural signals to be manipulated individually; therefore, it is an effective experimental tool in psychoacoustic studies related to spatial auditory perception. Many psychoacoustic experiments related to auditory distance perception have been conducted on the basis of VAD [56]. However, there are still some problems with the perceived distance control in binaural-based methods, especially in VAD. (1) Problems with dynamic cues in binaural-based methods  
Inappropriate binaural reproduction by headphone is prone to creating in-head localization (intracranial lateralization), a psychoacoustic phenomenon in which an auditory event is perceived inside the head. In-head localization often occurs for free-field virtual sources with a frontal target direction in static VAD. The possible reasons for in-head localization include errors in the reconstructed binaural pressures at the eardrums, the absence of reflections in the simulation of the free-field virtual source, and omission of dynamic cue in a static VAD. Externalization of an auditory event is essential to perceived distance control with the binaural-based methods. Although dynamic cues caused by head turning may not contribute obviously to distance perception for a real sound source, it is important for the complete externalization of an auditory event in VAD, especially the externalization of a frontal free-field virtual source [57]. Therefore, in addition to careful equalization of non-ideal transmissions in the signal and reproduction chain, a dynamic VAD is essential for perceived distance control, at least in the cases of scientific research with VADs. Otherwise, the in-head-localization effect may spoil the experimental results. However, some previous experiments were based on static VADs. (2) Problems with near-field HRTFs  
Near-field HRTFs are essential for recreating virtual sources at different proximal distances in a VAD. HRTFs depend on individuals.