## 6 Perceived Distance Control in Binaural-Based Methods
Although psychoacoustic experiments indicated that individualized features in near-field HRTFs have little influence on the distance perception of a proximal virtual source [58], they may influence the directional localization in a VAD. Therefore, individualized near-field HRTFs are needed for accurate binaural synthesis in VADs, at least for most scientific research purposes. Measurement is a direct and accurate method to obtain near-field HRTFs. However, near-field HRTF measurement is relatively difficult [8]. The distance-dependent characteristics of near-field HRTFs require time-consuming measurements for various source directions and distances. In addition, a special point sound source in the near field is needed. [PERSON] et al. measured the near-field HRTFs of a KEMAR artificial head in sparse directions in their pioneer work on auditory distance perception [9]. However, only a few other groups measured the near-field HRTFs of artificial heads and set up a database in recent years [59; 60]. Using a special spherical dodecahedron sound source and the first-generation measurement system established by our group, [PERSON] et al. measured the near-field HRTFs for KEMAR with DB 60/61 small pinnae [61; 62]. The sound source and the scene of the first generation of near-field HRTF measurement system are shown in Figure 3. The resultant database included HRIRs with 10 source distances of 0.20, 0.25, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90, and 1.00 m, and 493 directions at each source distance. Using the second-generation system of our group (the fast measurement system), [PERSON] et al. further measured the near-field HRTFs of 56 human subjects with seven source distances and 685 directions at each distance, establishing the near-field HRTF database of human subjects [8]. The layout of the second generation of near-field HRTF measurement system with many loudspeakers can be found in Figure 4. The above two near-field HRTF databases are for scientific research purposes, and the KEMAR database is available from the SOFA website since 27 March 2020 ([[https://www.sofaconventions.org/](https://www.sofaconventions.org/)]([https://www.sofaconventions.org/](https://www.sofaconventions.org/))). Notably, in some work, the source of the near-field HRTF measurement cannot be considered a point source, which may cause error in the resultant data. Therefore, the resultant database \"should be considered as a valuable set for auralization purposes rather than as a basis for sensitive listening experiments\", as stated by the authors of the database [60]. Moreover, due to the enormous amounts of data produced by measurements, all the near-field HRTF data should be carefully checked prior to use for scientific purposes to determine if artifacts are presented in some data. Calculation is another method to obtain near-field HRTFs. A spherical head is a simplified model for near-field HRTF calculation [63]. Improved models consist of a spherical head and spherical torso (snowman model) [64], or spherical head, spherical torso, and cylindrical neck [65]. However, these models neglect the influences of head shape and pinnae and, thus, are only valid below 3 kHz. To improve the accuracy of calculation, the geometrical surfaces of the head and pinnae are acquired by a scanning device and the HRTFs are calculated by some numerical methods, such as the boundary element method (BEM). The BEM is able to calculate the near-field HRTFs up to a frequency of 20 kHz [66; 67]. Figure 3: Photos of the sound source and the scene of the first generation of near-field HRTF measurements system: (**a**) spherical dodecahedron sound source; (**b**) scene of measurements. Near-field HRTFs can also be estimated from measured far-field HRTFs. One method involves decomposing HRTFs in arbitrary directions and distances using the spherical Bessel and spherical harmonic functions [68; 69]. The weights of decomposition are evaluated from a set of measured far-field HRTFs.