## 2 Proposal of a Position-Dynamic Binaural Synthesis System
### BRIR Synthesis
#### 2.2.2 Acoustical Shaping  
Acoustic shaping describes an approach to adapt a BRIR by changing single distant dependent acoustic parameters. The aim is to reach perceptual suitability of the synthesized BRIRs regarding spatial auditory perception. The basic idea is to change the acoustic structure of the early reflections of measured BRIRs based in a shift of the initial time delay gap (ITDG). The ITDG is the time between the incoming direct sound and the first reflection. The ITDG is a distance-dependent acoustic parameter. The ITDG is small for distant sources and bigger for closer ones. Figure 5 shows this distance dependency of the ITDG if the first reflection is a ground reflection. The principle of filter shaping (122) based on a manipulation of the ITDG is shown in Figure 6 as a block diagram. The presented approach is based on the work from [PERSON] et al. [15], and it uses one measured data set of BRIRs at one position in the room. Depending on the new listening position and head orientation to be calculated, the BRIRs corresponding to the new yaw orientation are selected from the recorded data set. The BRIRs are split into a direct part, the early reflections, and the late reverberation. The transition point between early reflections and late reverberation is defined by the perceptual mixing time [31] of the auralized room. The mixing time defines a point in time of a BRIR after which its content is perceptually independent from the head pose or the position in the room. The beginning of the early reflections is defined by the choice of the time index which is \((0.1\cdot ITDG)\) before the first reflection. The samples within the defined area of early reflections are now rearranged in time regarding the distance dependency of the ITDG. Therefore, the time of the first reflection is changed depending on the distance between source and listening position. In the presented case, the ITDG change originates from a measurement in a TV studio at our university (TU Ilmenau), but it can be adapted for a specific room or geometric arrangement. If a BRIR for a closer distance than the recording position is needed, the samples before the first reflections are stretched in time while the samples up to the mixing time are compressed in a linear manner. If a greater distance is desired, it behaves in the opposite sense. The last adaptation is a change of the energy of the shaped BRIR depending on the new distance following the inverse-square law for energy distributions. Figure 5: Principle of the distance dependency of the initial time delay gap (ITDG), with \(d_{\text{Dx}}\dots\) distance of direct sound, \(d_{Rx}\dots\) distance of reflection, \(\Delta d_{x}\dots\) distance of the ITDG; (after [15]). Figure 7 shows the ITDG-approach exemplarily on a BRIR shaping for a measured BRIR at approximately 4.2 m and a new BRIR at approximately 1.6 m. For clarity of the figure and approach, the traveling time between source and listening position is shown for the new BRIR and the measured BRIR is shifted to this time in the figure. In a real scenario, BRIRs would also be shifted along the time axis. In the case shown, the measured BRIR would have a longer traveling time than the closer, new BRIR. Furthermore, no distant-dependent energy adaptation is shown. The area where the BRIR is manipulated is indicated as a box in the figure. It covers the first reflections up to the mixing time (endpoint). Within this range the first reflection (usually the ground reflection) is detected and the ITDG is determined. According to the new distance all samples are mapped to their new times using a linear compression and expansion characteristic curve within the range of the early reflection. The transition point of the two curves is the time of the first reflection. The first curve shifts all samples before the first reflection, the second shifts all samples after the first reflection until the mixing time. In terms of Figure 7, the samples are compressed before the first reflection and expanded afterwards.