# Auditory Localization in Low-Bitrate Compressed Ambisonic Scenes
## 3 Methods
### Spatial Audio Rendering  
Evaluation was conducted using multi-loudspeaker and dynamic binaural rendering methods inside an acoustically treated room. The loudspeaker reproduction was done using a 50-channel full-sphere array based on the Lebedev quadrature [41], see Figure 4. The sound pressure level (SPL) at the center of the array was aligned for each individual loudspeaker using an automated SPL calibration script. The reproduction SPL in the center of the array was set to 65 dBA. The complex-scene stimuli levels were aligned subjectively to match the simple scene test loudness. The rendering of Ambisonic scenes was done using three different loudspeaker configurations: octahedron, 26-point Lebedev grid, and 50-point Lebedev grid, as optimal configurations for the 1 st, 3 rd and 5 th-order Ambisonics signals respectively [41]. Dual-band decoding was implemented by pre-filtering the Ambisonic input with a set of shelf filters ([[https://github.com/resonance-audio/resonance-audio/tree/master/matlab/ambisonics/shelf_filters](https://github.com/resonance-audio/resonance-audio/tree/master/matlab/ambisonics/shelf_filters)]([https://github.com/resonance-audio/resonance-audio/tree/master/matlab/ambisonics/shelf_filters](https://github.com/resonance-audio/resonance-audio/tree/master/matlab/ambisonics/shelf_filters))) and applying Max-Re correction weightings to the high-passed signals before feeding the decoder. The AmbiX ([[https://matthiaskronlachner.com/?p=2015](https://matthiaskronlachner.com/?p=2015)]([https://matthiaskronlachner.com/?p=2015](https://matthiaskronlachner.com/?p=2015))) Ambisonic decoder configuration files were obtained from the SADIE II database ([[https://york.ac.uk/sadie-project/ambidec.html](https://york.ac.uk/sadie-project/ambidec.html)]([https://york.ac.uk/sadie-project/ambidec.html](https://york.ac.uk/sadie-project/ambidec.html))). Listening test software for loudspeaker presentation was created using the visual audio programming environment Max ([[https://cycling74.com/products/max](https://cycling74.com/products/max)]([https://cycling74.com/products/max](https://cycling74.com/products/max))). To create binaural signals, loudspeaker feeds were convolved in real time with diffuse-field equalized HRTF sets obtained from the SADIE II database. Individual and generic HRTF sets were used. The individual HRTF-based evaluation required a participation of subjects who took part in the database creation. The generic HRTF evaluation was done using an HRTF set obtained \begin{table}
\begin{tabular}{l c c c c c} \hline
**Direction** & **1** & **2** & **3** & **4** & **5** & **6** \\ \hline Azimuth (\({}^{\circ}\)) & 0 & 180 & 72 & 324 & 216 & 108 \\ Elevation (\({}^{\circ}\)) & 90 & \(-\)18 & 18 & \(-\)18 & 18 & \(-\)18 \\ \hline \end{tabular}
\end{table} 
Table 1: Reference sound source directions during the localization performance test. \begin{table}
\begin{tabular}{l c c c c} \hline  & \multicolumn{2}{c}{**Total Bitrate**} \\  & **Bitrate per Channel** & **1 OA** & **3 OA** & **SOA** \\ \hline Compressed & 16 & 64 & 256 & 576 \\ Compressed & 32 & 128 & 512 & 1152 \\ Compressed & 64 & 256 & 1024 & 2304 \\ Uncompressed & 768 & & & 27,648 \\ \hline \end{tabular}
\end{table} 
Table 2: Investigated bitrates (kbps) at different Ambisonic orders. using Neumann KU100 binaural microphone. Sennheiser HD 650 headphones were used for the binaural tests due to the low variability in frequency response between coupling and decoupling of the headphones with the ears [42]. Frequency response of the headphones was compensated using inverse filters based on responses measured with the KU100 dummy-head. Binaural reproduction level was adjusted to match the loudspeaker reproduction level through calibration with a KU100 head. An Optitrack optical motion tracking system ([[https://optitrack.com](https://optitrack.com)]([https://optitrack.com](https://optitrack.com))) using six Flex-3 infrared cameras and reflective markers attached to the headphone headband was used for dynamic binaural rendering.