# Optimization of Virtual Loudspeakers for Spatial Room Acoustics Reproduction with Headphones
## 5 Discussion  
As the results show, the listening conditions drastically affect how well the differences in virtual loudspeaker positioning are noticed in binaural reproduction. The small room case appeared very susceptible to perceived image shifts and altered reverberance when the reproduction loudspeaker configuration was changed, whereas the changes in the setups were barely noticeable with the concert hall case.  
As visualized in Figure 3, perceived image shifts in the small room are explainable by the changes in the directions of early reflections. There are four strong reflections visible above the direct sounds, as well as reflections from the desk. The virtual loudspeakers of U14 that reproduce those directions are far from the reflections, whereas optimized setups have co-located loudspeakers for each of them. In U22, virtual loudspeakers are also off from those directions, but they are closer to them than in U14. This partly explains why U22 is as easy to discriminate from the optimized setups as U14 is. Visual inspection also suggests that O14 is a subset of O22, explaining why they are difficult to discriminate from each other. In short, strong early reflections affect the spatial image drastically, therefore requiring precise reproduction of their directions.  
A notable aspect in the concert hall case is that the directions of early reflections did not play as large role as they did in the small room. The direct sound reproduction loudspeakers were positioned identically between O16 and uniform setups, making discrimination between the setups hard. O24 had more loudspeakers optimized to reproduce direct sounds, thus being more distinctive than O16 when compared with uniform setups. Finally, U16 and U24 could probably have been discriminated from each other because of the combined effect of differences in the reproduction of side, ceiling and back wall reflections. To summarize, direct sounds dominate the spatial impression, allowing more drastic changes in reflection directions before the difference is heard.  
The small room case had only two sound sources with 60 degree separation. The reflections are then relatively scarce, therefore likely more audible and susceptible to changes in reproduction. The concert hall case in turn had 24 densely located sound sources, each having their own set of reflections and source signals. As the source positions are close to each other at the stage, most of the reflections are coming from adjacent directions. However, as these directions are not precisely the same, the reflections are spread over a larger area than in the two-channel case, therefore making the precise direction of the reflection fuzzier. However, more experiments are needed in different spaces to prove this theory.  
Another difference between the cases is the strength of the reverberation. Small room has only little if any diffuse reverberation, whereas the concert hall has prominently longer and more diffuse reverberation. Strong reverberant field may reduce the importance of correct spatial reproduction of earliest reflections. This psychoacoustic factor remains hypothetical and also needs more research.