Aural: world-fixed vs. user-fixed

Recall from Section 1.3 the trend of having to go somewhere for an experience, to having it in the home, and then finally to having it be completely portable. To understand these choices for VR systems and their implications on technology, it will be helpful to compare a simpler case: Audio or aural systems.

Figure 2.5: In a surround-sound system, the aural displays (speakers) are world-fixed while the user listens from the center.

Figure 2.6: Using headphones, the displays are user-fixed, unlike the case of a surround-sound system.

Figure 2.5 shows the speaker setup and listener location for a Dolby 7.1 Surround Sound theater system, which could be installed at a theater or a home family room. Seven speakers distributed around the room periphery generate most of the sound, while a subwoofer (the ``1'' of the ``7.1'') delivers the lowest frequency components. The aural displays are therefore world-fixed. Compare this to a listener wearing headphones, as shown in Figure 2.6. In this case, the aural displays are user-fixed. Hopefully, you have already experienced settings similar to these many times.

What are the key differences? In addition to the obvious portability of headphones, the following quickly come to mind:

• In the surround-sound system, the generated sound (or stimulus) is far away from the ears, whereas it is quite close for the headphones.
• One implication of the difference in distance is that much less power is needed for the headphones to generate an equivalent perceived loudness level compared with distant speakers.
• Another implication based on distance is the degree of privacy allowed by the wearer of headphones. A surround-sound system at high volume levels could generate a visit by angry neighbors.
• Wearing electronics on your head could be uncomfortable over long periods of time, causing a preference for surround sound over headphones.
• Several people can enjoy the same experience in a surround-sound system (although they cannot all sit in the optimal location). Using headphones, they would need to split the audio source across their individual headphones simultaneously.
• They are likely to have different costs, depending on the manufacturing difficulty and available component technology. At present, headphones are favored by costing much less than a set of surround-sound speakers (although one can spend a large amount of money on either).

All of these differences carry over to VR systems. This should not be too surprising because we could easily consider a pure audio experience to be a special kind of VR experience based on our definition from Section 1.1.

While listening to music, close your eyes and imagine you are at a live performance with the artists surrounding you. Where do you perceive the artists and their instruments to be located? Are they surrounding you, or do they seem to be in the middle of your head? Using headphones, it is most likely that they seem to be inside your head. In a surround-sound system, if recorded and displayed properly, the sounds should seem to be coming from their original locations well outside of your head. They probably seem constrained, however, into the horizontal plane that you are sitting in.

This shortcoming of headphones is not widely recognized at present, but nevertheless represents a problem that becomes much larger for VR systems that include visual displays. If you want to preserve your perception of where sounds are coming from, then headphones would need to take into account the configurations of your ears in space to adjust the output accordingly. For example, if you nod your head back and forth in a ``no'' gesture, then the sound being presented to each ear needs to be adjusted so that the simulated sound source is rotated in the opposite direction. In the surround-sound system, the speaker does not follow your head and therefore does not need to rotate. If the speaker rotates with your head, then a counter-rotation is needed to ``undo'' your head rotation so that the sound source location is perceived to be stationary.