In the literature on extended audio images, only horizontal extent has been observed. Vertical extents will be necessary in order to build complex shapes. Perhaps the reason that vertical extent has not been observed is that only sine tones and broadband noise were used. Sine tones are extremely ambiguous in vertical localization. White noise, although good for localization, does not reside on the frequency continuum.
Complex tones are more localizable, and have fundamental frequencies.
The use of complex tones might enhance horizontal extent, and enable vertical extent. The apparatus and system I used are described in chapter 6. Since I would be asking my subjects to perform a task that is known to be quite difficult, and I would be using in this task, a virtual sound system that is not tailored to these subjects, I decided it would be necessary to fit my subjects to the system. I ran a screening experiment that weeded out the subjects to whom the system did not provide adequate spatial information. The frequency of these errors appear to vary greatly from person to person.