Another obvious way to localize sounds is to have a separate channel
for every desired direction, including above and below, if wanted. This
is basically what is done with theater systems, such as Dolby Pro Logic
Surround Sound. In typically reverberant environments, one can take advantage
of the Franssen effect and
use small loudspeakers everywhere, except for one large speaker (the "subwoofer")
that provides the nondirectional, low-frequency content.
Although they produce impressive spatial effects, multichannel systems are
obviously expensive and inconvenient, and they are unlikely to play a major
role in HCI.
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