Spatial Sound for Computer Games and Virtual Reality - Game Sound Technology and Player Interaction: Concepts and Developments - page 291

Figure 3. a) Sound-wave arriving from the right. b) Sound-wave information reaches the right ear
11µsec before reaching the left. This corresponds to a sound-source detection precision of as little as 2º

2 kHz to 20 kHz), the techniques involving ITD
and its variant are not applicable. This is due to
the nature of high frequencies, where the periods
of each cycle are very short, meaning that many
cycles have occurred within the distance between
both ears. Therefore, when high continuous sounds
are presented to the listener, phase discrepancy
becomes unreliable. A very different analysis
procedure is required to determine the location
of continuous high-frequency sound source. To
this end, IID is employed.
IID is a technique employed by the auditory
system that describes the difference in intensity
levels between sound signals arriving at both
ears. In effect, this procedure takes into account
the interaction between the external sound-wave
and the listener's head and shoulders. As a solid
body, the head and shoulders will reflect and
absorb energy from a sound-wave as it travels
past. In essence, this means that a sound travel-
ing from the right will reach the right ear with a
particular intensity level, but reaches the left ear
with a lower intensity because of the head and
shoulder interference.
The combination of ITD and IID (known as
the Duplex Theory of sound localization) means
that the human hearing system is very efficient at

localizing sound on the horizontal plane. A very
different approach is believed to take place where
localization on the vertical plane is concerned.
The comparison between the inputs of a sig-
nal (time, phase or intensity) reaching both ears
is not effective when localizing sound on the
vertical plane. It is easy to comprehend why: A
signal coming from above or below the listener
is likely to reach both ears approximately at the
same time and the head and body shadows the
input of both ears almost equally. An alternative,
albeit technically more complicated, analysis is
used. The process entails the filtering of the sig-
nal before entering the auditory canal due to the
geometric features of the pinnae. (Refer to Figure
4 for a detailed view of a pinna). The folds of the
pinnae reflect certain frequencies of an incoming
signal and, as a sound source moves vertically,
the combined direct sound and reflected sound
changes dynamically. See Figure 4a showing
the structure of a pinna and 4b illustrating the
combination of direct and reflected signal paths
before entering the auditory canal.