The Effects of Decreasing the Magnitude of Elevationdependent notches in HRTFs on Median Plane Localisation

Accurate sound localisation in the median plane is known to require certain spectral cues caused by the
pinnae and upper body; which provides important information about a sources location in space, with
pinna cues in particular producing considerably prominent peaks and troughs in the spectrum. Iida et al.
(2007) suggest that the first peak and the first two pinna related spectral notches (> 5 kHz) provide
enough information for the source to be accurately localised. The importance of the magnitude of
spectral peaks and notches in median plane localisation is thought to depend on the relative rather than
absolute distribution across the frequency spectrum (Macpherson and Sabin, 2013). The present
experiment tested three subjects and explored whether a necessary magnitude of pinna related spectral
notches exists, if so, what happens to the perceived source position once the magnitude is reduced. The
results generally showed that for elevated sources, as pinna related notches were reduced in magnitude,
the source position moved upwards in space. This seems to be due to the overall frequency spectrum
becoming closer to that of sources located higher in space. For a non-elevated frontal source in the
median plane on the other hand, an increase in front-back confusion occurred as a result of notch
magnitude manipulation. This again is considered to be due to the altered frequency distribution
mimicking that of the HRTF for the opposite direction (i.e., relative dominance of frequencies around 1
kHz over those around 4 kHz).