64

The head-related transfer function (HRTF) is an auditory function that gives information about elevation. This function is monaural, meaning it takes place in each individual ear. By using information from both ears, one should be able to roughly identify the elevation of an external auditory cue. The speeds and intensities of the auditory cue are compared and contrasted at each ear, giving enough information for the brain to perceive the sound at the correct spot.

Each of us has a customized HRTF because of the shape of our pinnae: different frequencies are attenuated differently at different elevations. Thus, the HRTF provides information about elevation. It’s a complicated function and one that we’ve learned after years of living in our heads. A study by Hofman et al. (1998) changed the shape of subjects pinnae and discovered:

1. Subjects lost the ability to determine the elevation of sounds when the first received some fake pinnae.
2. Subjects learned a new HRTF after a few weeks.
3. Subjects still had their old HRTF (were immediately accurately determining elevation) when the artificial pinnae were removed.

There’s a thing called the cone of confusion, which is the cone-shaped region pointing out from the side of your head in which ITD and ILD are the same for all locations. This often leads to front/back confusions as well as left-right confusions (although less often). Elevation information provided by the HRTF is needed to break this up. Head movements have also been shown to help break up the cone of confusion.