How can we tell where a sound comes from? To completely localize a sound, we need to know three things: (1) angular location in the horizontal plane (0° is straight ahead, +90° is to the right, −90° is to the left), (2) angular location in the vertical plane (0° is straight ahead, +90° is directly above, −90° is directly below), and (3) distance. This section concentrates on angular location in the horizontal plane (called azimuth).
What cues do we have to horizontal location? Since we have two ears, we can compare the sound that each ear detects. See for an illustration of the binaural cues of interaural time difference (ITD) and interaural intensity difference (IID). This demonstration works best with headphones rather than speakers. Which cue (ITD or IID) seems to play the greater role in localization?
showed how ITDs arise and demonstrated that they are effective, but how does the brain measure those time differences? In 1948, Lloyd Jeffress proposed a way in which different ITDs could activate different neurons. gives a simple illustration of his model. Although widely accepted, there was no anatomical evidence for this model until 1988 when Catherine Carr and Mark Konishi reported such a circuit in the brainstem of owls (Carr and Konishi, 1988). However, it is not clear that this model applies to mammals (Campbell and King, 2004).
Of course, ITD and IID do not completely localize a sound. They give no cues to distance or vertical elevation, for example. With headphones, listen to B2.1 Localization cues again. As you change ITD, you may notice that the clicks seem to move back and forth inside your head. What is missing? When one localizes a sound in space (rather than with headphones), the shape of head and pinna (external ear) affect the spectral content of sound that reaches the ear.
Using empirically determined “head-related transfer functions,” sounds can be digitally altered to mimic the acoustic effects of the head and pinna to give a realistic impression of location in three-dimensional space. Listen to with headphones. Next, listen to . The latter, which uses the 3D simulation, gives a more realistic impression of a bumblebee flying around your head. , and give another example.
The 3D-simulated sounds presented here are reproduced courtesy of Sensaura Ltd. (www.sensaura.com), which developed this technique.