structural:summarization, information:highlighting
Pinna filtering effect

Duplex theory shows that ITD and IID play significant roles in sound localization, but they can only deal with lateral localization problems. For example, if two acoustic sources are placed symmetrically at the front and back of the right side of the human head, they will generate equal ITDs and IIDs, in what is called the cone model effect. However, human ears can still distinguish between these sources. Besides that, in natural sense of hearing, one ear alone, without any ITD or IID, can distinguish between them with high accuracy. Due to the disadvantages of duplex theory, researchers proposed the pinna filtering effect theory. The shape of the human pinna is concave with complex folds and asymmetrical both horizontally and vertically. Reflected and direct waves generate a frequency spectrum on the eardrum, relating to the acoustic sources. Then auditory nerves localize the sources using this frequency spectrum.