Question

Localization of seen objects to some degree naturally falls out from representing the real world as an image on the retina, and subsequent retinotopic mapping. There is no direct analog for sound, and you we do a reasonable job of localizing sounds as well. How do we localize sounds? Your answer should include both monaural and binaural methods, different methods for high vs. low frequencies, and should mention the sorts of confusions (e.g. front-back, top-down) that we may make and why we might make them.

Answer 1

Sound localization is our ability to find direction of the sound from which it is originating. It is an important aspect of hearing as it helps safe movement around environment. It includes two aspects, the absolute localization and relative localization. Absolute localization is the ability to tell the direction the exact position of the sound source in a three dimensional space. Relative localization is the ability to find if there is shift from absolute position of the sound source.

The sound localization is basically depending on cues called monoaural and binaural cues. Monoaural cues needs the use of only one ear. This cues are captured by the head related transfer functions or HRTF which is the characterization of how an ear receives a sound from a source. This depends on the size and shape of the ears like shape of pinna and external ear canal, head, nasal cavities, oral cavities etc. that changes the way a sound is perceived by attenuating some sounds and boosting some others. This varies from person to person. All these characteristics act as direction-dependent frequency filters for the sound reaching the eardrum. Binaural cues sound information from two ears used. This depends on two types of cues called Interaural time difference or ITD and Interaural level difference ILD. The localization of sound works when the brain compares the time taken for a sound to reach the left and the right ears. This is called Interaural time difference or ITD. The difference in the loudness and frequency distribution between the sound information of two ears is compared to cause the ILD.

The sound localization of low frequency sounds depends on the time phase at the two ears or ITD and the high frequency sound localization depends on ILD. The distance between the two ears, frequency of sound and the azimuthal position of the sound depends on the ITD. For low frequency sounds below 800Hz our auditory system can localize the sounds by determining the phase delays between both the ears as the length of sound waves are larger than dimensions of head .As the frequency goes below 80 HZ it is difficult to determine the phase difference to evaluate the sound. For high frequencies above 1600 Hz the length of sound waves gets smaller and dimensions of head are larger so ITD cues are not taken to localize sounds. Instead a greater value of ILD is evaluated by human auditory system to localize the sound. Head creates an acoustic shadow for the sound waves and more prominent in high frequency sounds.

There is region called cone of confusion where all the sounds produce same ITD and ILD. This may cause confusions in sound localization. Sounds that lies in the cones of confusion can produce same diffrences in cues . One such cone is in midsagittal plane. The front back confusion makes it difficult to determine whether the sound is originating from the front or back. The sources of sound which is located at positions in the cone like above right, in front and to the right, behind and to the right, closer or further cannot obtain differences in value of ITD and ILD.