In my last post, I explained the importance of Interaural Time Differences (ITDs) and Interaural Intensity Differences (IIDs) in aiding our ability to accurately locate sounds. These time and level differences provide the brain with sufficient information to determine whether a sound came from the left or the right (Hofman and Van Opstal, 2002), however they are sometimes insufficient for the brain to deduce whether a sound originated from in front of, or behind the head. There can be circumstances where a sound emanating from the front produces exactly the same ITD and IID as if the same sound source was positioned behind the head. Additionally, changing the elevation of a sound source whilst maintaining its position on the horizontal plane does not necessarily affect the ITD, nor the
The Cone of Confusion
The area where ITDs and IIDs are identical regardless of sound source elevation or front-back position can be visualised as a cone-shape which radiates out from the ear, known as the Cone of Confusion. A Cone of Confusion exists at each ear. Any cross-section of these cones represents a set of locations where a sound source will produce identical phase delays and transient disparities, thus making the use of such binaural cues futile.
Cones of Confusion should be thought of as extending outwards from the ear indefinitely. Fortunately, there are mechanisms which can be used by the human auditory system to resolve confusion. These are:
- Head-Related Transfer Functions – Spectral changes caused by the natural filtering effects of head, torso, and pinnae (explained below in more detail). HRTFs have the most effect on higher frequencies.
- Head Movements – For sound waves of lower frequencies, head movements can be relied upon. Rotating the head changes interaural cues which in turn provide unambiguous information about the position of a sound source (Figure 5).
Head-Related Transfer Functions (HRTFs)
As stated above, HRTFs are relied upon for ascertaining the elevation of a sound source as well as whether the sound is coming from behind or from the front. The term HRTF relates to how the head, torso, and pinnae interfere with sound waves before they enter the auditory canal and reach the sensory structures within the middle and inner ear. Each of these external body parts modifies incoming sound waves, altering their frequency spectra and perceived volume through specific diffractions and reflections depending on the angle at which the sound waves strike them. These natural filtering effects produce subtle
colourations that are very important in allowing the auditory system to determine the origin of a sound.
HRTFs are specific to an individual. Every human possesses different shapes and sizes of head, torso and ear, the variation of which can be great. The structures of the pinnae are arguably the most important outer ear function associated with localisation. A pinna acts as an acoustic antenna, capturing and collecting sound. Its geometry is made up of unique skin folds and cartilage which cause certain frequencies to be attenuated, while resonant cavities, such as the concha, amplify other frequencies. Hofman and Van Opstal (2002) conducted studies which demonstrated the significance the role of the pinnae in sound localisation through running listening experiments involving the insertion of moulds into the pinnae of subjects in order to alter their shape and concurrently disrupt original spectral cues. Results suggested that the moulds hindered the ability of subjects to accurately identify the elevation of sound stimuli in both bilateral and unilateral test conditions.
Just how these binaural cues are incorporated and implemented in the creation of binaural recordings will be explained in next week’s post.
Hofman, M., Van Opstal, J. (2002) Binaural Weighting of Pinna Cues in Human Sound Localisation. Berlin: Springer-Verlag.
Plack, C. J. (2005) The Sense of Hearing. New York: Psychology Press.
Wenzel, E., Begault, D. R. (2012) The Role of Dynamic Information in Virtual Acoustic
Displays. Available from the NASA website: http://humansystems.arc.nasa.gov/groups/ACD/projects/dynamic_info.php.