Hearing is one of the most amazing things in the world (just my opinion, but I think it’s probably better than sliced bread). The human ear can hear improbably quiet sounds, but hearing can equally be destroyed by loud noises (jet engine by your ear, shotgun going off near your head (assuming that it wasn’t actually aimed at your head)).
One of the cleverest things about hearing is the ability to calculate to an incredible accuracy the location of a particular noise. How does the brain do this?
It’s actually a combination of a number of things. Two of which are the interaural time difference (ITD) and the interaural intensity difference (IID). As the names suggest, these two are measures of the difference in time between when each ear hears the sound and the intensity difference of the sound arriving at the ear. How can that be? I hear you ask, the head is less than a foot across. We all know that sound intensity decreases in an inverse square relationship with the distance. So you would have thought that this would only have meaning for sounds very close. Equally, a sound source arriving at both ears only a few milliseconds apart will surely not have any effect on our perception of where the sound comes from.
However, both of this are contributing factors to how we perceive where sounds originate. The other is the head related transfer function (HRTF). This is something which scientists have ‘invented’ to describe how the brain interprets sounds as they arrive at the ear. It is important in determining location due to the way the individual facial and head characteristics warp the sound before it arrives at the ear. The HRTF cannot be solved, but must be measured using dummies and so on. And yet, the brain can calculate the HRTF, the ITD and the IID within a split-second to tell you where a sound came from.
Amazing huh!
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