Having some insight into the design of our human hearing system will not only give us a good understanding of how people are able to tell which direction sound is coming from, but also aid in creating a realistic and natural sound system that takes human hearing into account.
Sound localization occurs on 2 planes : The lateral plane (left, ahead, right), and the medial plane (front, above, back, below). This means that different aspects of physics as well as the ear are at work in both planes to give our brains a picture of where a sound source is located.
In the lateral plane, we have 2 concepts to consider: i) Inter-aural time differences ii) Inter-aural intensity differences
i) Inter-aural time differences: This means that sound waves reach the left and right ears at slightly different times. This is simply because if a sound were to come from our right, it would reach the right ear before it’s reflections get to the left ear. The brain can use the delays in arrival to tell which direction the sound came from.
ii) Inter-aural intensity differences: Sounds originating from one side of the head will have a higher level or volume, due to head shadowing, which simply means that the human head is blocking the path to the other ear. So sounds on our head’s left side will reach the left ear directly, but will have to be reflected off surrounding surfaces into the right ear, thus arriving later, and losing energy as well as intensity as a result. The brain can thus tell that sound must have originated from our left.
High, low and medium frequencies tend to be partial to either one of these mechanisms more than the other. For frequencies below 800 Hz, mainly inter-aural time differences are used, whilst for frequencies above 1600 Hz mainly inter-aural level differences are used. This is because Between 800 Hz and 1600 Hz there is a transition zone, where both mechanisms play a role.
In the medial plane, the outer ear (pinna) is responsible for localizing sound. It does this through it’s unique shape. Being irregularly formed with ridges and bumps, the pinna acts a direction-sensitive filter, which, depending on the direction of the sound, causes a slight time delay in the direct and reflected sound that enter the ear.