Sound Cues and Virtual Surround Sound
Most people have had the experience of sitting in a very quiet room, like a classroom during a test, and having the silence broken by an unexpected noise, like change falling from someone's pocket. Usually, people immediately turn their heads toward the source of the sound. Turning toward the sound seems almost instinctive -- in an instant, your brain determines the sound's location. This is often true even if you can only hear in one ear.
A person's ability to pinpoint a sound's location comes from the brain's analysis of the sound's attributes. One attribute has to do with the difference between the sound that your right ear hears and the sound that your left ear hears. Another has to do with the interactions between the sound waves and your head and body. Together, these are the aural cues that the brain uses to figure out where a sound came from.
Imagine that the coins in our quiet classroom example hit the floor somewhere to your right. Because the sound travels as physical waves through the air -- a process that takes time -- it reaches your right ear a fraction of a second before it reaches your left. In addition, the sound is a little quieter by the time it reached your left ear. This reduction in volume is because of the natural dissipation of the sound wave and because your head absorbs and reflects a little bit of the sound. The difference in volume between your left and right ears is the interaural level difference (ILD). The delay is the interaural time difference (ITD).
Time and level differences give your brain a clear idea of whether a sound came from your left or your right. However, these differences carry less information about whether the sound came from above you or below you. This is because changing the elevation of a sound affects the path it takes to reach your ear, but it doesn't affect the difference between what you hear in your left and right ears. In addition, it can be hard to figure out whether a sound is coming from in front of you or behind you if you're only relying on time and level differences. This is because, in some cases, these sounds can produce identical ILDs and ITDs. Even though the sounds are coming from a different location, the differences in what your ears hear are still the same. The ILDs and ITDs are identical in a cone-shaped area extending outward from your ear known as the cone of confusion.
ILDs and ITDs require people to be able to hear in both ears, but people who cannot hear in one ear can still often determine the source of sound. This is because the brain can use the sound's reflection off of the surfaces in one ear to try to localize the sound's source.
When a sound wave reaches a person's body, it reflects off of the person's head and shoulders. It also reflects off of the curved surface of the person's outer ear. Each of these reflections makes subtle changes to the sound wave. The reflecting waves interfere with one another, causing parts of the wave to get bigger or smaller, changing the sound's volume or quality. These changes are known as head-related transfer functions (HRTFs). Unlike with ILDs and ITDs, the sound's elevation, or the angle at which it hits your ears from above or below, affects its reflections of the surfaces of the body. The reflections are also different depending on whether the sound comes from in front of or behind your body.
HRTFs have a subtle but complex effect on the shape of the wave. The brain interprets these differences in the wave's shape, using them to find the sound's origin. We'll look at how researchers have studied this phenomenon and used it to create virtual surround-sound systems in the next section.