Perception of Sound Positions

In the real world, the perception of a sound's position in space is influenced by a number of factors, including the following:

• Volume. The farther an object is from the listener, the quieter it sounds. This phenomenon is known as rolloff.
• Arrival offset. A sound emitted by a source to the listener's right will arrive at the right ear slightly before it arrives at the left ear. (The duration of this offset is approximately a millisecond.)
• Muffling. The orientation of the ears ensures that sounds coming from behind the listener are slightly muffled compared with sounds coming from in front. In addition, if a sound is coming from the right, the sounds reaching the left ear will be muffled by the mass of the listener's head as well as by the orientation of the left ear.

Although these are not the only cues people use to discern the position of sound, they are the main ones, and they are the factors that have been implemented in the positioning system of DirectSound. When hardware that supports 3-D sound becomes generally available, other positioning cues might be incorporated into the system, including the difference in how high- and low-frequency sounds are muffled by the mass of the listener's head and the reflections of sound off the shoulders and earlobes.

One of the most important sound-positioning cues is the apparent visual position of the sound source. If a projectile appears as a dot in the distance and grows to the size of an intercontinental missile before it roars past the viewer's head, the listener does not need subtle acoustical cues in order to perceive that the sound has gone past.