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.
·Interaural intensity difference. A sound coming from the listener's right will sound louder in the right ear than in the left. This effect is familiar to anyone who has listened to a stereo sound system.
·Interaural time difference. 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 sound 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. Hardware optimized for 3-D sound can support other cues as well, such as the effect of the earlobes on the pitch and timing of sounds arriving from different directions. The mathematics behind this effect are known as the head-related transfer function.
One of the most important sound-positioning cues, however, is still 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 sense the position and velocity of the sound source.