Final answer:
The duplex theory refers to the use of both time differences (Interaural Time Differences, ITDs) and level differences (Interaural Level Differences, ILDs) in sound localization.
Step-by-step explanation:
According to the duplex theory of sound localization, which is often associated with the way humans perceive the location of sounds, we use both time differences and level differences (intensity differences) to locate sounds in space.
These two types of differences are referred to as Interaural Time Differences (ITD) and Interaural Level Differences (ILD), respectively. ITDs are the differences in the time it takes for a sound to reach each ear, and ILDs are the differences in the sound pressure level reaching each ear.
The duplex theory explains how humans use these differences to localize sound sources in the horizontal plane.