Sounds are time-varying when their intensity changes over time

Evidence

The representation of the temporal features of sounds is a key component to auditory processing. Time is an extremely important aspect of both sound and its perception, as all sounds vary over time to some degree, and information in sounds unfolds over time. Note that because sound is a vibration, all sounds, by definition, are characterized by pressure changes occurring over time. However, if the overall intensity and frequency of a sound remain constant over time (e.g., a pure tone or white noise), the sound is considered a steady one. Sounds with changes in intensity over time are considered time-varying, and the limits on the perceptual ability to represent those variations is the focus of this chapter. — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 924

Categorization

• DEFINE: Defines steady versus time-varying sounds by whether intensity remains constant.