Slow sound-level changes are heard as fluctuation and fast ones as roughness

Evidence

If the level of sound fluctuates relatively slowly over a certain range of rates, between about 1 Hz and 16 Hz, a significant fluctuation strength may be perceived. When the modulation rate far exceeds 16 Hz, our hearing is unable to follow the level of sound, and the sound is perceived as rough, associated with the psychoacoustic quantity roughness. A fast-rising sound is perceived as impulsive, and the corresponding psychoacoustic quantity for this perception is impulsiveness. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1217

Categorization

• DEFINE: Defines fluctuation strength, roughness, and impulsiveness as distinct psychoacoustic quantities tied to modulation rate.