Above 250 Hz modulation, frequency-modulated sounds behave as steady state.

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. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1217 (#expands, via linked claim)

Categorization

• XREF: connects to: Low-frequency noise fluctuates audibly while high-frequency noise is steady; Modulation frequency determines whether sound is heard as fluctuation, roughness, or multiple tones — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1217; Frequency-modulated tones lose fluctuation strength above 1 kHz centre frequency

Broader Connections

• Modulation frequency determines whether sound is heard as fluctuation, roughness, or multiple tones #expands (0.75) — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1217
• Frequency-modulated tones lose fluctuation strength above 1 kHz centre frequency #qualifies (0.75)