Human hearing is most sensitive to sounds around 3-4 kHz.

Evidence

The sensitivity of hearing is best at about 3–4 kHz, and the threshold increases at low (below 250 Hz) and high frequencies (above 5 kHz). — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 978

Categorization

• SHARE: A classic fact for anyone who works with speech/audio engineering — hearing peaks near 3-4 kHz.
• XREF: connects to: Human hearing sensitivity peaks near 4 kHz and falls off at both extremes — David Howard, Jamie Angus, coustics and Psychoacoustics, 600