A sound can only be heard by a normal human ear if its frequency falls between:

While the normal hearing range extends from 20 to $20,000 \mathrm{Hz}$, our hearing is most sensitive between $2000$ and $5000 \mathrm{Hz}$.
Humans can detect sounds with frequencies ranging from $20 \mathrm{Hz}$ to $20\mathrm{kHz}$.
Human infants can hear frequencies slightly higher than $20\mathrm{kHz}$, but as they mature, they lose some high-frequency sensitivity; the upper limit in average adults is often closer to $15-17\mathrm{kHz}$.