Which is the lowest frequency a human ear can hear?

The human ear is incapable of detecting sound frequencies lower than 20 vibrations per second, or $20 \mathrm{Hz}$.

Humans can normally detect sounds in the range $20-20,000 \mathrm{Hz}$ , and it is well known that sounds in this range can cause hearing damage.

However, sounds below the frequency of $20 \mathrm{Hz}$ can also affect our ears even if we cannot hear them.
A young ontologically healthy person in the workplace can detect a signal sound in the frequency range of 20 to $20000 \mathrm{Hz}$, but the level of the signal sound required for detection is dependent on variables such as the frequency of the sound, the duration of the sound, and the nature of any other sound present at or near the same time as the signal sound that may mask the signal sound.