Although it changes a little for each frequency, the air pressure at 0dB is about .0002dynes/cm2
At 0 Bells (dB), a tone would equal the intensity that is accepted to be the standard normal threshold for the healthy adult ear.
Zero, in an arithmetic scale, would mean a total absence of sound.
But 0dB is a logarithmic scale and hence does NOT mean a total absence of sound. It represents the pressure of the sound at threshold, which for most frequencies is close to .0002 dynes/cm2 .
The dynes/cm2 unit is a more appropriate measure here than pounds/in2, because of the delicate nature of the eardrum and its sensitivity to very minute pressures.
If 0dB is a positive intensity level, then there must be smaller (lesser) intensities.
Hence, a person with a threshold of -10dB can hear a tone at 1/10 of the intensity level of a person with 0 dB threshold.