If the sound intenstiy level is measured relative to
thethreshold of heary, what does it mean for the intensity level to
benegative, e. g., -20 db?

Explanation:

The relation that is used to measure the intensity of sound is given by :

I is the intensity for a particular state

is the human threshold intensity

If the value of intensity level is negative, it means that even though some of the sound waves are present but we are not able to hear them. Hence, -20 dB shows that some of the sound waves are present at a point but we can't hear.