If we calculate the loss of sound pressure level as we go away from a loudspeaker using the inverse square law (-6 dB per doubling of distance), we will get a theoretical value that is valid for short distances but not for longer ones. This is due to air absorption. This is greater with dry air than it is with high humidity (water is a better conductor of sound than air). The curves evidence this behaviour at 30 m (100 ft).
Also, air absorption varies with frequency. It is well known that the very high frequencies are the first to go at long distances. Curves for different frequencies are therefore presented.
Related »» Air absorption calculator