When an acoustic wave travels through a medium, its intensity decreases exponentially with the distance travelled.

P_d = P_oe^-ad/2

where P_d is the pressure at a point d away from the source P_o, and a is the absorbtion coefficient.
 
(For example:  at 50 kHz, a sound emitted at about 110dB can travel about 30 meters before becoming absorbed)
 

We can stand to lose about 60dB, and still receive our signal.
If we want to send a signal 20 meters, we cannot go above 100 kHz.
If we want to send a signal 10 meters, we cannot go above about 200 kHz.
Distances shorter than that start to get "not useful" for LAN's, so let's say that our upper frequency limit is around 200 kHz.
 

Humidity and temperature play a big role in the attenuation constant.