It is a directed weapon, so the we don't know how much loss of dB is as a function of distance, within reason of course. It may be like a laser beam at short distances, before atmospheric interference forces more normal sound behavior.
Here's a better story on the weapon.
http://washingtontimes.com/national/20040307-120634-6220r.htm
DK
I doesn't matter in this case. It is not like collimated laser light.
The beam width has to expand with distance - it is air pressure waves. So double the distance, double the beam width (at best). Hence 1/4 the pressure density at double the distance (or 6 dB of attenuation).
Notice the "300 yards or less" statement in the link you posted in order for it to be affective?
That's the reason why.
...silent noise.