What we did at the recent Prosound Shootout was to move our speakers and microphone far away from obstructions in an anechoic environment, meaning it's reflection-free (because it's outdoors). We also measured at a distance that scales the path length of the horn down. It's hard to measure 1 meter away from a sound source that is 3 meters long but you can get 10 meters away and calculate back to the 1 meter level. That averages the difference and makes the best data.