Anechoic far-field measurements need at least two meters of distance

Evidence

At present there is widespread agreement that 2 m (6 ft) is a reasonable distance for making anechoic measurements to characterize the performance of conventional domestic and studio monitor loudspeakers. Large loudspeakers must be measured at greater distances, including any that involve adjacent boundaries in their function—the reflections are part of the source. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1271