Does anybody lnow anything about the sound system used at the March on Washington?
Looking at various photographs of the event, it appears the main PA consisted of a pair of large re-entrant horns mounted on scaffold at around 12 to 15 foot high pointing straight across the Reflecting pool to the Washington Monument. Thread on compression driver re-entrant horns were made by Electro-Voice/University, RCA, and many other companies that looked nearly identical to those in the photo below.
A pair of large re-entrant horns each driven with around 30 watts, fairly typical for "high powered" drivers for that time, would be capable of around 130 dB at one meter.
Given the calm day and the Reflecting Pool avoiding ground absorption that pair of horns could produce around 52 dB at 2048 meters (1.26 miles).
Although 52 dB is a low conversational level, with an attentive audience the PA may have been clearly audible past the Washington Monument.
Other photos show what may be a pair of column speakers (similar to the Shure Vocal Master, though the Vocal master was not introduced until 1968) covering the area around the Lincoln Memorial.
Art Welter