That's exactly what it was. There's vents on the back of the speaking chamber of Diaphones which is why you see what puff of condensation. If conditions are right you can also see condensation shoot out of the projector. Here's Adam Smith's nicely restored Type B Diaphone (same as Wethersfield's) with a brass piston instead of the standard aluminum where you can see it puffing out both the rear vent and the projector (also note the lower pitch from the heavier brass piston).Rheems1 wrote:It looks like condensation that was built up being purged just prior to them sounding, if you have a humid day they will build up the condensation pretty well.
https://www.youtube.com/watch?v=GuOHKyuHikk
Also I was about 250 feet away, it took a few seconds for the sound to travel from the Diaphones to me (and the Barn Swallows sitting on the fence in front of me).