I remembered hearing that on QI, which was enough to turn up this[0]:
> "The panel are shown a photo of a man and are asked how his bare bottom helped Britain win World War One. The man is William Lawrence Bragg, a physicist and the youngest ever Nobel laureate. In 1915, he was in Flanders trying to figure out a way of using sound to locate enemy artillery. One day, he was sitting on the latrine in the house where he was billeted, and noticed that when there was nearby gunfire his backside momentarily lifted off the seat, even if he didn't hear the explosion. Another physicist he was working with, William Tucker, was billeted in a tar paper hut, and noticed by his cot that there some little holes, and even on windless days little puffs of air were blowing through. The two compared notes, and concluded it was due to inaudible, low-frequency sounds of artillery. They thus began to build detectors, and by 1917 the Allies had really devastating advantage when locating and targeting enemy guns."
Unfortunately, I think it's probably made up; I couldn't find a solid source for the story, which seems unlikely if it were true (given that it's a great story, Bragg was decently well known at the time, and 1915 is fairly recent [by "sources disappearing in the sands of time" standards, at least])
Edit: although I suppose the story would've been highly classified for years, so maybe it's plausible that there would be no solid sources.
[0] https://www.comedy.co.uk/tv/qi/episodes/15/14