II. Note on General Duane’s soundless zones
John Tyndall · Proceedings of the Royal Society of London · 1883
Abstract In reference to one of the powerful fog-whistles established on the coast of Maine, General Duane rem arks as follows :—“The most perplexing difficulties, however, arise from the fact that the signal often appears to be surrounded by a belt varying in radius from 1 to 1½ mile from which the sound appears to be entirely absent. Thus, in moving directly from a station the sound is audible for the distance of a mile, is then lost for about the same distance, after which it is again distinctly heard for a long time. This action is common to all ear signals, and has been at times observed at all the stations, at one of which the signal is situated on a bare rock 20 miles from the mainland, with no surrounding objects to affect the sound.“