I am trying to determine why diesel locomotives have always used horns (or seemingly so) while steam always had whistles. No theories, just real answers would be appriciated.
Simple Whistles are operated by steam pressure blowing throught the whistle, Diesel electrics have no steam, thus the electric horn replaced the steam whistle
Horns are actually air-operated. Air is piped into the horn, through a sprung diaphragm and out the bell. The vibration results in the noise, the tone being set by diameter and length of bell (and dia of diaphragm?). Loco horns have 2-5 bells, typically.
Air horns were not uncommon on steam locomotives, e.g., SP’s, incl. GS-4 4449. Whistles often leaked steam by and were more maintenance intensive. The horn, by comparison, is as loud or louder and aimable (sound is projected in path of travel).
Whistles can operate with compressed air, but need a larger volume of air than a horn. Some US roads did use an “air whistle”, made by Hancock (New Haven, for one). At the risk of upsetting someone out there… it sounds pretty pathetic.