Locomotive Air Horn Guide
A very complete guide available online and served as one of the sources of reference for this page: http://atsf.railfan.net/airhorns/index.php
Information Pages
Check the sections below for more information. This is an incomplete list, and more sections remain to be added.
Nathan/Airchime
Horns
Leslie Horns
Horn Basics
Terminology
The proper term is "horn", not "whistle". Whistles were used on steam engines, but on modern trains, a "whistle" is the sound created by blowing the horn.
Manufacturers
There are only two major airhorn manufacturers from which a variety of different horn models have been produced: Leslie and Nathan/Airchime. Horns aren't classified according to railroad or locomotive builder.
Valves
Older locomotives often have manual air valves where the engineer can control the amount of air going to the horn, which generally means softer-starting horn blasts. It also means an engineer can blow the horn at less than full volume. Many newer locomotives have automatic button-actuated valves that give the horn immediate full air pressure, with resulting immediate sound—no half-measures. The result is a difference in the character of the horn—compare the two recordings below of the same horn type (Nathan K5LA) with different valves.
SLR
RM-1 #805 (June, 06, 2005) - manual valve
LLPX
GP38-2 #2235 (June 08, 2001) - automatic valve
Pitch and Echoes
The Doppler effect causes a horn to sound higher-pitched that its actual sound when approaching, and lower-pitched when receding. However, the echo of a horn blast is not always affected by the Doppler effect. As a result, the echo will generally sound lower-pitched than the horn blast on an approaching train, and higher-pitched than the horn blast on a receding train.
CN
train (1993) - train approaching
LLPX
GP40 #3207 (August 26, 2002) - train receding