The horn shown above and at left was at one time designated the A-125-L, indicating that it was designed to be blown on air, had a 125 mm diameter diaphragm, and had the "low pitch" bell that measured 19 and 1/4th inches in length. There would also be additional letters on the end of the designation indicating the materials from which the diaphragm and the horn itself were made.
In later years, the horn was redesignated the A-125-247, with the final number giving the horn's fundamental frequency in hertz. As before, a pair of letters following the last number indicated the diaphragm material (M for metal, P for plastic, or R for rubber faced) and the metal used to cast the horn (A for aluminum, B for bronze).
The A-125-247 was the most popular of the A-125 line, but other pitches with longer or shorter bells were available.
Factory tuning for the A-125-247 was: B
The A-125-247 was commonly used on early EMD switchers and sometimes also in tandem with the A-200-156 on road locomotives. As with the larger single chime "honkers," the A-125's fell out of favor with the advent of the multi-chime airhorn. Some may still be heard on older shortline and industrial locomotives and on museum engines.
A-125-247 sound samples:Courtesy Ron Chamberlain:
Sample 1...167kB...10 seconds