Nathan Manufacturing, Inc. is a division of Micro Precision Group which makes Airchime, Ltd. train horns for North America. It is one of two major train horn manufacturers in the United States, Leslie Controls, Inc. being the other. Nathan horn manifolds are "stout and solid-looking", while Leslie's are "open and lacy." 
The K-series Airchime horn is sold to manufacturers of locomotives and to fleet operators.
- The K-1 single tone horn includes a die-cast aluminum bell and has frequencies ranging from 261 hertz to 660 hertz.
- The K-3L-CDF three-chime assembly includes three die-cast aluminum bells with the same frequency ranges, and they may be reversed. A shield to protect from snow and other debris may be included. The original K3L was sand-cast, made by Holden Ltd. of Canada.
- The K5LA five-chime assembly has five bells whose musical chord helps the horn to be heard and lessens complaints. The bells may be reversed for trains that go backwards and help those working at the back of the train hear the horn on the front. The K5LA is the most popular horn in use today, with a B major 6th chord (D-sharp, F-sharp, G-sharp, B, D-sharp). Though first used by Chessie System, it was developed for Amtrak as a variation on the original K horn, and is the standard horn for Amtrak, CSX, Norfolk Southern, and Illinois Central as well as commuter and passenger trains.
Nathan also makes P-series and M-series horns for railroads and CS-Series and KJ-series horns, KSV vertical mount horns, steam whistles, heated horn assemblies, electronic pressure regulators, control valves, flange lubricators and glo rod gauges.
K Bell Numbers:
1- D#- 311hz
1L- B- 262hz
2- F#- 370hz
3- A#- 470hz
3L- A- 440hz
3A- G#- 415hz
4- C- 512hz
4A- B- 490hz
P-series horns also include single tone (P1), three-chime (P3) and five-chime (P5) versions. The bells are sand-cast. Frequency ranges may be from 220 to 554 hertz. The P-series horns have longer bells and a heavier manifold than the M-series. The name of the horn is a P followed by the bells that face forward, followed by R if any are reversed, and then the numbers of the reversed bells; a P12345 is a true five-chime horn with all bells facing forward, while P135R24 has bells 2 and 4 reversed.
Robert Swanson started Airchime Ltd., starting out by making custom steam whistles in his British Columbia home. He liked the sound of steam whistles better than the single-chime horns made by Leslie and Westinghouse Air Brake Company. In 1949, he introduced the Hexatone 5 Chime (H5), of which some odd (88) were made according to Robert Eugene Swanson's personal manufacturing/sales records. The Hexatone H5 was only preceded by the Hexatone H6, of which (4), including a cast iron H6 prototype known as the 'Iron Maiden', were only produced and never sold in any quantity to any railroad and were considered experimental. The H-series and N-series, also rare, preceded the M-series manufactured by Nathan in the United States and Airchime Mfg. Co. of Vancouver, B.C., Canada. Airchime Mfg. Co's M3H became the standard 3-chime horn used in Canada. The Holden Co. Ltd. was the Canadian Distributor of Airchime Mfg. Co's locomotive air horns and steam whistles and did not manufacture any horns or whistles, a very common misconception.
Captain Charles Benter, who followed John Philip Sousa as Marine Corps Band conductor, played a role in developing what is called "the most melodic horn in railroading." The five-chime horn Swanson and Bentor came up with became the H5 and H3-series horn, characterized by "short, fat bells that were welded together by their flares, the power chambers not being on an even plane, and copper air tubes supplied air to all five bells from an air-chest below the largest bell. The H3 had three chimes, and the H5 five chimes. This is the horn that Bentor had Swanson modify to A 7th major from C# diminished 7th.
The H5 and N3 were evolved into the superior M-series horns. (M standing for "Modulation") Both the 3-chime and the 5-chime had all power chambers on an even plane, and the M5 eliminated the air tubing, instead using a manifold that internally supplied the air to all 5 (or 3) bells. The M5 is still to this day considered by historians and collectors as the most musical of all locomotive air horns. Although supposed to have been originally tuned to A 7th major, 1st inversion with doubled third, few if any M5's ever blew that diatonic chord. M5 bells tended to be varying amounts of hertz off from the diatonic keyboard, just like a chime steam whistle, so about any sound close to various chords could be heard on a healthy M5. Another well known fact is the old M5 was the true "king" of locomotive horns, both for wonderful musical tones to absolute loudness, the M5 with 5/16" inlets being the most powerful horn of all.
In the early 1950s, Swanson introduced the Nathan Truck Horn in T-E and T-5 versions. In 1953, the truck horn was refined for use on locomotives, becoming the P-series. Swanson, whose Airchime never made the horn, sold the rights to Nathan. He never liked the P-series considering the K-series to be the ultimate horn, with the P (President's whistle) being a cheaper alternative. The P-series did its job, and was easier to repair than the M-series because each M-series bell had a separate diaphragm, while the P-series horns had the same diaphragm for each bell, a practice Leslie was already using. M-series horns also needed more frequent maintenance. The P-series was Nathan Mfg. Co's equivalent to the Leslie SuperTyfon.
All generations of P-series horns "used a steel diaphragm disk. Also, the orifice opened up into a fairly wide, oval-shaped hole in the horn's internal chamber." The second generation P-series diaphragms remained the same stainless steel, however the inlet to the power chamber was now round, due to elimination of the core in the casting pattern. The changes made no difference in the sound.
The P3 was used by the first diesel locomotives of Illinois Central and Southern Pacific. The P5 was used for the passenger trains of Illinois Central, Rock Island and Southern Pacific, along with various freight units, especially those on the old Southern Railway.
In 1954, the K5H/K3H made their debut, using a D-sharp minor chord (D-sharp, F-sharp, A-sharp) because of Canadian regulations. "H" stood for "high-pitched" because none of the low-pitched bells available were used. Later, "H" referred to high-profile manifold, while L stood for low-profile. The K5H is Swanson's best yet imitation of a steam train chime whistle, heard at a distance it was described as "unresolved" and "haunting". It was used by Norfolk Southern and CSX (some of whose older engines still use it), and by C&NW. Early K-series horns were sand-cast, like the P-series, but later ones would be die-cast.
In the early 1960s, Nathan introduced the low-profile P3 manifold, replacing a five-chime manifold used previously even though the horn had only three chimes. Southern Railway used this design.
Deane Ellsworth of Amtrak developed the P5a (A for Amtrak) in 1976. While the P5 blew an A7th Major chord, the P5a reverted to Swanson's original C# diminished 7th by shortening the #4 bell the same 7/16" that Swanson lengthened the #4 bell on the H5 in 1949 at Bentor's request. Also used by Amtrak was the P01235, with an "0" bell which was an octave lower than the #4 bell it replaced.
In 1975, Deane Ellsworth, who was in charge of locomotive appliances for Amtrak, wanted to bring the superior K5 to America. However, he did not like the D# minor 6th of the K5H and personally asked Robert Swanson what could be done. His designer came up with a novel way to lower the pitch of the #3&4 bells to lower their pitch, altering the K5H chord from D# minor 6th to B major 6th, and which reminisced of the old M5 chord in structure, but was higher in pitch, which Ellsworth liked. At the same time, Airchime came up with a lower-profile bracket and the new chime was called the K5LA, and it cheery chord has become very popular from the old Chessie system (first user of the K5LA) to Amtrak, Southern and many other roads The K5LA designation included a "K" for the double-diaphragm kettle drum design, an "L" because of its low-profile manifold, and an "A" for American tuning, to differentiate it from Canadian tuning. By the later 1980s, the K5LA was North America's most popular locomotive horn, and today it remains the most-used 5-chime horn in the world.
In 1976, Nathan had two other foundries cast P bells, but new castings were used that did not have the right pitches. The 1, 2 and 3 bells had a slightly higher pitch, and the 5 bell a lower pitch. The result was D, F, G-sharp, A, C, not considered a chord. These horns are considered the third generation P series.
In 2005, Federal Railroad Administration regulations specified a maximum decibel level for horns, resulting in the development of the K5LLA (the extra "L" means low-pitched, with the first bell tuned to middle C) for EMD and K5HL for GE.
- Thompson, Keith (2006-05-01). "Introducing the horn section". Trains: THE Magazine of Railroading. Retrieved 2009-05-20.
- "Model K – Locomotive Air Horns". Micro Precision Group. Retrieved 2009-05-20.
- "Choosing the Right Sound System". Soundtraxx. Retrieved 2009-05-28.
- "AirChime K5LA". railfan.net. Retrieved 2011-07-21.
- "Nathan/Airchime K5 series". trainiax.net. Archived from the original on 2010-05-02. Retrieved 2009-05-28.
- "Nathan Manufacturing Inc". Micro Precision Group. Retrieved 2009-05-20.
- "Model P – Locomotive Air Horns". Micro Precision Group. Retrieved 2009-05-20.
- "Nathan P5". railfan.net. Retrieved 2011-07-21.
- "History of the Air Chime P Series". Retrieved 2011-07-21.
- "Leslie S3-K". railfan.net. Retrieved 2009-05-28.
- "Nathan P01235 and P3 Train Horns". Retrieved 2011-07-21.