M horns are the first series of horns from AirChime that had a large production run. Introduced in 1950, these horns can still be purchased today, for a price. AirChime produced these horns, as well as licensed them out to Burnette, Holden, and Nathan, and so several variaions have formed. Nathan is the only one of these companies that has continued producing M horns into and past the 1960s, including AirChime itself!

M horns are arguably the prettiest-sounding horns ever made. They have a very mellow, yet commanding, tone, and are tuned to "pleasing" chords. However, the maintenance needed and the rising costs for replacement parts forced the railroads to abandon the M series. Newer models, such as the K and P series, and Leslie's SuperTyfon, were cheaper and more maintenance-free. The last large order of M horns was placed in the late 70s. A few horns have been cast at Nathan since, but not many. The last M horn cast that I'm aware of was an M5 special-ordered from Nathan in 2001. Owned by Brent Lee, this horn had a pricetag of $6600, head and shoulders above the 1950 pricetag of $100.

identification and features

M horns can be identified by their short, wide-mouthed bells with almost no flare through most of the bell length. Easier to pick up is that the heads, or diaphragm housings, are all different sizes. Also, these horns have a screw-on back cap that can be used to adjust tension on the diaphragm. The back caps have two holes which are used by a spanner wrench to adjust the cap. Another unique mark is that the manifolds have the bell number for each position cast into the manifold. Since each bell size is different, bell placement must be exactly as specified, or the bells won't all fit. Bells can still be reversed, though, without problems.

M horns, like Leslie SuperTyfons, have separate heads and bells, which bolt to opposite sides of the horns' manifold. However, the insides of the heads are nothing like the insides of a Leslie power chamber. The M horn uses a multi-leaf phosphor-bronze diaphragm assembly (3 leaves on all but the 5 bell, which uses 2 leaves), which is bolted together. Also, there is a separate clapper disk on one side of the diaphragm assembly, since the seat is not in the same plane as the diaphragm leaves. In fact the diaphragm assembly is very similar in construction to a Westinghouse E-2 diaphragm assembly, but with fewer parts.

The nozzle, which the clapper seals against, is cast bronze as well, and is seated on top of an O-ring. Part of what gives an M horn its mellow tone is the heavy bronze clapper which vibrates against the bronze seat - the bronze actually helps to mellow the tone. The other factors which help give the M horn its unique tone are the heavy castings (which means that there is more metal to resonate when the horn is sounding) and wide throats on the bells (which allow for a more pure and open tone).

		diaphragm_040215_1.jpg	the diaphragm assembly off of an M4 bell
		head_040215_1.jpg	the insides of the M4 head showing the bronze nozzle and diffuser, along with the inside of the back cap showing both clamp rings

designations and chords

M series horns were offered in single, two, three, and five chime configurations. The tuning for these horns was for the most part based on the bell numbers used, though this also can vary slightly. The variations will be explained later. For now, suffice it to say that the 1, 2, 3, 4, and 5 bells sound C#, E, G, A, and C#, respectively.

The single-note M horns were offered on either a three-chime manifold with two ports "blanked off", or on a single-bell manifold designed for the 1, 2, or 3 bell. Horns using the single-bell manifold are designated as MS1, MS2, or MS3, depending on whether the 1, 2, or 3 bell is used. The single-note horn on the three-chime manifold was only offered with a 1 bell. The 2 and 4 bell ports have caps bolted on each side of the manifold, which can be removed to add bells at a later date. This horn is designated as an M3-1.

The only two-note M horns ever offered used a three-chime manifold with one port blanked off, just like the M3-1. The only bells offered on the two-note version were 1 and 2, and so this horn was designated M3-12. This horn plays C# and E, a minor third interval. Like the M3-1, the 4 bell could be added to the M3-12 at a future date in order to make an M3. Also, if one of the bells is reversed, the designation includes an 'R' preceding the bell reversed. With one bell in each direction, the M3-12 changes to an M3-1R2.

Next, there are several different configurations for standard M3s. First, the basic M3 is a three-note horn using bells 1, 2, and 4. This horn plays an A major triad - C# E A. If any bell is reversed, the designation includes an 'R' followed by the bells reversed. An M3 with the 2 bell reversed would be classified as an M3R2. The M3 could also be configured on the five-chime manifold with bells 3 and 5 blanked off, and would be classified M5-124. With the 1 bell reversed, it would be an M5-24R1. One unique configuration of the M3 was the M3RT1. This horn is basically an M3R1, but the 1 bell is tilted upwards so as to clear any obstructions that may be just behind the horn. It sounds identical to an M3R1, and was used extensively on Alco RS2 and RS3 models, as the reversed 1 bell would just clear the cab roof.

Finally, the five-note M horns are designated M5s. With bells reversed, add an 'R' and the bell numbers that are reversed. Common reversals are the 2 and 4 bells - an M5R24 - and the 2 and 3 bells - an M5R23. The M5 plays an A major 7th chord - C# E G A C#. However, the tuning on the 3 bell was often flat, and so an A major 6th chord was also common - C# E F# A C#.

One common variation on the M horn is called the M3H, which was designed for use on Canadian engines. This horn plays the government-mandated D minor chord - D F A - and is achieved by using bells 2, 3, and 4 with heads 1, 2, and 4. The M3H was a common horn sold by Burnette, AirChime, Nathan, and Holden, and even saw some service on the Grand Trunk and a few other lines here in the US.

A few other variations are quite rare and were mainly manufactured by Burnette. For whatever reason, Burnette cast some of their bells slightly longer or shorter than AirChime's specs, and so different chords were produced. One chord, found on the Burnette M5H, is believed to be C# F G A# C#. This configuration uses a longer 2 bell and a shorter 4 bell. A few other known Burnette M5 chords are C# E G A# D#, C# E G A D, and C# D F# A C#. Burnette also offered a non-standard M3H, tuned to D# F A.

		kaspriske_030614_3.jpg	among the horns Ed Kaspriske brought to Oak Ridge 2003 was an M3RT1, in the center of this picture - notice the tilted #1 bell - this is a stock configuration
		m3_020518_3.wav	my scallop-base M3 recoded at the Altoona 2002 blow
		m3_030614_1.wav	an M3 blown during the Oak Ridge 2003 horn blast
		brw752_010704_5.wav	my dad's M3H while in active freight service on Black River & Western GP9 #752
		brw752_010704_10.wav	this clip shows how the M horns modulate volume and pitch just by varying the amount of air to the horn
		m5_weiler_1.wav	an M5 in active service (Copyright Normal Weiler, used with permission)
		m5_030614_4.wav	my M5 mounted on a GE at Steve Forrest's blow
		m5h_030614_1.wav	Ed's M5H, though one of the bells may be silent in this particular recording

evolution

When the M-series horns were first introduced in 1950 to replace the H and N series horns, they were produced in what is known as a "round-base" configuration. What this means is that the base of the bell, which is bolted to the manifold, is completely round. AirChime, Holden, and Burnette all retained this "round base" through their production runs. However, as early as July 1952, Nathan changed its castings to a "scallop-base" style. This means that the base of the bell is no longer round, but "scalloped", since it was found that the extra metal used to keep a round base was not needed. Cutting this out reduced the weight of the horn, as well as the cost to manufacture the horn. This is the largest difference visible in early versus later-production horns. (AirChime, Holden, and Burnette are all each slightly different in their castings, but are not covered in great detail here. The remainder of this section will deal primarily with Nathan horns except where otherwise noted.)

There are other changes that have taken place in the Nathan line of M horns, and I will try to cover them all here. Dates have been obtained by looking at the earliest M horns, which had the manufacture date stamped into them until about 1952. On the back of the heads, or sometimes on the manifold, there is a three-character code of a letter followed by two digits. The letter represents the month of manufacture (A = January, B = February, etc.), and the two digits represent the last two digits of the manufacture year. Therefore, my M3 with "H50" stamped into the heads was manufactured in August, 1950. Some early Ms had other letters stamped, though I don't know what these would have stood for.

The oldest M horns from Nathan in early 1950 had used some leftover parts from the A5 horn, and so some very early M horns may be found with H/N-style back caps, which predated the M horn's use of a spanner wrench. Also, some of these early horns used bells that had "Nathan AirChime" cast into the bell mouths, rather than on raised blocks on the sides of the bells, which is found on newer bells. There are even two existing M5s which have no marks on the bells except for the bell number and "Nathan Airchime" cast on the top of the #1 bell, just behind the bell mouth. I don't know production dates for these horns.

From the start, M horn back caps were designed to take a spanner wrench. However, the earliest of these were designed by Nathan with spanner holes set at the outside edges of the back cap, meaning you would need a different-sized spanner for each back cap! Thankfully, AirChime changed this very early on (and Nathan quickly followed) to a style which uses a spanner with the two holes set at exactly two inches from center to center. This allows one spanner to be able to be used on each back cap. The latest I've heard of the "non-standard spanner caps" in production were in May of 1950, when AirChime's improved cap was introduced. Also, the earliest of these "AirChime caps" have "Pat Pend" cast into the cap. Caps cast from late 1950 on instead have a part number cast into the back cap in place of the "Pat Pend" marking.

About the same time, Nathan started casting their name on the side of the horn bells instead of the mouth. At this time, the words Nathan and AirChime were each in separate raised blocks. This continued until late 1950, when Nathan started casting the complete "Nathan AirChime" name on only one raised block. This was the last major change to the bell style up until the change to scallop-base castings in 1952.

		m3h_2001_2.jpg	my dad's scallop-base M3H - notice the bell bases and the manifold where it meets is not round
		m3_040215_1.jpg	my round-base M3 - compare the round bases of the bell and manifold to the scallop-base picture above
		m3_030614_1.jpg	this M3 belonging to Jim Smith is a very early and rare (May 1950) M3, which uses different spanners for each back cap
		m5_031231_3.jpg	a picture of our scallop-baes M5R24
		m3_040215_2.jpg	my 1950 M3 also has "Nathan" and "AirChime" on separate raised blocks, and the upside-down casting on one side of the #1 bell is also shown here (the #2 bell is not original to the horn)
		m3_040215_4.jpg	I know this horn is from August 1950 because H50 is stamped into each head

last updated 02.17.2004