The diaphone in all of its variations, from mu-nicipal alarm to the various foghorns, is one of the most interesting of all air horns. Its sounding principle is similar to that of a siren in that it has two elements with corresponding sets of ports which alternately open and close, although in the diaphone this is done by a reciprocating piston rather than a spinning rotor.
       Diaphone production has, with perhaps the exception of a few individuals who have foundry and machine shop knowledge and resources available, ceased in this country, no doubt because the diaphone requires a considerable amount of accurate machining and as such is more difficult and expensive to manufacture than a simple diaphragm air horn. Large marine dia-phragm horns have just as much power as the biggest lighthouse foghorns and are much less complicated to build.
      In a sense, the diaphone is like a jet engine in terms of output vs. weight. The jet engine or gas turbine has the best HP to weight ratio of any typical prime mover which is why it is the preferred power source on most airplanes. The jet engine is by no means the most efficient engine, but it’s the lightest. In like manner, although the diaphone is not the most efficient signal device, it is the loudest for a given size and weight. Air consumption of a Gamewell type B will rapidly draw down a 60 gallon receiver after just two or three relatively brief honks, but it nevertheless consumes much less air than a whistle of equivalent power.
      The typical fire station installation consists of one or more large, probably 250 to 300 gallon receivers maintained at 125 to 150 PSI. The air is conveyed at this pressure to a second, smaller receiver of perhaps 80 to 120 gallons capacity close to the horn, usually just inside the attic below the roof of the fire station. From here, the air goes through a high capacity solenoid valve used to sound and code the horn. The output of the SOV then passes through a high-flow pressure regulator and is dropped so that when blowing, the pressure measured in the diaphone air jacket is in the range of 35 to 45 PSI. Typically a 5 to 10 HP air com-pressor maintains the big receivers at the proper pressure. This set up allows ample air for sounding several rounds of a coded fire alarm box number signal or other warning while always maintaining sufficient pressure at the horn itself. If the air pressure in the horn drops too much below 30 PSI, the diaphone operation becomes erratic and it may fail to work at all.

      In some installations, the pressure reducing valve is placed before the small receiver near the horn so that this local receiver holds air at the typical operating pressure for the diaphone. In other installations, likewise the diaphone is just above the valve at roof level and in still others, it may be on top of a length of pipe, perhaps ten or more feet away from the valve or regulator. Therefore, even though the B diaphones are essentially all the same, you will find that many installations sound entirely different.
      Some produce a simple, one-pitch blast. Some start at a somewhat higher pitch and then drop very quickly to the steady-state pitch. Some start at a lower pitch and quickly rise to the steady-state pitch, and some end with a pitch drop or grunt as do the bigger foghorn diaphones. Some type B diaphones may exhibit several of these aberrations in succession.
      The steady-state pitch of a B diaphone is quite dependent on the operating air pressure in the air jacket of the horn itself, so depending on how the regulators are set, you may find considerable variation from one location to another. In a mu-nicipality with several diaphones, it’s possible to have the horns sound different enough from each other that emergency response personnel can tell by the sound of the horn which location is signaling.
      All of these diaphones have a test port on the air jacket which is tapped for standard 1/4" NPT for attachment of a gauge when initially setting up or testing the unit. The important thing with these diaphone installations is to make sure that the air jacket pressure is in the recommended 35 to 45 PSI range for reliable operation of the horn.
       Some of these B diaphones were fitted with a longer resonator. They sound approximately the note Eb or 156 Hz instead of A 220 but are otherwise the same.

For those of you who collect signal devices and are interested in finding a diaphone, it’s our recommendation that you begin your search right now. B diaphones are no longer manufactured. Some communities have stopped using theirs, either because they can’t get them repaired or because of complaints from residents. These horns are quickly becoming the equivalent of the steam whistle; scarcer and scarcer and more and more expensive.
—ECL