Figure 8. Diaphone piston-cylinder assembly show-ing one of two motor air exhaust slots on the side of the unit.

Figure 7. View of piston inserted in diaphone cylinder as seen with the back cover removed.

      The obvious implication of this is that the fre-quency of the diaphone’s signal should be twice that of the piston. However, for that to happen, the piston would have to move uniformly back and forth and would also have to move exactly the same distance beyond the open slot position on both the forward and reverse strokes. Neither of these ideals prevail, however. There is inferred evidence that the piston travels faster on the return stroke than on the initial stroke, and also it is very unlikely that the motion of the piston is exactly centered about the cylinder slots. Therefore, the two re-sulting puffs of air during each piston cycle are not equal in duration or amplitude which results in a strong component in the resulting tone of the piston frequency.
      An actual test of a Gamewell type B diaphone shows that the free air resonant frequency of the resonator when attached to the horn is close to 220 Hz, which is the pitch A below Middle C on the musical scale. When blowing normally with a full 40 PSI measured in the air jacket, the pitch of the resulting blast is very close to A 220, showing that the basic piston frequency and pitch of the signal match that of the attached resonator. However, there is an extremely loud octave component to the sound of A 440, resulting from the two slot coincidences for each complete piston cycle.
      The impression that you get from listening to this type of diaphone from approximately 200 feet off to one side is that you are hearing two simultaneous and exactly in-tune signal tones an octave apart. This may be one of the reasons why the tone carries so well. A 220 is low enough to carry for a long distance, and A 440 is app-roaching the range at which our ears become very efficient. So the horn can be said to use a lower frequency that traverses long distances easily as a carrier for a higher one that approaches our ears’ maximum efficiency.
      The sound of the diaphone actually contains many harmonics, which are integral multiples of the basic or fundamental frequency. This property also helps to make it easily heard even above con-siderable background noise, a characteristic which also applies to the larger diaphone foghorns.

      This next picture, figure seven above, shows you the piston in the cylinder as it appears with the back plate removed. Notice the opening in the center of the piston. This does not go all the way through, but extends into the piston for about 1/4" and is tapped for a 1/4 × 20 screw. To facilitate removing the piston for cleaning and oiling, you can insert a screw or bolt into this opening and use it as a convenient handle to withdraw the piston from the cylinder.
      Notice also in figure seven that there’s a very small hole at the 6 o’clock position of the piston. This port leads through to the other side of the piston in the motor section. There is some spe-culation about its purpose, but the general con-sensus seems to be that it aids in starting the diaphone piston for the next signal if it should have stopped after the previous blast in a certain part of its stroke where starting might otherwise be difficult.
      The motor air that drives the piston does not contribute to the blast of the diaphone. No doubt it makes some noise, but the diaphone signal greatly overpowers any other noises that the diaphone might make. The motor air is exhausted from the diaphone through two ports in the cast iron housing where the back cover bolts to the unit. In the larger diaphones, such as the C and CC and of course the F and F2T foghorns which have at least eight or ten studs and nuts to hold the back in place, the exhaust ports are between the studs. This next photo, figure eight shows the diaphone assem-bled, and also shows one of the two motor air exhaust slots on the side of the unit.
      From the preceding, you can easily see how the rapid oscillation of the piston opens and closes the cylinder slots, effectively functioning as a alter-nately opening and closing valve to chop or modulate a stream of compressed air into a sound wave. When the diaphone is operating normally at full power, the slots open and close twice for each piston cycle; opening once as the piston moves to the rear of the horn, and then opening again as it returns to the front. Therefore, there are two slot coincidences for each complete cycle of the piston.