The preceding describes the so-called “speak-ing” section of the diaphone piston and cylinder. There is a second, larger-diameter section of the cylinder referred to as the “motor” section, and it is here that a relatively small percentage of the total compressed air supplied to the diaphone is used to make the piston vibrate rapidly back and forth in the cylinder.     
       In the large diaphone foghorns, the motor air comes to the horn independently of the speaking air, having its own supply pipe and valve. In the type B diaphone, there is just one single master air supply inlet for the entire horn. A small portion of the total air is diverted through ports from the speaking section to the motor section to drive the piston. With this sole exception, the type B dia-phone’s action exactly parallels that of the bigger type F diaphone foghorn.  
      The air motor section of the diaphone is simple and quite cleverly designed. As we will see from the accompanying photos and computer ani-mation, the piston has two sections of different diameters. The cylinder likewise has two sections of different diameters. Here is a look at a B diaphone piston.

The next view [figure four] is a look at the diaphone cylinder which is press-fitted into the cast iron housing that also serves as the air jacket.

Figure 4. Diaphone cylinder showing the speaking slots and also the three air slots in the motor section and the motor air supply ports and, all around the motor section, the ports that connect slots one and three.

Figure 2. Diaphone piston and approximate dimen-sions

      Figure two shows the slots in the wall of the speaking portion of the piston and the reinforcing ribs on the inside. Figure three [below] is another look which shows the shape of the larger diameter part of the piston. The purpose of the roughly triangular groove in this part of the piston will become apparent when we look in detail at the operation of the motor section of the diaphone cylinder and see how the motor air drives the piston rapidly back and forth as the piston simul-taneously acts as its own valve to control the motor air, resulting in piston oscillation.

Figure 5. This view, with both piston and resonator removed shows the speaking slots in smaller dia-meter speaking portion of the cylinder.

Figure 6. This picture, looking into the air inlet in the flange of the diaphone's base pedestal shows a small portion of the outside of the cylinder in the air jacket. You can see the cylinder slots and also a little of one of the reinforcing ribs that are part of the cylinder casting. When compressed air is admitted through this opening, it surrounds the entire cylinder, and some of it also enters the motor section through the motor air supply ports that are visible in figure four.

Figure 3. Diaphone piston showing the two sections and the bypass groove in the larger diameter section.