One of the notable qualities of the diaphone foghorn is the so-called “grunt” at the end of each blast. The production of the grunt sound is explained in detail in the article in H&W #103 on the diaphone foghorn, so I’ll just mention it briefly here, with a reference to that article. The foghorn’s grunt results from the fairly rapid but not instant-aneous lowering of the motor air pressure at the end of the blast as the air in the piping between the motor valve and the motor section of its cylinder decays while the speaking air valve is still re-maining open.
      Because there is only one common air supply and thus one valve for a Gamewell B diaphone, both motor and speaking air are cut off simul-taneously. Therefore, according to numerous ac-counts of various types of diaphones which I have read about in both manufacturers’ literature and also tests by the US government on fog signals, the smaller diaphones which have just one common air inlet for both speaking and motor functions are not supposed to produce a grunt.
      This however is not necessarily true. Much depends on the piping and operating valve details of a specific B diaphone installation. If the air pres-sure can be lowered gradually at the end of the blast, the B diaphone will produce a grunt, where its pitch drops fairly quickly from A 220 to F 175. [Note that pitches are approximate and vary with applied pressure, thus rounding off the frequencies to the nearest whole number is perfectly accep-table. ed]
      How does the diaphone motor section work to make the piston vibrate back and forth at a rapid rate? Again, we can refer to the article in H&W 103, but for the sake of convenience we will repeat the significant points of that information here.
      The most important thing to consider is that whenever compressed air acts on a surface, the total force on the surface depends on both the pressure and also the surface area. The same pressure on an area that is twice as large as another area will develop twice as much force on the bigger area.
      Now let’s look at the diaphone piston, con-centrating on the motor section. The motor section of the B diaphone piston is three inches in diameter. The speaking part of the piston is 2 1/8 inches in diameter. Because the interior of the speaking  part  of  the piston  remains  essentially

at atmospheric pressure, the only area on the forward side of the piston that motor air can act on is the ring-shaped area that represents the difference of a three inch circle minus a 2 1/8 inch circle.      
     This area is 3.52 square inches. Forty PSI motor air appears in the forward part of the motor section all the time whenever the diaphone is sounding. It pushes the piston back with a force of:

(40 PSI)(3.52 sq. ins) = 130 pounds

      The rear surface of the piston has an area of 7.07 square inches and if we put 40 PSI pressure in the rear portion of the motor section of the cylinder, it will push the piston forward with a total force of 282.8 pounds of force, which obviously is more than twice as much as 130 pounds. There-fore, even if we have 40 PSI in the front section of the motor region of the cylinder, if we simul-taneously put 40 PSI into the rear section, the piston will move forward because the force on the rear of the piston is 282.8 pounds which easily overpowers the force on the front of the piston which is only 130 pounds. 
   This greatly simplifies the motor air valving requirements, because it is not necessary to do anything about the air on the front of the piston. It is only necessary to connect the rear section alternately to 40 PSI or exhaust it to the atmo-sphere. As the piston moves to the rear of the cy-linder, it uncovers a slot through which the 40 PSI air from the front section travels to the rear section, building up pressure there. This develops the ne-cessary force on the rear of the piston to push it forward. As it does, it acts as its own valve, shuts off the supply of 40 PSI air to the rear section of the cylinder and opens an exhaust slot through which the 40 PSI leaves the rear section of the cylinder. Once the pressure in the rear section of the cylinder is removed, the 40 PSI air which is always on the front of the piston pushes it back again.  
    The entire operating sequence of the diaphone is beautifully illustrated in the animation produced by HWEG member Adam Smith and used here by permission. The arrows show the airflow during progressive stages of the diaphone cycle. The link is highlighted below.

To see Adam Smith's Diaphone cycle animation, click this link.
www.airraidsirens.com/posts/diaphone_animation.gif