Diaphone Piston Close Up Photo - Submitted by Roger Elliott, City of Orange CA
Piston types, stator types and diaphone tone/pitch Info researched by Adam Smith.
"I am still working on finding out everything possible about the variations of the type B, and I'm hoping the folks here who work with these diaphones can help make som concrete connections between the configurations and the sound. I have also been consulting Thiessen's patent for the fog diaphone horns, and it has some information about the various holes in the stator and piston that I will share here also. Firstly, here are the known variations of the piston, as documented by Brett Jones (thanks Brett!). These are all his photos, I've just cut them out and given them arbitrary designations so that I can refer to them (A1 and A2 for aluminum, B1 and B2 for the brass variants)."
Adam Smith - "As you can see, the pistons were made in two shapes, and each of these in two materials (aluminum and brass). Piston style 1 has this extra flange with 40-something holes machined in it, whereas style 2 does not have this ring. So far, what I have heard on this group regarding these differences are: Mike Maderia tells us that piston style 1 is the high-tone version, and style 2 is low-tone (for a given material). I would speculate that the brass pistons are both lower in pitch that the aluminum ones also, because they are about 3 times the mass, but this is pure speculation. I need to hear from someone who has compared the sound of the aluminum and brass variants." "Someone also mentioned a gamewell brochure that suggested two different "tones" of horn, organ and trombone. At this time, I'm not certain how this would come about, except perhaps by a different type of oscillation of the piston, e.g. one more square-wave-like, and one more sine-wave-like. Perhaps this is due to airflow and "cushioning" withing the chamber? I do not know. This brings me though to the question of the 20-rear-holes stator with round intake holes, vs the 10-rear-hole stator with a slotted intake. Here again is Brett's comparison photo:"
"The stator on the right has brass plugs in every other hole. My presumption is that the entire hole is filled with the plug, thus this 10-hole stator has half as much vent area connecting the front of the piston to the rear of the piston. it seems to me that less connecting vent area would give a more sinusoidal piston movement, because it would take longer for air to fill in behind the piston and reverse it. But would this also mean lower pitch? Thoughts? If this theory is correct about the tone, the 10-rear-holes stator would have more of a sinsuoidal "organ tone", and the 20-rear-holes stator would have more high harmonics like a brass instrument. Again, speculation on my part, I need to hear from observers/operators of known types. Ideally, it would be great if we had a sound file to match each combination here on David's site. Lastly, I'd like to show this diagram again from Thiessen's patent. It's the diagram for a single-inlet fog diaphone, and it the same pattern used for Gamewell's horns except that Gamewell's do not have the center alignment rod (#27) through the piston:"
"In the patent, the holes in the style 1 piston (#13) are part of the pathway between the incoming air and the chamber behind the piston, as are the 10/20 holes in the stator (#21). The patent also shows that holes #21 must be capped off by the rear cover gasket, so that they do not vent to the outside. Only the vent holes (#22) are supposed to connect to the horn's rear vents. Now look at the intake holes (#3), on the 10-rear-hole horns, these holes connect to slot #4, as shown. In the 20-rear-hole horns, Gamewell did not just slot #4, they just drilled the holes right through. I don't know what effect this has either." "To sum up, I think the core unanswered questions are these: 1. Is the piston ring with 40 holes to give a different pitch, different tone, or does it affect both? 2. Do the 10 vs 20 holes in the stator affect pitch or tone, or both? 3. How much does the mass of the brass piston lower the pitch? Lastly, if someone could identify the variations in each horn that we have a sound sample for, that would be of tremendous help. Glen Cove, Larkspur, Hudson falls, Farmingdale, etc.. all I know so far is what Northport has, thanks to Brett. I have taken the sound files and analyzed them, and here's what have for pitch data: Hudson Falls: 186 Hz Glen Cove: 162 Hz Northport: 210Hz falling to 170Hz as air runs out Larkspur: 210 Hz Salem, VA: 180Hz 180Hz is the nominal pitch for the type B according to John Pell Northey."

Adam from Airraidsirens.com has been a great source of Diaphone info. He recently created the following animation and gave a description to show how the Diaphone works:

Adam says "I made this little animation to show how a single-valve diaphone operates. I've taken the illustration from Robert Hope-Jones's patent, and given it motion. In an effort to keep this animation small, I did not include arrows showing the direction of air flow, I am showing here only the pressures (darker red = higher pressure). From this animation, it's easy to see how the sound is produced. The slots in the moving piston line up periodically with the slots in the stator, chopping the air stream. What is more difficult to tell, even with the animation, is how the vibratory motion comes about. I'll try to describe it as simply as I can: When high pressure air comes into the horn from the pipe (from the left in the diagram), it starts to push down on the rim (ring shaped) of the rear wide part of the piston. It pushes the piston all the way back, at which point air is suddenly able to get around to the space behind the piston. Now, with equally high-pressure air on the front and back of the piston, it starts to move up again since the back of the piston has more surface area than the front 'rim'. The piston moves forward until it reaches the point where the air behind the piston can escape through the slot in the wide part of the piston and into the mouth of the horn. Note that the piston is symmetrical, the diagram shows only the cross section on the left side, for illustration purposes."

"Pitch of the diaphone is determined by a combination of factors. Pitch is raised by: Higher pressure air Lighter Piston Shorter resonator (horn) Smaller air chamber volume behind the piston Of course, the opposite of any of these things will lower the pitch. If you listen to the "Dying diaphones" recording on the sounds page, you'll notice that as the pressure drops, so does the pitch of the horns. Also, most fog diaphones are 2-valve types, where the air to the rear "motor" section of the piston is controlled by a separate feed than the bulk of the air to the "speaking" front section of the piston. Operation is otherwise fundamentally the same."

Adam also notes "Variance in sound is a factor of the horn's condition (how well maintained) and the supply of air. At different feed pressures, a Gamewell horn will have different pitch and timbre. At the pressure where the piston's motor frequency and the horn's natural frequency are the same, the horn is in tune and has a smooth mellow note like an air horn. It higher pressures, they have a harsh, more siren-like timbre. With too little air pressure, the piston frequency drops below the horn's natural resonance, and it becomes really flabby and inefficient, sounding like a fart (sorry!). Different sounds come about as the period of the piston goes in and out of phase with the natural resonant frequency of the horn projector. The resonating column of air in the projector exerts a force on the piston that definitely coerces it towards the horn's natural frequency. But too much air pressure, and the natural period of the piston's motor section becomes dominant, at which point the piston alone is setting the pitch, and the horn is not operating in a resonant condition anymore. As with any other kind of air horn or whistle, this is called "overblowing". Some stations have more than one horn, which is especially common here in Massachusetts, where 2-4 horns per station is typical. In this case, the horns are never quite in tune with each other, and there is always a strong 'beat frequency' that pulses at a rate which is the difference(s) in pitch between the two (or more) horns. For example, if one horn is barking at 400 Hz, and another is 405Hz, there will be a strong and audible 5Hz throbbing."