About air horns and their construction

Air horns were one of the first puzzle pieces that gave me the idea for this project, and while I don't remember what originally got me interested in them, I remember finally figuring out how they work and being surprised by how simple the design actually is.

The basic construction of an air horn is pretty simple. There's a tube that's open at both ends, which is the bell of the horn. Sometimes it's flared, but if you're building one yourself, it's easier to make it straight. There's a diaphragm, which is stretched across the back of the bell and also separates the base of the horn into two areas, which I call the input and back chambers. The input chamber is a donut-shaped space between the diaphragm, the bell, and the shell of the horn's base.

These images are a cross-section through a cylinder;
think cutting a cake in half the usual way.

If you pressurize the input chamber, it puts tension on the diaphragm and pushes it upward, away from the end of the bell (see dotted line in image below left). This allows the pressure in the input chamber to be released out the bell, which allows the tension in the diaphragm to pull it back against the end of the bell again (see right).

Closed diaphragm

Open diaphragm

As the diaphragm snaps back against the end of the bell, it acts like slapping the end of a pipe with your palm; the bell resonates (think Blue Man Group), which is the result of a pressure wave bouncing back and forth within the tube (standing pressure wave). If you supply a flow of air to the input chamber, the pressure wave will help synchronize the diaphragm's pressure-release cycle, and you wind up with a continuous note at the resonant pitch.