How does an air horn work?

What is the operating principle, and why is it so loud.

How does an air horn work?
Wow, someone actually beat me to linking to my own website, amazing! That animated GIF above is something I did to show a group of horn collectors how a Gamewell Type B fire diaphone works. This is NOT a typical air horn like you would find on a truck, boat, train etc... it's an early 1900s design that was used only for fog horns and fire station horns. It is an air horn nonetheless, so that answer is correct.





The most popular and common design for a vehicle air horn consists of a flared resonator like a trumpet (this is the actual horn), and a small enclosed chamber at the throat end that contains a metal disc called the diaphragm which caps off the throat of the horn. Air pressure pushes on the edge of the diaphragm to lift it off the throat of the horn, and let a little puff of compressed air enter the horn itself, simultaneously causing the pressure in the chamber to drop. The diaphragm is springy, and it snaps back to cover the throat of the horn, but pressure builds up again and this cycle repeats. So you have periodic pulses of air entering the horn. The diaphragm performs the same function as the reed in woodwind instrument such as a clarinet.





Now, the horn trumpet itself does a couple of things. Firstly, it acts as a resonator, which is to say there are standing waves setup in it, the frequency of which (the pitch of the horn) is determined by the dimensions of the horn. Longer and wider generally gives you a lower tone. The standing waves actually exert force on the diaphragm as well, so that its valve like oscillation matches the pitch of the horn. Secondly, the horn is an acoustic impedance matching device which helps transform the small high-pressure puffs at the throat to large amplitude sound waves at the mouth, increasing the efficiency of sound production.





Air horns are louder than musical instruments because they operate at much higher pressure than what your lungs can produce (typically 200-300 times as high). Higher pressure fluctuations at the horn throat gives more powerful sound output at the horn mouth.
Reply:I think it depends on the kind of horn you're asking about, but the general principle is to use compressed air to move a cylinder back and forth to create the tone you hear. It's loud because there is a lot of air being moved and pulsed by the cylinder.





This animation might help understand better. The compressed air comes in at the bottom and splits into two parts, in this case the left path of air is used to move the cylinder left and right (by virtue of the air paths that push it in, then out. At the same time, a larger amount of air moves through passages that align with the movement. This creates a pulsation of air flow. The pulses create the sound you hear.





http://www.airraidsirens.com/posts/diaph...
Reply:The air is compressed the it's released through a valve that makes the sound. Think of it as a whistle, where the can with the compressed air is the "lung," the valve and horn are the "lip/tongue" that form the proper shape, thus emitting the sound. It's loud because the air is extremely pressurized, so it's basically an effect of a lot of air trying escape a tiny valve at the same time.



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