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Thread: Figuring the flow rate and pressure of an air horn

  1. #1
    Associate Engineer
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    Figuring the flow rate and pressure of an air horn

    First off let me state I'm out of my wheelhouse with this and it's been many years since high school physics. (I had a soft science major in college). That said, I'm working on a project and I keep getting the same question. What flow rate and psi do you require? Essentially I need to mimic the flow rate and psi of that created by a compressed air horn. So how much air, and what psi are needed to make the horn blow? I can easily get figures for air horns with mechanical compressors, but it seems much more difficult to get this information for air in a can. I imagine that there are slight variations between horns but I'm talking about your basic signal air horn that you would buy from West Marine.

  2. #2
    Principle Engineer Cragyon's Avatar
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    Flow and pressure would depend on the size geometry of the horn design.

    According to Wiki:

    n air horn consists of a flaring metal or plastic horn or trumpet (called the "bell") attached to a small air chamber containing a metal reed or diaphragm in the throat of the horn. Compressed air flows from an inlet line through a narrow opening past the reed or diaphragm, causing it to vibrate, which creates sound waves. The flaring horn serves as an acoustic "transformer" to improve the transfer of sound energy from the diaphragm to the open air, making the sound louder. In most horns it also determines the pitch of the sound. When vibrated by the diaphragm, the column of air in the horn vibrates in standing waves. The length of the horn determines the wavelength of the sound waves generated, and thus the fundamental frequency (pitch) of the note produced by the horn. The longer the horn, the lower the pitch.


    Larger air horns used on ships and foghorns function similarly to a whistle; instead of a diaphragm the air escapes from a closed cylindrical resonator chamber through a precisely shaped slit directed against a knife edge (fipple). The air blowing past the knife edge oscillates, creating sound waves. The oscillations excite standing waves in the resonator chamber, so the length of the chamber determines the pitch of the note produced.

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  3. #3
    Associate Engineer
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    I appreciate all of the information you posted however the question wasn't how a horn works. I asked what the flow rate and pressure for a compressed air horn (from a can not mechanical compressors) would be. Again this would be one of the handheld deals you might see at a sporting event, commercially available at Big 5, Walmart, West Marine etc.

  4. #4
    Lead Engineer Cake of Doom's Avatar
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    I believe compressed air cans have a safe range of 60-80 psi. Don't ask for proof of that though, it's just one of those random things I've picked up but can't remember where. Where an air horn falls in that range will depend on how much pressure is needed to move the baffle and so on and so forth. Buy one and put an air flow meter on it.

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