That was my thought. The bag reed is for a human powered version, but the concept is the same. I would have thought that HDPE (most food containers) would have been two supple. I want to up it a bit with shim material or sheet brass. I have a suspicion that the stiffer the read the faster the vibration and thus possibly the longer the horn. Really cool though to have something cheep to learn and play with.
by KennyJ on Astrosafari.com
Here's some notes on horn theory and design:
What makes a horn work is the way the expanding column of air in the horn couples the small movements of the relatively heavy diaphragm to much less dense air outside the horn. Think of it as an acoustic transformer coupling the high impedance diaphragm to the low impedance air. Horns have a very high acoustic efficiency which is why they can be so loud.
The opening of the nozzle at the diaphragm is the throat of the horn, and is typically smaller than the diaphragm. The ratio of the diaphragm area to the throat area is the compression ratio of the horn. This compression helps transfer the diaphragm's movement to the outside air, increasing efficiency. Some of the train horns I've looked at on the internet have a compression ratio as high as 25:1.
The length of the horn from the throat to the mouth, or bell, is the horn path. Depending on the frequency range to be covered different horn flares are desirable from a long exponential flare for a bass horn to a more rapidly expanding flare for higher frequencies.
The length of the horn path determines the low frequency cut-off and in an air horn is the frequency at which the horn resonates. The ideal horn length is one wavelength of the desired frequency, in other words for a 400 hz horn, the speed of sound traveling through air is approximately 1,130 feet per second, divided by 400 hz, one single sound wave would be 2.825 feet long. In actuality a horn can reproduce frequencies over a horn path of 1/4 or 1/2 a wavelength so that 2.825 foot horn can also reproduce 200 hz and 100 hz at lesser volumes.
In real terms you can estimate the frequency or note of your horn by dividing its length into 1,130, roughly the speed of sound in free air.
The size of the mouth, or bell, of the horn determines how well the lowest frequencies are reproduced. Ideally the area of the mouth should translate to one wavelength of the lowest frequency to be reproduced. This ideal mouth area can be very large for a given horn, and in reality smaller mouth sizes can be used at the expense of efficiency.
Well, that was a little long winded and kind of technical but may be of interest to some.