Thingiverse
Air Raid Siren by ZX80
by Thingiverse
Last crawled date: 4 years, 7 months ago
(MakeItLoud submittal is 'r3' versions)
Too loud for iPhone microphone--over 94dB at 2 meters. (also measures 94dB at 1m, so I'm sure the microphone is swamped.) You can add any number of sirens, so you can double to quadruple the sound level easily (3dB or 6dB more). But it gets harder to crank without binding up.
It's a little picky on printing. I suggest 0.3 mm layers, and medium quality. I've built in some compensation for inaccurate printing (I use Afinia, which commonly prints 0.07mm too large). It might take a few tries to get it just right. If the .STL files make the crank too sloppy, let me know, and I'll make some with lower 'undercut' (when I can).
I had misread the due date (thought it was Dec 16 till today), so I'll be a few try/fix iterations short of what I'd like. My first misprint had the siren 16mm smaller than the crank, so I had to drill new holes for it to attach. My second rev was too large for the contest rules (by the same 16mm).
Theory of operation: The squirrel cage fan (rotor) has 60 blades (50 in r1). The stator just outside it has 59 blades (49 in r1). As the blades line up on each side, the air flows, and the air is blocked on the opposite side. This happens about 1500 times/sec. The wavelength of 1500Hz is about one circumference. (I used some approximates, so might have the freq. a little off). That corresponds to 2.5-3 cranks on the handle each second, and 25-30 turns of the rotor each second.
Rev 2 added. This fixes a size mis-match in the siren & crank. It is designed for a higher frequency, but lower crank speed (about 3/sec should howl). Rev 2 eliminates the need for supports on the siren-rotor. Nothing else needed supports. So this version is very printable.
Rev 3 added. All this rev does is conform to the 10x10x12.5cm print area rule for the MakeItLoud contest. (I'm glad I noticed that rule in time!)
Too loud for iPhone microphone--over 94dB at 2 meters. (also measures 94dB at 1m, so I'm sure the microphone is swamped.) You can add any number of sirens, so you can double to quadruple the sound level easily (3dB or 6dB more). But it gets harder to crank without binding up.
It's a little picky on printing. I suggest 0.3 mm layers, and medium quality. I've built in some compensation for inaccurate printing (I use Afinia, which commonly prints 0.07mm too large). It might take a few tries to get it just right. If the .STL files make the crank too sloppy, let me know, and I'll make some with lower 'undercut' (when I can).
I had misread the due date (thought it was Dec 16 till today), so I'll be a few try/fix iterations short of what I'd like. My first misprint had the siren 16mm smaller than the crank, so I had to drill new holes for it to attach. My second rev was too large for the contest rules (by the same 16mm).
Theory of operation: The squirrel cage fan (rotor) has 60 blades (50 in r1). The stator just outside it has 59 blades (49 in r1). As the blades line up on each side, the air flows, and the air is blocked on the opposite side. This happens about 1500 times/sec. The wavelength of 1500Hz is about one circumference. (I used some approximates, so might have the freq. a little off). That corresponds to 2.5-3 cranks on the handle each second, and 25-30 turns of the rotor each second.
Rev 2 added. This fixes a size mis-match in the siren & crank. It is designed for a higher frequency, but lower crank speed (about 3/sec should howl). Rev 2 eliminates the need for supports on the siren-rotor. Nothing else needed supports. So this version is very printable.
Rev 3 added. All this rev does is conform to the 10x10x12.5cm print area rule for the MakeItLoud contest. (I'm glad I noticed that rule in time!)