The Siren Board

I'm trying to make a siren just like this one: http://www.youtube.com/watch?v=2Tr6hCZAVhI

I'm just wondering how to fine tune it to make it as loud as possible. I'm wondering where is the best position of the blades. I know that a siren works without blades, and the purpose of them is to make it loud. But imagine this rotor is turning clockwise. Would it be best to put the blades on the right or left of them? This is what I mean, compare the drawings I made in MS paint that I attached. In siren #1, the blades are right behind the holes, and in #2, they are farther back. Which would be better for blowing the most air out of the holes?


Also, I'm wondering why the 2T22 port ratio is 12/10, if 6/5 is the same frequency ratio of a minor third. Does it simply make the siren an octave higher at the same volume? Or does it just lower the required motor speed? I like lower pitched sirens better so I'll do 6/5

Last thing, how much does the space between the rotor and stator matter? I don't want them to be too close together or i'll risk them rubbing together and self destructing

Do a centrifugal blower design, with the blades 'pushing' the air forward (forward curved vanes) to get most sound. Make the stator forward curved and use a dremel tool to make the chopper and stator sharp. Hope that helps

The sound output frequency (pitch) of a mechanical siren is equal to the number of chopper ports times the chopper rotation speed in revolutions per second. So a 10/12 port chopper will generate the same pitch as a 5/6 running at twice the motor speed.

For example, I've got an idea of a 220/550/990Hz three-tone siren I want to build sometime. I could do it as a 2/5/9-port chopper with a motor speed of 6600 rpm, or a 4/10/18-port chopper at 3300 rpm.

Your best off making it a 10/12 siren with a 3450 RPM motor. Or you can make it 20/24 with a 1750 RPM motor. I wouldn't want a wood siren going over 4000 RPM.

My tip: use the strongest glue you can find, maybe even screws and make sure it's well balanced .

*deleted, read post below*

rdfox wrote:The sound output frequency (pitch) of a mechanical siren is equal to the number of chopper ports times the chopper rotation speed in revolutions per second. So a 10/12 port chopper will generate the same pitch as a 5/6 running at twice the motor speed.

For example, I've got an idea of a 220/550/990Hz three-tone siren I want to build sometime. I could do it as a 2/5/9-port chopper with a motor speed of 6600 rpm, or a 4/10/18-port chopper at 3300 rpm.

So the pitch of the 10/12 siren would be an octave higher, but would the volume be the same?

Also what I plan to do is use a two sided motor like the castle castings, so I can simulate hi-lo by manually blocking the two intake holes (i don't understand how solenoids work).

If I successfully make this siren I'll post a video when I use it on halloween

Liquid_Pooterbear wrote:So the pitch of the 10/12 siren would be an octave higher, but would the volume be the same?

My theory was that the faster a chopper spins, the more air it pumps through it. I got this theory from the way a chopper spinning at 30 RPM makes very little noise (or none at all) compared to the full volume of full RPM. I'd go with Holler and make sure the RPM doesn't exceed 4000.

A higher number of ports will increase the pitch, but the air will have less time to pressurize. When the air can pressurize properly, the sound is nice and crisp. An example is how a Model 2 sounds more crisp than an HOR Siro-Drone, though the chopper design is exactly the same. The Model 2 has larger ports, so the air has more time to pressurize before being depressurized and chopped. The Thunderbolt has tons of time to pressurize because of the single stator slot, however the air does not have enough time to fully depressurize before it is chopped because the stator slot is thinner than the ports of the rotor.

If you were to make a 5/6 port 3450RPM siren, it would sound louder than the 10/12 port though the volume would increase just slightly, if at all.

sirendude2012 wrote:

Liquid_Pooterbear wrote:So the pitch of the 10/12 siren would be an octave higher, but would the volume be the same?

My theory was that the faster a chopper spins, the more air it pumps through it. I got this theory from the way a chopper spinning at 30 RPM makes very little noise (or none at all) compared to the full volume of full RPM. I'd go with Holler and make sure the RPM doesn't exceed 4000.

A higher number of ports will increase the pitch, but the air will have less time to pressurize. When the air can pressurize properly, the sound is nice and crisp. An example is how a Model 2 sounds more crisp than an HOR Siro-Drone, though the chopper design is exactly the same. The Model 2 has larger ports, so the air has more time to pressurize before being depressurized and chopped. The Thunderbolt has tons of time to pressurize because of the single stator slot, however the air does not have enough time to fully depressurize before it is chopped because the stator slot is thinner than the ports of the rotor.

If you were to make a 5/6 port 3450RPM siren, it would sound louder than the 10/12 port though the volume would increase just slightly, if at all.

Thx, since I like lower pitched sirens better I'm going with 5/6.

Just one last thing before I get started with this project: In the youtube video I posted in the original post, the rotor has ports and solids between them of the same size. Meaning if the circle had a circumference of 10, and it was a 5 port siren, it would be a 1 inch wide port, then 1 inch of solid, then 1 inch of port, etc. Then the holes on the stator are the same size as the solids on the rotor, making the ports on the stator a bit smaller than the solids. Is this the best way to make the siren, or could it be better? If you don't understand, you can find the video on how he made the wooden siren here: http://www.youtube.com/watch?v=XAfvOjdZpkg.

Basically i'm wondering how big I should make the ports on both the rotor and stator.

It makes sense, because if the rotor solids are smaller than the stator ports, it won't completely block the air

EDIT: I watched this homemade air raid siren video where the stator ports were REALLY big and it still worked, so it seems that sirens aren't as picky as I thought

The pitch of the 10/12 would be an octave higher *if the rotor RPM is the same*.

If you're spinning a 10/12 at half the speed of a 5/6, they'll put out the same pitch. Not sure on volume, but I suspect it depends on the efficiency of the air pump fan design and many other variables.

Short version: Rotor speed in revolutions per second (i.e., rpm divided by 60) times number of ports equals sound frequency.