OrganGuyJohn
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Real Name: John Miller

Siren Physics

Wed Feb 22, 2017 10:32 pm

Hello all!

I'm looking for information regarding the physics of sirens. Formulas help me a lot. I think I understand pitch, but what other variables affect timbre, volume, etc.?

Ok so pitch is pretty straightforward
Frequency (Hz) = (Chopper/siren disk RPM / 60) x n
Thanks @landmobile!

Does a formula exist for determining how intake and output air volume affects sound intensity?

How about how air pressure affects sound intensity or pitch?

How shape of stator and rotor openings affects timbre? (aka what kind of soundwaves are produced? sawtooth, square, etc.)

How shape of horn affects timbre?

How length of horn affects pitch?

If nothing has been published on these topics, then it looks like a ton of experimentation is in order. I know all of these questions can be quantified! I'm just wondering if someone has already done the work.

Thanks all!
John Miller
Pipe organ builder's apprentice with Orgelbau Schulte in Kürten, Germany
Originally from Milwaukee, WI
Living in Cologne, Germany
Avid TBolt fan :crazy: :TBolt:

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DrDanthrax99
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Re: Siren Physics

Thu Feb 23, 2017 5:27 am

I'm pretty sure the R&D folks at Federal Signal, American Signal, Sentry, etc. (maybe not ATI xD) have done this research, the real question is is this information publicly available.

I'm no mathematician, far from it, honestly, but I can explain in layman's terms what you're asking.

Pitch, as you already know, is dependent on the rotational speed of, as well as the number of ports in the rotor.
The higher the number of ports, the higher the frequency of the sound produced.

Now to move on to air pressure. The entire premise of the Thunderbolt siren is centered on air pressure. As a mechanical siren's rotor coasts toward a stop, obviously less air is being drawn in and forced through the stator and the intensity of the sound decreases. To combat this, the Thunderbolt uses what is essentially a water treatment aerator to send a consistent amount of air pressure to the chopper rotor, thereby causing the intensity of the sound waves to remain largely consistent regardless of the rotational speed of the rotor. The air pressure affects pitch to some degree, (listen to a Thunderbolt with and without a blower) but it affects the intensity of the sound waves produced more than frequency of said sound waves. To recap, the higher the air pressure, the more intense the sound waves are.

Next, shape of the stator and rotor ports effect on timbre. I'm unaware of any sirens that don't have round rotors with square/rectangular ports, so I really can't provide a definite analysis of this. If you want to know what type of sound waves a siren produces, the only thing I can really tell you is take an oscilloscope to a siren and analyze the sound waves for yourself.

The shape of the projector has a definite affect on timbre. For example, the Thunderbolt and Model 2 have the exact same chopper mechanism. If you listen to a Thunderbolt 1000 chopper outside of the housing (only the stator cap, no projector) it will sound nearly identical to a Model 2. However, once you bolt that projector onto the stator the sound gets a little more "boomy". (I'm citing videos I've seen of Thunderbolt choppers outside of the housing tube with and without projectors attached, vs. Model 2 choppers also outside and inside the housing.)

On a side note, the model blower that a Thunderbolt uses also affects the timbre, the 6M blower has a kind of deep, bass-y, warbling sound it contributes, whereas the 5M makes more of a subtle humming and makes the sound kind of, raspier for lack of a better term, and the 4M sounds largely similar to the 5M (at least to me) only less intense.

Moreover, the shape of the projector also has an effect the propagation of the sound waves from it. For example, going back to the Thunderbolt yet again, the projector is an exponential horn, (meaning the opening gets exponentially larger from the base value.) It collects the sound waves into a confined space, which increases the intensity as well. (i.e. cupping your hand around your ear to hear something more clearly.) The wider the output end of the projector is, the higher degree angle the sound waves are dispersed at. For example, the Thunderbolt's projector propagates the sound waves approximately 40 degrees off axis from the center of the projector.

The length of the projector has absolutely no affect on the pitch, I've seen a Thunderbolt that has about 1/4 of the horn cut off, and it sounds no different than a normal Thunderbolt with a full 52" horn. The only thing the length of the projector affects is the actual physical propagation of the sound from the projector, (i.e. the angle of dispersion, the distance the sound carries, etc.)

Well, I hope I've helped you at least somewhat, anyone who is more knowledgeable than myself feel free to correct me if I have made any errors in any information I have dispensed and chime in with what you have.
"I'm a Clinically Depressed Fecalpheliac on Prozac."

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coastalsyrolover
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Re: Siren Physics

Fri Feb 24, 2017 1:13 am

I don't have the formulas but things that affect siren volume include but are not limited to: the ratio and relationship between the tolerance (distance between the chopper and the stator), the speed/HP of the motor and the size of the ports and even the chopper and stator itself. If you have a siren that has GIANT ports but a motor that can't turn the chopper fast enough to create vibration or send enough air through the ports isn't gonna do squat. Now let's add tolerance to it and shrink the ports. If the chopper is too far from the stator then even if we increase the motor speed/HP, it's just gonna whip around like a carousel on steriods. Too close and you have a mess to clean up. But the main thing is how much air is being chopped by the ports... not sure if I understood any of that right but... yea...
Own and love a Thunderbolt 1000 and a Model 5.

I have many hobbies and interests. And I love them all.

Christian, Lima, Oscah, November, Golf.

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archizackture
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Re: Siren Physics

Fri Dec 27, 2019 7:47 am

Reviving an old thread, because I'm wondering about siren timbre as well and was going to start another thread, but the posts above are a good place to start. I'm mainly wondering how the design of the rotor and stator affect the timbre - what effect does the shape of the ports have? Would round ports create a more sine-wave-like sound? How about the placement and shape of the vanes? What effect would changing the ratio of opening to closed space have?
Architecture preservationist. I enjoy obscure history and mapping things, and grew up around sirens. Not planning to own one.
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