Do port shapes make a noticeable difference?

[Robert Gift]

From Daniel, just learned of old East German industrial sirens with round ports.

Thanks to many of you, I've seen various sirens with square, rectangular, and trapezoid-shaped ports.

Also, some with sharp beveled port edges, others with blunt squared edges.

Any other shapes: triangle, oval, etc.?

Do they sound differently?
Thank you.

[Daniel]

Those old DDR sirens had regular square rotor blades, but the stator ports were round. You can find these on Ebay if you select "worldwide" and search for "alarm sirene" or "motor sirene." Those Greek sirens have angled rotor blades, but they are the only other ones I know of that aren't square or rectangular. I wonder what would happen if a siren's rotor and stator ports were placed at a 45? angle like the blades of a jet engine.

[orestis]

I can't remember where I have found this picture, but it's an other example of non-square shaped stator and rotor

Those old DDR sirens had regular square rotor blades, but the stator ports were round.

Those IMI sirens are quite effective, but I assume that if the stator ports were square, they would have been much more effective. I wonder what would be the effect of round shaped rotor blades running in the same siren.

I think that the blade's edge (rotor), which is in the rotation direction, (the leading one) should have the same angle whith the stator's edge, so as to achieve the highest possible compression.

[CABLEVision]

Erik Greens mini T-Bolt has round ports and seems to work efficiently. It has the same rich sound the regular 1000T has. But it seems like if it was "the way to go" federal, sentry, and asc would be using it.

[Travis]

Erik Greens mini T-Bolt has round ports and seems to work efficiently. It has the same rich sound the regular 1000T has. But it seems like if it was "the way to go" federal, sentry, and asc would be using it.

It probably has to do with production costs. Whatever is cheaper and works.

[SirenMadness]

The larger both the rotor and the stator ports are, the louder your siren will be. Round ports give you a more mellow sound, I'd say, because the volume of air expelled out of the stator is rather small in most cases, and because the outburst of air is not as sudden as in a square or rectangular port, so the square wave produced would probably be a bit more smoother around the corners than that produced by regular mechanical sirens, though at such a high speed, the timbre wouldn't really make a difference to the human ear.
However, in pneumatic sirens, the smaller the port, the better, because the power of the blower is outputted most efficiently, whereas in a rotor, the smaller the ports, the smaller the sound output is, because a rotor will not try to force more air out than what can naturally go out at such and such a specific speed-to-output-area ratio, so the vacuum at the intake will most likely be smaller by a little.

Darn, I guess I wasn't the only one to have thought of the idea of using angled blades and ports! Whereas the design would most likely be very good in use, I don't know about the manufacturing costs and methods.

[Robert Gift]

Ideally, I presume one wants to produce a square wave.
I assume a square or rectangular port would produce more of a square wave by creating a greater expellation of air more suddenly.

A round shape allows a little air to be released as the opening begins to open.
Then the opening increases to max, then begins to decrease to closure.
Straight-walled stator ports would be instantly 100% open at the beginning and 100% closed at the end compared to round openings.

The openings of the siren orestis kindly provided, though angled, open and close "instantly".
Would be interesting to study the aerodynamics of the air through the ports. My guess is that it is a more ideal and superior design.

[Daniel]

I suppose for the ultimate "round port" siren, one could put a flat metal chimney-type damper inside of a pipe and spin it at high speed while blowing compressed air through the pipe. Perhaps a large, rotating ball valve could do this, but without that instantaneous "snap" of a large surface area port suddenly opening and closing, the sound would be more muffled.

[acoustics101]

I have even entertained the idea of building the chopper of a siren based on the old optical soundtrack principle on film, etched from a sampled waveform. For any high frequency resolution the chopper would need to be large and the slot width to the horn small. A slot width of .25" x 8" should give good results if the chopper speed was high enough. It's analogous to the gap width on a tape head vs tape speed.

If you made it too wide you'd limit the high frequencies and if you made it too narrow you'd limit the output (less air flow). Making the slot long would give it both high frequency and high output capabability. You could then have a completely mechanical siren that could make a vowel sound. If there was a way to make such a chopper based on the configuration of a belt sander, you could even have it repeat single syllable words, such as "fire".

In these days of electronic sirens I suppose most would consider all of this a waste of time.

[Robert Gift]

Yes, it is a waste of time, but fun.

I thought of making it speak using a large diameter chopper in which the shapes were formed.

Also thought of making the sound bumps on interstate shoulders say something.

I'm surprised anyone makes a mechanical siren.
Would not exponential horn speakers be more efficient - more dB/watt?

Thank you.