FS Modulators

[ACAP10]

I saw my first Modulator today at Chicago Midway Airport. Let me just say the things are absolutely GIGANTIC! For those of you who have never seen one before, the individual cells ARE NOT the size of donuts! I'm still dumbfounded by how big it was, and it was only a 4 cell model.

But it got me wondering, why is it that most Modulators are mounted so low to the ground. We see some mechanical sirens mounted 50 ft. up, but this one was only about 20 ft. up. Also, how exactly do they work? Does the sound move up towards the sky or straight out of the cells?

Very interesting sirens...it's a shame they aren't as reliable as the older FS sirens.

[acoustics101]

FS Modulators operate as a stacked phased array, analogous to a stacked antenna array. Due to this, the more elements in the stack, the more flattened the vertical dispersion pattern throughout the horizontal plane and the higher the directivity index. Each cell of the stack consists of a 360 degree radial horn driven by multiple compression drivers.

[Mr.Thunderbolt]

I saw my first Modulator today at Chicago Midway Airport. Let me just say the things are absolutely GIGANTIC! For those of you who have never seen one before, the individual cells ARE NOT the size of donuts! I'm still dumbfounded by how big it was, and it was only a 4 cell model.

But it got me wondering, why is it that most Modulators are mounted so low to the ground. We see some mechanical sirens mounted 50 ft. up, but this one was only about 20 ft. up. Also, how exactly do they work? Does the sound move up towards the sky or straight out of the cells?

Very interesting sirens...it's a shame they aren't as reliable as the older FS sirens.

So you found it too? Yeah, there pretty big. Never found it until I go interested in sirens, and I go to Midway alot!

[ACAP10]

FS Modulators operate as a stacked phased array, analogous to a stacked antenna array. Due to this, the more elements in the stack, the more flattened the vertical dispersion pattern throughout the horizontal plane and the higher the directivity index. Each cell of the stack consists of a 360 degree radial horn driven by multiple compression drivers.

Wow thanks for the info! You sound like you designed it yourself

Mr. Thunderbolt, I'm at Midway all the time too, but never BEHIND the terminal. I'm always dropping people off or getting picked up. Today we had to go to the rental car park and on the way out I looked up and there it is and I was dumbfounded by the size. Very weird looking siren.

[Federal Signal fan]

Dear ACAP10,
I believe if you look around on Google Patent search you'll find a patent for the Federal Signal Modulator. It will give you information and diagrams of it.

Sincerely,
Josh B.

[Oldiesmann]

There are two of those near where I work and I will agree that they are quite large.

[holler]

They are not little.

[piski125]

I live near a few 4-celled Modulators. They have a lower tone then I thought they would have.


But you're right-they are mounted low to the ground. And they are big.



The bottom cell has no speakers in it(which is why you see no maintenance doors on it). I believe it helps direct the sound from the second to bottom cell outward better.

[Joey P 3]

There are modulators along Atlantic City's boardwalk, and they as well, are about 25 ft off the ground, and i mean right on the boardwalk. I've stood next to a WPS 2805, and the 3 cell(2 active) mod's in A.C. are almost the same height.

[acoustics101]

The reason the FS Modulator sirens are as big as they are is that a 360 degree radial horn must have dimensions to its center as large as a standard 60 or 90 degree radial horn for a given low frequency cutoff. Just one of these "omni" horns is thus as large as four to six standard radial horns of the same cutoff frequency! To make them even larger, these "omni" horns are arranged in a vertically stacked phased array!!
http://www.federalwarningsystems.com/pr ... p?prodid=2

Whelen's WPS-2900 series is another example.
http://www.whelen.com/outdoor/omnisirn.htm

Vertically stacking an "omni" horn array increases the directivity index throughout the horizontal plane by limiting the vertical dispersion. The more units/stack, the tighter and narrower the vertical pattern. This makes the array loudest at the elevation of the center of the stack, with nulls on axis (directly above and below the array).

Because of this, a vertically stacked array will sound louder from a distance closer to the horizontal plane of radiation than from directly beneath. This is how they can get by with mounting these units as low as they do, although I would still recommend a minimum mounting height of 50 feet myself.