Why do many sirens have cone into air intakes?

[Robert Gift]

On eBay I have noticed many small sirens; auto alarm sirens, some old fire engine sirens and others, which have a flared cone into the FRONT opening.
Instead of: []D it is: >[]D with stator ports uncowled.

Does enough sound emerge from the rotor intake for a cone to be.neficial?

Or is this justradition -for looks, fooling everyone into thinking it is amplifying and trumpeting sound forward?

Years ago I gave a fellow volunteer firefighter a white siren,
probably from a military ambulance, which had a bell into the air intake,
and also a sound reflective cowling around the stator ports: >)D

Doesn'that make much more sense?

Thank you,

[Nelso90]

With stationary sirens, it is a horn, like on the Allertor, as sound comes out that horn as well. It gives it a throatier quality, and serves as a venturi, to speed up the air. On vehicle sirens, it does this and serves as a Ram-Air duct, to force more air through, and thus make a louder sound.

[Robert Gift]

I would love to knowhat % of sound comes from the horn compared to the output from the stator ports.

I presume the stator is many times greater.

Would it not be.tter to amplify the stator output, as the FS 2001 does?

Yes, I thought abouthe ram air issue.
Would these sirens generate some sound just driving down the street?

Thanks,

[kx250rider]

I too am a bit curious as to why there is sound coming from the intake. I am weak on the theory & engineering, but I do know that sound waves are loudly emitted from both the intake and the chopper. I first noticed this when I removed the air box and hoses from a turbocharged car, and noticed a VERY loud increase in compressor whine. Almost equal to the exhaust turbine whine when there is no muffler.

I know JUST the person to ask... I know a fellow with a strong background in acoustical engineering, so I will try to get a "plain English" answer on this.

Charles

[StonedChipmunk]

Would it not be.tter to amplify the stator output, as the FS 2001 does?

Yes, that was what the military ambulance you were talking about. Think of a FS Model A (this might have been the siren you were actually talking about): http://www.federalsignal-indust.com/def ... &lookup=29

Yes, I thought abouthe ram air issue.
Would these sirens generate some sound just driving down the street?

Actually, they would, unless they had a brake on them. Most sirens shaped that way do (newer ones, at least) that were made to be mounted on vehicles. Because think... if you have a firetruck going on a non-emergency call (no lights/siren... just imagine here) then if it was going around 45 MPH then you would be looking at around 45 MPH of wind coming into the cone of that siren... plus you have the wind pressurized by the cone itself and the force placed upon it, causing more air to enter. This would surely get a siren going by itself, which is why to prevent confusion many vehicular siren manufacturers have a brake on their sirens to prevent them from spinning while the vehicle is just moving.

[StonedChipmunk]

Would it not be.tter to amplify the stator output, as the FS 2001 does?

Yes, that was what the military ambulance you were talking about. Think of a FS Model A (this might have been the siren you were actually talking about): http://www.federalsignal-indust.com/def ... &lookup=29

Yes, I thought abouthe ram air issue.
Would these sirens generate some sound just driving down the street?

Actually, they would, unless they had a brake on them. Most sirens shaped that way do (newer ones, at least) that were made to be mounted on vehicles. Because think... if you have a firetruck going on a non-emergency call (no lights/siren... just imagine here) then if it was going around 45 MPH then you would be looking at around 45 MPH of wind coming into the cone of that siren... plus you have the wind pressurized by the cone itself and the force placed upon it, causing more air to enter. This would surely get a siren going by itself, which is why to prevent confusion many vehicular siren manufacturers have a brake on their sirens to prevent them from spinning while the vehicle is just moving.

[Robert Gift]

When I have a chance, I'll slightly rev our engine's Q2b and listen and compare the difference.
Wish I had a dB meter.

Recently, a firefighter pressed the switch and reved it up much higher than intended.
Everyone was annoyed, especially since the siren has no brake.

I folded my coat and placed it against the front to seal off air input.
That cuthe sound output WAY down and I did not remove my coat until
it was a low growl.

[StonedChipmunk]

Haha, loser. Hope he gets fired.

Compare the difference between what?

[Robert Gift]

Would it not be.tter to amplify the stator output, as the FS 2001 does?

Think of a FS Model A (this might have been the siren you were actually talking about)

Yes! Thank you, Chip.
Same design, but case and reflectors seemed not as heavy and in white.
24 volts which I rewired to operate on 12. (Can't remember what I did.)

Wish I could ask the volunteer firefighter who put it on his "light bar"
in 1976. On the top of his vehicle it would receive ram air.

Actually, they would, unless they had a brake on them. Most sirens shaped that way do (newer ones, at least) that were made to be mounted on vehicles. Because think... if you have a firetruck going on a non-emergency call (no lights/siren... just imagine here) then if it was going around 45 MPH then you would be looking at around 45 MPH of wind coming into the cone of that siren... plus you have the wind pressurized by the cone itself and the force placed upon it, causing more air to enter. This would surely get a siren going by itself, which is why to prevent confusion many vehicular siren manufacturers have a brake on their sirens to prevent them from spinning while the vehicle is just moving.

Huh. I never knew they had a brake that was passively engaged while not in use. Thought it was only a stopping brake activated by solenoid.
Would that not cause a faster run down, or does it somehow engage only after stopped?
But withe stator reflector also gathering air, do they equalize enough NOT to spin the rotor sufficiently?
Thank you,

[Robert Gift]

Haha, loser. Hope he gets fired.

Compare the difference between what?

Sorry Chip. I was away and then posted after your 2nd question.

Don't be so hard on everyone.
He is a jolly volly just like me.
He merely meant to "get" me because I wastanding right in front of the Q (parked in the station)
Something I would have done to him - but I would just tap the switch.

I want to compare sound levels at front intake and side of the Q.
I expecthe stator side to be MUCH louder than intake.

My original design for a "Q" would be to have the intake from the rear
where it would help cool the motor (if that was a concern) and lessen water, slush and dirt entering as it does from the front.
Then place a big red light ([]D where the front intake would have been.