Which rotor sirens most efficient? (Most dB/watt)

[Robert Gift]

Look at the distance the sound coming out the top goes, compared to the sound coming out the bottom. That would cause some out-of-phase waves, because of the slight time delay of the waves hitting the open atmosphere. But, I don't think it's enough to make a difference. But, what would make even more of a difference, is the effect that 2 tones mixing together makes. Instead of one pure wave, you have 2 different waves. Inherently, they will cancel each other out somewhere in the waveform. Am I right?

My understanding is that two waves of unequal frequency will null and add
where pressure nodes coincide.
This will produce a resultant frequency.
For example: an E-natural and G-natural (minor third) will produce a C-natural RESULTANT just below the E.
C and E-natural (Major third) will produce a G-natural below the C.
C to G (perfect fifth interval) will create a C natural a whole octave below that 1st C.

When waves of the same frequency get reflected and become out of phase, they may null. Being the same frequency, they will not produce
resultant frequencies.

The problem (I presume) with the Allerter stator chamber may be that some cancellation will occur INSIDE the chamber. The question is: How much?

In the FedSig 2001, they have no chamber. Only an exponential flare
cleverly formed by the siren motor housing and outside projector.

I suspectheir design makes the most efficient use of the output of the motor.

[SirenMadness]

Yeah, but that resultant frequency might be caused due to cancellation, most likely cancellation of the higher waves; the lower waves have a higher capacity for energy, so the higher waves saturate their energy into the lower waves, causing them to have a higher profile and therefore a higher pitch, but not as high as the pitch that got canceled.

[Robert Gift]

Yeah, but that resultant frequency might be caused due to cancellation, most likely cancellation of the higher waves; the lower waves have a higher capacity for energy, so the higher waves saturate their energy into the lower waves, causing them to have a higher profile and therefore a higher pitch, but not as high as the pitch that got canceled.

I don't understand.

The pitches cannot change.
Other resultant pitches are produced.

Two pitches produce resultants below and ABOVE the two frequencies.

Is this lower "artificial" frequency important to siren warning because it carries farther than the higher frequencies?

I would love to know if a lower resultant tone created by two higher
pitches would carry further than either of the two pitches alone.

In recordings, I hear the "sour resultant" more than the two notes (minor or major third interval) of the dual-toned siren producing the resultant!
Is this on purpose, or accident?

If this be true, the best siren would produce a perfect fifth 8/12 to produce a resultant a full octave below the lower of the two notes.
EX: C2 + G2 = C1 which supposedly carries farther.

But wouldn't a single-toned siren of C1 be even louder and carry further
than investing energy into two tones C2 & G2?

Would love to do a sound level at a distance on a 2t22 in high and low and then both together.

[SirenMadness]

But I don't understand as to how two types of pitches coming out of one source can cause a resultant frequency without something happening to one type of pitch.

[Robert Gift]

But I don't understand as to how two types of pitches coming out of one source can cause a resultant frequency without something happening to one type of pitch.

One graphic analogy would be to have two combs with different number
of teeth per inch (representing frequency)
Place one over the other and observe the areas where the teeth coincide and where they fill in the space between teeth.

Each still retains it's same teeth, but you can see how their co-incidence
creates another series of waves.

Same can be done with a window screen, one tilted compared to the other.

[SirenMadness]

So the area of where the waves coincide is the resultant frequency?

[Robert Gift]

You're right!
Their energies add at those places.

If the difference is very slight, the resultant frequency is so low it will not be heard as a tone, but more of a "beat".
As the frequencies differ more, the resultant frequency becomes higher in pitch - up to a certain point.

If you can place two window screens against each other, you can simulate a lower "frequency" of one by tilting it compared to the other.
(until you get to 45 degrees)
Then you can see the "waves".

Better:
You can tune guitar strings and listen to the "beats" or resultant
frequency.
It is pleasing to tune one string a pure or perfect minor third
above the other.
If the lower string is C and the other E-flat a minor third above, the resultant will be A-flat, producing an A-flat - C - E-flat chord with justwo strings!

If you raise that E-flat a half step to E-natural, making a perfect Major third, the resultant lowers to G making a G-C-E chord.

I you raise that E up to G, making a perfect 5th, the resultant drops a whole actave below the C string creating a "non existant low C", middle C string and G string.

Have I made it confusing enough?
(Wish I had a guitar to check myself on all of this.)

[SirenMadness]

So, waves that are almost exactly the same would be heard as a very low frequency at a large distance?

Also, so it is only the peak of a wave that we can hear?

[Robert Gift]

So, waves that are almost exactly the same would be heard as a very low frequency at a large distance?

You would hear both tones, with another beating sound.
Now, if you travelled just beyond the audibility of the two tones,
would you still hear the beat? I believe so, because we are hearing
the SUMMATION of energy of each tone's waves when the coincide.

Also, so it is only the peak of a wave that we can hear?

Our eardrums detect the whole wave, both peak and valley, with any harmonics among them.

The magnitude of the peak to negative peak is the loudness (volume) of the sound.

[chris]

Yes, I don't get how it avoids wave cancellation either.

Why would the waves cancel? Doesn't the air pulse out of all the ports
of the stator at the same time. Wouldn't pulses have to be out
of phase to cancel the pulses coming out of the stator?

Here's how I see it. The airflow towards the intake cone is pretty even; there may be minor fluctuations but it does not pulsate violently like the flow leaving the projector cone. Some distance from the siren the intake airflow will just be a very slight breeze. I would expect the effect on the sound output to be the same as a very slight, almost imperceptible breeze. My gut reaction is that the positioning of the air intake won't affect the output much, although I can see why people think it might.

Best wishes,

Chris