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

[va_nuke_pe]

[A [b]new standard[/b] such as a 70 dB or 80 dB radius/HP (or watts) [b]needs[/b] to be established. For watts, simply multiply HP by 746. This would be much more meaningful than the currently used [i]and long outdated[/i] rating in dB at 100 feet. This new rating would automatically take factors such as atmospheric absorption loss into account and give you a much better picture of the unit's real world performance.

And exactly HOW would a new standard take into account atmospheric absorption, terrain effects and weather effects. The reason that dBC at 100 feet is used is because it is a known and fixed value that is corrected for in the near-field by spherical divergence - 6 dB/DD.

Atmospheric absorption due to change in temperature and humididity doesn't have much effect on range for a given siren model operating at a given frequency. However, it can be a large effect on range when comparing one siren model (say one that sounds at ~800 Hz) compared to another model (say that operates at ~500 Hz) that have the same or similar dBC at 100 feet ratings.

How far a siren will sound depends on atmospheric conditions - wind speed and direction, atmospheric stability and turbulence. All you fire men out there know the conditions that make smoke plume rise and disperse and that make a smoke plume stay concentrated close to the ground. The same thing for sound waves - atmospheric stability can either disperse sound relatively quickly or keep it close to the ground. Range is reduced 20% in bright sunny no- to low-wind conditions (that's an area reduction of 36%) compared to stable atmopheric conditions. By the way, if you look at some standard methods used to compute range (e.g., ISO 9613:2), it doesn't account for atmospheric stability effects.

Wind speed and direction also have a big effect on sound propagation - especially in the upwind direction. A steady wind of 10 mph will reduce siren range by ~30% in the upwind direction. Again, some standard methods used to compute range (e.g., ISO 9613:2) doesn't account for this effect.

Terrain has a large effect on siren range as well - not only due to elevation changes but the ground covering as well. Again, some standard methods used to compute range (e.g., ISO 9613:2) doesn't account for this effect because it is only good for ground that is essentially a plane. Forested areas have a bid deadening effect on siren range. Surprisingly, not for the reason you would think - viscous drag on the leaves tends to be very low for the ~1.5 to 2 foot wavelengths that sirens typically produce. However, the large amount of dead leaves on the ground is a very large absorber of sound and the scattering introduced by trunks and branches can also have a significant effect.

Then there is the issue of how things are rated - is it based on peak measurement (which may be an artifact of the time response of the microphone), average measurement, minimum steady level, what? If you use peak measurement, then you typically will get anywhere from a 2 to 5 dB "boost" compared to a weighted average, for example.

The standard you propose is subject to the same or more "gaming" as what is used now. Sound propagation outdoors is not simple to predict. It requires understanding of basic principles to be sure; but it also requires understanding of real-world effects. No standard will address all conditions to which people may think it should be applied. And over reliance on, with lack of understanding of, what the standard covers inevitably leads to poor design, maintenance, testing and operating practices.

For example, all you folks in tornado country, is there a prevailing direction from which tornadoes come for your community? Are those areas upwind from the closest sirens? If that is the case, then that portion of your alerting area should have sirens right at that edge of the warning area. That part of the system should also have "booster" omni's to get the general sound level up. What are the weather conditions that most correspond to tornado conditions? Then your siren system should be designed based on the most likely weather conditions that correspond to what they have to alert people about.

[Robert Gift]

... Atmospheric absorption due to change in temperature and humididity doesn't have much effect on range for a given siren model operating at a given frequency.

Interesting.
I thought humidity and air temperature affected sound propogation.
Colder air would conduct better.

... Range is reduced 20% in bright sunny no- to low-wind conditions (that's an area reduction of 36%) compared to stable atmopheric conditions.

I would expect the oppositeffect.

Wind speed and direction also have a big effect on sound propagation - especially in the upwind direction.

I know it happens but do not understand why. The [eastern] movement of the air is so minute compared to the speed of the [west propogating] sound wave.

... Surprisingly, ... viscous drag on the leaves tends to be very low for the ~1.5 to 2 foot wavelengths that sirens typically produce. However, the large amount of dead leaves on the ground is a very large absorber of sound and the scattering introduced by trunks and branches can also have a significant effect.

I expected flat leaves to reflect and absorb less than grasses and bushes.

Then there is the issue of how things are rated - is it based on peak measurement (which may be an artifact of the time response of the microphone), ...

The mass of the microphone diaphragm?

Thank you.

[ver tum]

My first Doss Highschool Thunderbolt thread proves that colder air definitely makes higher frequencies travel for less distance. http://airraidsirens.com/forums/viewtopic.php?t=3106

Listen to the recording made in colder weather, then compare that to the one made during the tornado warning, where it was warm, and very humid. There's a world of difference in the sound. I think that the cold weather recording is on the second page.

As far as DB at 100 feet is concerned, it doesn't mean that much to me by itself if I'm trying to figure out how far the siren's sound will carry. You definitely have to take the siren's pitch and resonance into account. Most of the time, lower pitches carry a lot better than higher ones. However, if the tone doesn't resonate in the projector or speakers well, I think that that can limit the distance that a siren can be heard. That's exactly why Whelen went with the slightly higher 560 HZ Alert tone for their newer 2910's and 4004's, instead of the 465 HZ Alert tone. If the tone resonates more, the higher harmonics aren't taking up as much energy. Therefore, the original frequency of the tone is louder.

I believe that in an electro-mechanical siren, the key is to get the rotor to produce a fairly low tone that will resonate in the projector. Maybe, the frequency of the siren should be a factor in the design of the projector.

[va_nuke_pe]

[quote="Robert Gift"][quote="va_nuke_pe"] ... Atmospheric absorption due to change in temperature and humididity doesn't have much effect on range for a given siren model operating at a given frequency. [/quote]Interesting.
I thought humidity and air temperature affected sound propogation.
Colder air would conduct better. That is true. But the difference in range due to terrain, wind speed and atmosheric stability is much greater.
[quote="va_nuke_pe"]... Range is reduced 20% in bright sunny no- to low-wind conditions (that's an area reduction of 36%) compared to stable atmopheric conditions.[/quote]I would expect the opposite effect. Air is heated by conduction and convection from the ground. Air near the ground is warmer than the air above it under direct sunlight conditions that warm the ground. The speed of sound through a medium is inversely related to the density of the medium and warmer air is less dense than cooler air. That means that the bottom of the sound front travels slightly faster than the top of the wave front which results in the sound front being curved upward away from ground level. This effect starts becoming significant beyond about 1 mile or so. [quote="va_nuke_pe"]Wind speed and direction also have a big effect on sound propagation - especially in the upwind direction.[/quote]I know it happens but do not understand why. The [eastern] movement of the air is so minute compared to the speed of the [west propogating] sound wave. It is a vector addition issue. The sound is already starting to curve away from the earth for the reason delineated above. The wind vector accelerates the rate that the sound is curved upward in the upwind direction. [quote="va_nuke_pe"] ... Surprisingly, ... viscous drag on the leaves tends to be very low for the ~1.5 to 2 foot wavelengths that sirens typically produce. However, the large amount of dead leaves on the ground is a very large absorber of sound and the scattering introduced by trunks and branches can also have a significant effect.[/quote]I expected flat leaves to reflect and absorb less than grasses and bushes. As I said, surprisingly. There are studies that show that leaves can actually focus sound and decrease the rate of attenuation in the 250 Hz octave band and below. The effect is not much on 500 Hz sound and doesn't really have a big effect until the wavelength is about the size of the leaves - 2000 Hz and up. There have been several recent studies that show that scatter from trunks and branches is a bigger factor. [quote="va_nuke_pe"]Then there is the issue of how things are rated - is it based on peak measurement (which may be an artifact of the time response of the microphone), ...[/quote]The mass of the microphone diaphragm? I am not an expert on instrumentation, however, if you are taking "fast" measurements (1 millisecond) with a fast ramp-up characteristic of electronic devices, the measured rate may be subject to overshoot.

Thank you.[/quote][b][/b] You're welcome.

[Robert Gift]

Why is quoting not working above?
Can a moderator fix it and delete this message?
Thank you.