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

[acoustics101]

Here are two websites that will provide answers to some of the questions:

http://www.dogstar.dantimax.dk/acoustic/acoust-1.htm
This website explores the world of psycho acoustics. Many of the ways we perceive sound are based on auditory illusions.

http://www.media.uio.no/personer/arntm/Testsounds.html
I recently discovered this neat website. I now have problems hearing tones of 15 kHz and above. At 58 I guess that's not too bad.
The biggest limitation to hearing the tones below 100 Hz are your
computer speakers. If possible, patch your computer's audio output
through your stereo or listen through a good set of headphones. If loud
enough you can hear pure tones down to 25 Hz without much difficulty,
although very few speakers can accurately reproduce them.

[acoustics101]

Your statement of low frequencies cutting through the air better is basically correct. It is the phenomenon known as atmosperic absorption. In the case of higher frequencies, it is not the pressure, but the higher frequencies of the shorter wavelengths of sound being more readily absorbed with distance. It is analogous to the atmosphere also absorbing the shorter wavelengths of light, resulting in the reddening of the sun's light during sunrise and sunset, when it passes through more of the atmosphere. The lower frequencies will always propagate further with less loss, but a point will also be reached where the ear's reduced sensitivity to them will make sound signals below 200 Hz impractical.

At a hundred feet away from itself, the 2001 SRN-B is the best, because a high pitch has a higher pressure in a specific space, than a lower-pitch siren, such as a T-128, assuming that you pump the same amount of energy to each type of wave; the higher the pitch, the higher the concentration of the waves. However, at a long distance, the rotor of the T-128 would perform more efficiently, because you let the length and lower pressure do the work, by cutting through the air easier. In contrast, it is also probably the high pressure of higher pitches that reduces their length of travel, because of the resistance that is caused by the pressure of the sound against the medium of conducting.

[acoustics101]

http://www.sentrysiren.com/pdf/Web_40V2T.pdf
As far as total radiated output goes, it would be very hard to beat Sentry Siren's 40V2T 40 HP 130 dB rated omnidirectional siren. That's 5 dB more output than American Signal's 50 HP 125 dB rated Cyclone siren.
http://www.americansignal.com/PDF/Cyclone%20C-125.pdf

The higher output at a lower HP is achieved by the 40V2T's spaced dual phased array of radiating horns as opposed to the Cyclone's single array. It's analogous to stacking arrays of loudspeakers or antennas to achieve a higher effective radiated power. You get more dBs at the expense of a flatter horizonal plane of radiation. Translation: a higher directivity index

[jmev]

And any of this matters because of.......

[Robert Gift]

And any of this matters because of.......

I'm curious to knowho makes the most noise for least horsepower.
Who has designed the most efficient rotor/stator and possible projection system to yield the most dB/Watt.

[va_nuke_pe]

The metric being proposed is not a good comparison basis. As previously pointed out, the alerting frequency has a big effect on range and it takes less power to put out a higher frequency (at least for electro-mechanicals in any case). For example, for a typical summer day, attenuation due to atmospheric absorption of sound is about 2 dB per kilometer higher at 800 Hz than at 500 Hz. That means that a siren at 800 Hz (for example a 2001-130) that is 2 dB higher in sound output than a siren that is at 500 Hz (say a T-128) will sound equally loud at 1 km away and not be as loud beyond that.

That is also why dual-tone is used for electro-mechanical sirens, it is a compromise between power required and range obtained for the maximum dB at 100 feet.

Second, and more importantly, dB is not a linear measurement, it's logarithmic. A 3 dB change corresponds to a 50% change in sound pressure level. So, dB per watt is not a good measure to compare one siren design to another.

Third, for an electro-mechanical device, using the name plate rating of the motor doesn't necessarily reflect the actual HP (or watts in metric) at the steady state operating point for the motor and its associated rotor assembly. Even for electronics, the dB output versus frequency is not a linear or even necessarily a monotonic (meaning it can be a curve but it only curves in one direction) relationship.

Fourth, it could only be used for alerting tone. Many siren signals are wail or attack signals, which vary frequency and use different methods of accomplishing the variation in tone.

All that can really be said truthfully is that for a given output (defined by dB AND frequency), electronics are generally more efficient (require less electricity consumption) than electro-mechanicals.

[JasonC]

And any of this matters because of.......

Hey...it's a whole lot better than those "OMG!!! A TBOLT!11!!!" posts. It's just how those crazy engineers like to talk

Anyways, I wish I could add more to this but I haven't had any classes yet that go into sound dynamics for any good length of time.

[Robert Gift]

...All that can really be said truthfully is that for a given output (defined by dB AND frequency), electronics are generally more efficient (require less electricity consumption) than electro-mechanicals.

Thank you.
Though I have no interest whatsoever in electronic sirens, that is what I expected.
I'm talking dB at 100 feet.
Not real-world attenuation due to frequency atmospheric absorption.
I assume the Alerter loses dB output because of cancellation in its stator plenum. So I want to compare dB/Hp among different makes.

Still curious if a dual-tone siren's lower resultant carries further.
The 10-12 2t22 produces 575/690 Hz with a 115Hz resultant.

[acoustics101]

A new standard such as a 70 dB or 80 dB radius/HP (or watts) needs to be established. For watts, simply multiply HP by 746. This would be much more meaningful than the currently used and long outdated 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.

As far as electronic sirens are concerned, the total efficiency includes the efficiency of the amplifier as well as the speakers. A low efficiency amplifier would greatly reduce the overall efficiency. The actual efficiency of an electronic unit vs a well designed mechanical unit need not be all that different. Stacked phased arrays increase the effective radiated power by increasing the directivity index, thus increasing the dB rating for a given power usage. To convert watts to HP, simply divide by 746. Sentry Siren's 40V2T unit is proof that it is also possible to use a phased array in a mechanical unit.

It is easy to maintain phase in a speaker array. In my opinion, a directivity index beyond 13 dB (20:1) or so is undesirable, as it restricts the sound propagation to an overly narrow beam (or horizontal plane in the case of so called omnidirectional units). This is not actual efficiency in terms of total radiated power, but simply a way to increase the SPL per watt, albeit over a narrower angle. Some electronic sirens have a directivity index as high as 20 dB (or 100:1)! Overly narrow dispersion causes skip, dead areas over varying terrain and shorter periods of audibility during rotation.

All that can really be said truthfully is that for a given output (defined by dB AND frequency), electronics are generally more efficient (require less electricity consumption) than electro-mechanicals.

[acoustics101]

The new standard I'm proposing would be stated in square miles/HP (or square kilometers/kW), based on a 70 or 80 dB radius. Once the specifics of the standard could be agreed upon (whether English or metric and based on a warning radius of 70 or 80 dB), it could then become the new universal standard for rating all large scale warning signals. You could figure the coverage area by the formula A = pi x radius squared.
It would be much more meaningful than simply knowing the SPL at 100 feet, as it is really the far field performance that counts.

A new standard such as a 70 dB or 80 dB radius/HP (or watts) needs to be established.