Question for the acoutic engineers

[holler]

I've been pondering this for a while. With a dual rotor siren that has the same port ratio on both rotors/stators, which arrangement is better, two motors each driving seperate rotors or both rotors on one motor.

Reason why I'm asking is I've always been curious as to whether or not the dual motor setup has more cancellation than the singe motor setup with both rotors turning the exact same speed.

Videos for reference: 8 port dual rotor siren with two seperate motors


https://www.youtube.com/watch?v=zCAEDLHPtxs

8 port dual rotor siren with a single motor


https://www.youtube.com/watch?v=zeQhQuS1714

[landmobile]

A good question. Lots of equipment has been designed with dual power sources over the years (buses, boats, sirens) with varying degrees of success. The toughest part is getting the motors to work well together.

All sirens will have small variations in bearing friction, winding resistance, air flow, and manufacturing tolerances. Because of this, two separate identical motors, running from the same power source, driving identical rotors will not run at exactly the same speed. This can be heard quite clearly in the first vdeo, as a "flutter" or "beat freqency" which is equal to the difference between the pitch (speed) of the two motors.

Even if the motors were, by chance, turning at the exact same speed, they are not necessarily turning "in phase", which means that one set of ports might be open while the other is closed. This "out of phase" operation results in sound waves that might cancel each other out depending on the listener's position.

When a single motor drives two rotors on a common shaft, they must obviously turn at the same speed. Since the port ratios are the same, the frequency produced by each rotor must be the same. In addition, since the rotors are fixed in their position relative to each other they are also "phase locked" which means that the sound waves produced by each chopper will leave the siren as the design engineer intended, regardless of speed.

As far as which design is better, I really don't know. Since I'm an electrical engineer, not acoustic, my description of the two designs only explains what is going on from a basic physics point of view. The cancellations inherent in a dual motor might be preferable or detrimental, depending on the desired sound. Hopefully some of the acoustics folks on the board will offer a more detailed analysis.

Chris

[coastalsyrolover]

All I can tell you is that studio engineers have a guitar player record their parts twice, both times going on separate tracks, and then they get panned to the left and the right. The reason is to build stereo. If you just copied and pasted the guitar like on 2 tracks and panned them the sound would just cancel out and you would end up with no better quality than when you started. The reason you record it twice is because of the slightly early or late strums, the slight pitch and the slight volume differences going to 2 sides. Now going back to the copy and paste thing you could knock it off and it sounds better... But not best. I would imagine a siren running on one motor with both choppers in sync or even just slightly off is like the copy and paste and having 2 motors is like the recording it twice.

[SirenMadness]

Don't take my word for this, but I'm thinking the "beat" effect is for the purpose of better getting the attention of people across a wider range of effective hearing conditions, where some might be more in tune with a certain phasing than others.

As for cancellation, I'm sure there are instants where the two rotors are pulsing air 180* apart from one-another and maximum nulling occurs, but I think it's that fluctuation between the null and the "sweet spot" that grabs attention.

[landmobile]

... I'm thinking the "beat" effect is for the purpose of better getting the attention of people across a wider range of effective hearing conditions,..

I agree Peter.

I read somewhere that the original EBS (Emergency Broadcast System) alert tone ("the attention signal you just heard would have been followed by official news, information, or instructions") consisted of two tones whose beat frequency was designed to attract the greatest attention.

The implied message (that none of us who grew up in the cold war wanted to think about) was: "the attention signal you just heard would have been followed by a blinding flash of gamma rays, followed, at 1100 feet/second, by a sonic boom.

[Adam Pollak]

The difference in frequency in Hertz between the 2 sound sources will be the frequency of the pulses. If one rotor is doing 499 Hz and the other is doing 501 Hz, then there will be 2 'pulses' per second. Once you get above a certain range, the pulses become somewhat indiscernible as other harmonics take over.

Like other have said, it's dependent upon bearings, wire resistance, minor manufacturing differences in the motors, etc.

As far as the ports being open or closed out of phase with each other, this will not affect it or sound cancellation. Sirens are not a single point source of sound. There is sound coming from each port, as well as from the intake. It would also be affected by the distance between the rotors, the listener's distance from the sirens, as well as the angles the sound is reaching you. You would never be able to find a position where the crests and troughs destructively interfere with each other to create any meaningful net effect.

[Taterworks]

I think holler's original question was regarding two rotors and stators with the same port ratio, and whether it is better to have both rotors driven by the same motor or separate motors.

In terms of achieving the highest acoustic efficiency, both rotors need to be synchronized (phase-locked), otherwise the output from one rotor will have a nonzero phase shift and summation of the acoustic outputs will not be perfect. If you have two rotors that are in phase, such that the ports open and close at the exact same time, at the same speed, then you get perfect summation, and an additional 6 dB of output from the system. If the rotors are not in phase, the phase shift means that you will wind up with something less than that 6 extra dB. The easiest way to produce a phase-locked system is to use a single motor with dual shafts, or to mechanically couple two motors together if two motors are necessary to achieve the needed power.

In the case of a dual-tone system, everything is different. You can either create a dual-tone system by adjusting the ratio of ports, or by varying the speed of one of the rotors. In that case, you get phase shift that is moving between 0 and 180 degrees all the time, with its own periodicity, and that gives you a beat frequency equal to the difference between the operating frequency of each rotor. If the rotor speeds are only very slightly different, you'll get a very long beat period, and a low beat frequency - even less than 1 Hz. Another thing you can do with a dual-tone system is that the beat frequency can be used to produce a third 'phantom' tone, adding more distinctiveness to the sound of the siren and potentially allowing the sound to carry over a longer distance due to less atmospheric loss.

Phase shift can be used to acoustically 'steer' a beam of sound by creating phase summation in one direction relative to the source, and phase cancellation in the opposite direction. Imagine a siren like a dual-head Sterling or HOR Sirex that can be rotated on a turntable. If the phase shift between the rotors is about 90 degrees at the operating frequency (achieved by installing one rotor so it is not perfectly synchronized with the other), then in one direction the output will sum to +6 dB, and in the other direction, the output will be reduced by greater than 6 dB. If you could not use an acoustic horn to form the siren beam, you could use this method to create a steered sound beam that could then be rotated around.

I hope this all makes sense, and is helpful.

[holler]

Wow, you must be a wizard or something.

So would this explain why 5/6 and 10/12 sirens seem to carry so far? For example the 2t22, which is rated for less than the STH-10 but outperforms it in the real world.

[coastalsyrolover]

And why the P-50, (Besides the fact that it's a giant 50 HP siren) the hurricane, the 5/6 1000T and the, as Jeb said, 2T22 carry so far as far as the 70 DB ranges are concerned.