[soho54]

Use more stable control arm gear to float the flange, or use a linear rotary bearing.

 

[SteveCallas]

Yeah...I was thinking the shaft rotates the flange, but it doesn't have to - the blade housing will. So I can use this between the flange and shaft and save even more money:

 

[Moonfly]

What is the size of the shaft from the fan motor, from memory, the IB motor housing has a vent of about an inch or so, perhaps slightly bigger, so that might be off the shelf for Fi if its suitably large enough.

I have a question on volume control for these devices. I take it the rpm is what dictates the spl output, and you would need a system where by your master volume control can also increase and decrease the rotor speed. The big issue here would be getting the variable rpm to increase and decrease at the correct rate so the rotary woofers spl increases and decreases exactly as the rest of your system does.

I simply might not understand this area, not sure, so how will this work?

 

[soho54]

The fan speed doesn't change. You pick a speed, and then it runs at it constantly. Think of fan speed as the rotary subs "cone" size.

SPL is a function of how large the cone is, and the amount of throw(blade pitch here) at the time.

 

[Moonfly]

Hold on a second. You say the fan speed is constant, and fan speed is representative of cone size. You you say variation of cone size (so rpm?) equals variation of spl, doesnt that translate into variation in speed equals variation in spl?

I thought the blade pitch is what altered the frequency reproduced?

 

[soho54]

Say you have the same motor on two different drivers. One cone is larger than the other. Now you play a tone on both, and have the VCs move the exact same distance. The larger cone will produce more SPL.

That was what I meant. The RPM is the cone size. The higher the RPM the more air the sub can potentially move, literally with this driver.

The blades pitch to produce frequency with their pitching period, and the SPL of that frequency with the degree of the pitch. Think of the blades as part of the traditional VC and suspension here.

 

[Moonfly]

What I am not getting here, is how the pitch of the blade can control the frequency as well as the spl output. I understand that the pitching is controlled with a VC, and the VC linear movement will tilt the pitch of the blade. I get that as the pitch changes, the frequency produced will change, (if this is indeed what you mean), but I dont get how that system can control both frequencies produced and spl level at the same time, which is how I am interpreting your explanation of how this works.

What is interesting though, is that with the relationship between rpm and effective cone area being what it is, this small blade has the potential to represent a very large cone area, which must be why such low frequencies are produced so well.

 

[soho54]

Its a little complicated to wrap your head around, until the light bulb comes on.

OK, RPM is cone area, and its throw potential.
The blades pitching are just extensions of the VC, and work exactly the same way here.

In a regular speaker the VC throw distance alters the amount of cone displacement, and thus SPL. THe farther it moves the more SPL/amplitude produced.

With the fan, the greater the VC moves the greater the blade angle. At zero signal the fan blades are flattened with the angle of rotation, and there is no air movement. Even with them turning a constant ~700RPM. As the VC starts to move the blades leading edge starts to move out of the flat plain, and begins to pull air. The great the pitch the more air the fan can move per cycle of the wave.

Once you understand that, you can see how the rotary woofer actually has more displacement the lower the frequency, once you factor in RPM and frequency cycle times. This is what makes the rotary woofer so potent down low.

Then you move into acoustics, fan aerodynamics, and stall points to explain why the max SPL doesn't increase below the RPS' frequency parallel, and where the high frequency rolloff comes into play.

 

[Moonfly]

OK, I understand how fan blades work. I understand more air moved can equal more spl. The difficult bit for me is understanding how this could play say a particular frequency, at variable spl levels. If the blades have to be at a set angle to play a particular frequency, how does the system vary the spl level of that frequency if this is also tied into blade pitch. It seem that a change in the angle of the blade, to try adjust the spl, would also then by default change the frequency produced.

The reason I figured the fan would change speed to alter spl, is that changing the speed of the fan will also change the amount of air displaced. I understand what your saying, I just cant get my head around the technology, and how it could independently control variable spl and frequency both from just the blade pitch.

Hopefully the bulb will light up sooner rather than later with this :bigsmile:

 

[soho54]

The difficult bit for me is understanding how this could play say a particular frequency, at variable spl levels. If the blades have to be at a set angle to play a particular frequency, how does the system vary the spl level of that frequency if this is also tied into blade pitch. It seem that a change in the angle of the blade, to try adjust the spl, would also then by default change the frequency produced.

Frequency is not determined by the pitch angle. Pitch angle is VC throw, or amplitude.

Frequency is determined by the amount of time it takes to to move from positive to negative VC positions/and pitch angles here +&-. Just like with a traditional driver. The throw amount is irrelevant. 20Hz is 20Hz whether the VC is moving .1cm Peak to Peak, or 8cm PtP.

 

[Moonfly]

Ah, thats where I was going wrong, misinterpreting what you were describing. All makes perfect sense now :T

Given a typical cinema system is only good to say 30hz or so, especially here in the UK, these things have got to be the new standard for theatres surely. They would work very well with current systems given they operate the 2-40hz range (IIRC).

I really want to watch this one progress now, if it works I think I'm gonna give it a shot. Everyones subs will imporve if they are only required to go down to say 20hz maximum, with a rotary doing the really touch bit.

 

[BustedCrank]

Thanks for that explanation on the rotary Steve. Ive been looking into these things for a while and couldn't quite get it figured out on how to build one. Your little explanation made the light bulb turn on up in the attic. Almost seems easy. I will be watching your progress on this, maybe this will spark everyones interest. And someday we will be building these things cheaper then buying a decent ported/sealed unit. And maybe I will build one of these instead of an IB in my future theater project.

Good luck, just wish I knew enough to lend a hand on this endeavor.

 

[Moonfly]

The rotary subs only work along side a subwoofer, they dont replace them. The rotaries only operate upto about 40hz, so you still need a subwoofer. The rotary sub is also an IB installation, albeit one a little easier to implement that a multi driver array. The upside, is that if you are running a rotary woofer, your normal sub only really need be good to about 20-30 hz, making our current subs jobs that much easier, and their resulting output that much better.

It seems a rotary woofer would essentially have the same effect on a normal sub, as the addition of a subwoofer has with your speakers. On top of that, they would essentially totally complete the frequency range meaning low end limits would be a thing of the past. Extremely attractive indeed if this is the case.

My only concern would be getting one to work with a normal IB, if both the IB drivers and Rotary woofer shared the same wall (baffle). No issue with any other type of sub though.

 

[SteveCallas]

Soho nailed it, but just to beat a dead horse, the speed at which the blades change pitch is the frequency, the amount of pitch is the amplitude. If the fan is spinning at 1800 rpm, or 30 rotations per second, and we feed the sub a 10hz sine wave, the blades will change pitch once every three revolutions. If we feed it a 30hz sine wave, the blades will change pitch every revolution. If we feed it a 1hz sine wave, the blades will change pitch every 30 revolutions.

The amplitude is controlled by how much pitch they are changing at the given frequency. If they are only twisting a tiny amount, your output won't be very high relatively speaking, and the driver motor has a really easy job, as the acceleration is minimal. But if the pitch of the blades is moving to nearly a full 90 degrees on every cycle (180 degrees peak to peak if you will), you are at the max amplitude level the device can create at that given rpm.

As alluded to, if this works out well, which I think it will, "regular" subwoofers become really easy to overkill.

 

[BustedCrank]

My only concern would be getting one to work with a normal IB, if both the IB drivers and Rotary woofer shared the same wall (baffle). No issue with any other type of sub though.

I am curious about this as well. Could you have them both in the attic, for example. Or would you have to have one in the attic and the other in the basement, or next room.

If you can use the same room, could you use the same manifold/box/baffle. Take the "DIY rotary" setup mentioned earlier, with the big box he used, could you throw a couple drivers in there for an IB setup and only use the one "manifold"?

As alluded to, if this works out well, which I think it will, "regular" subwoofers become really easy to overkill.

Looking forward to it

 

[soho54]

But if the pitch of the blades is moving to nearly a full 90 degrees on every cycle (180 degrees peak to peak if you will), you are at the max amplitude level the device can create at that given rpm.

Remember, you will never want to have them pitch that much though. The fan will not pull air at anywhere near those extremes.

It has been a few years since I hit this hard, but it seems like not quite +45deg to -45deg was about all that was possibly useful, so ~90deg PtP. Any more than that and the blades start to stall, and amplitude drops until you get no useful air movement in the forward/rear direction. Look at a cheap oscillating fan with straight blades. If a steeper angle pulled more air they would use it.

 

[soho54]

My only concern would be getting one to work with a normal IB, if both the IB drivers and Rotary woofer shared the same wall (baffle). No issue with any other type of sub though.

It will work fine. You would need to HP the IB at the RW's LP point though. This is would be for the same reasons as any other 2-way xover in a loudspeaker

If you forgo the xover you just risk possible funky phase interactions in-room where they overlap. There is not a lot you can do about rear wave bleed through with waves under 20Hz, so sharing the same enclosure space is no problem.

 

[soho54]

Delete Night all.

 

[Moonfly]

Soho nailed it, but just to beat a dead horse, the speed at which the blades change pitch is the frequency, the amount of pitch is the amplitude. If the fan is spinning at 1800 rpm, or 30 rotations per second, and we feed the sub a 10hz sine wave, the blades will change pitch once every three revolutions. If we feed it a 30hz sine wave, the blades will change pitch every revolution. If we feed it a 1hz sine wave, the blades will change pitch every 30 revolutions.

The amplitude is controlled by how much pitch they are changing at the given frequency. If they are only twisting a tiny amount, your output won't be very high relatively speaking, and the driver motor has a really easy job, as the acceleration is minimal. But if the pitch of the blades is moving to nearly a full 90 degrees on every cycle (180 degrees peak to peak if you will), you are at the max amplitude level the device can create at that given rpm.

As alluded to, if this works out well, which I think it will, "regular" subwoofers become really easy to overkill.

This is the bit I was missing from Soho's explanation. I thought he was implying the angle of pitch itself would create the frequency. No probs now anywho :T

In the video Soho posted, they note the fan only runs at about half its rated speed in order to ensure the motor can handle load changes without the rpm being affected. I think this will make motor choice important, so your experience will be invaluable for people to follow.

My concerns about mixing an IB are whether or not the operation of the IB for example, will affect the operation of say the fan motor speed. Chances are there would be no affect, not least as the 2 devices will operate in different ranges, but its something I thought about.

Have you done any work on the modelling of the chamber the rotary woofer needs fire into?

 

[atledreier]

I'd be concerned with distortion levels from the blades flexing and rpm fluctuations. As pitch increase, the rpm will drop until the frequency drive compensate. Also, as the load on the blades increase they WILl flex, even if it's only a little. With the current dynamic driver technology being as good as it is these days I'd say they would be hard to beat.

of course, inovation is GREAT, and this may well be the way forward for ultra-low frequency woofers, but this early in the game I think you can't beat the dynamic driver. Of course, you would need a great many drivers to get the samme efficiency as this rotary!