Re: DIY rotary woofer

I'm sure you could build something but I imagine most of the research goes into the aerodynamics of the rotor. A dc motor being fed the ac signal or a servo would do the trick I'd think.

 

Re: DIY rotary woofer

There was a start of a diy rotary sub on the diyaudio.com forum a while back. I don't know if it was ever completed.

 

Re: DIY rotary woofer

Doing a DIY version (COPY) of the rotary sub is a grand waste of money and time. You have to have connections with people who can custom manufacture a few parts and understand fully all the inns and outs of the design(I am sure many think they do but have little idea).

Waste of time and doomed to fail,unless you are ready to invest serious time and money.

 

Re: DIY rotary woofer

Here's a link to one type that was not finished.

http://www.betteraudio.com/geolemon/servoproject/

 

ok, it looks like they use a voice coil mounted in between the motor and the rotor to control the blade pitch. I think I could make the linkage, fans, and hook up the motor but I don't know about the voice coil as I've never taken one apart. My questions(and possible deal breakers) are:

1. Would a voice coil taken from a large speaker be able to exert the force required to move the blades? This will exert more back pressure on the coil than just a cone and I wouldn't want to design and build everything it just to find that I would need a high powered custom voice coil that would be inhibitavely expensive.

2. In order to mount the voice coil directly in between the motor and the rotor, you would need to run the shaft directly through the center of the coil. Has anyone ever tried to drill a hole through a speaker magnet?

Another option that might be a little easier would be to mount the voice coil on the rotor itself. You'd need a slightly bigger motor since you now have more mass to rotate, an it would probably take longer to turn on, but since the rotor spins at a constant speed, this shouldn't really cause a problem. In fact, it might help even out the rotational speed of the rotor as it would have more mass.

 

I can't help you with any of your questions. If you're interested in this, then you definitely have some research to do.

 

For one thing you are going to need to figure out how to pass the electrical connection from the amp to the spinning voice coil assembly as you obviously can't just use wires. As for the motive force to turn the fan blades I imagine you will have to research and test this though a powerfull enough motor should do the trick and you can always design the linkage assemblies to amplifiy the tourque. As far as drilling through the magnet.. I dont see the need. Why not just connect everything on the 'top' of the motor? You'll also want to make sure that the forces exerted on the linkage assembly is in line with the motion of the motor so you don't cause it to rub and damage itselft. In all this sounds like a pretty complicated project!

 

It is a very complicated project. http://www.hawthorneaudio.com/forums/viewtopic.php?f=2&t=1454&start=615

 

I've decided I will be building one. It's definitely not doomed to fail - I have a firm grasp on the concept and the actual mechanics of it aren't very complicated. The $14k is paying for the R&D and the fact that they developed the idea. More power to them.

Replace the CNC'd aluminum fan housing with MDF and plywood layers, and your diy costs go way down. This will be fun.....and possibly game changing.

 

This is definitely one thread I will follow closely.
May the DIY Deities smile upon you in your endeavours!

 

This will be interesting. Can you explain the mechanics? What will you use to change the "pitch"?

 

Interesting in deed.

Dont the commercial ones run of 3 phase power though, for a quieter smoother and more efficient motor? If so, how will one tackle this issue. How would the right rotational speed be determined. I think they also need an IB installation, is this correct? I'm sure I also read they need to fire into a chamber to act as a filter, has anyone figured how best to design that yet.

Steve, good luck with this one, if you pull it off I'll worship you forever :bigsmile:, and possibly grovel at your feet for the design info :R.

Here's hoping this can be pulled of, a game changer indeed :T

 

There better be some pictures forth comingaddle:

 

This will be interesting. Can you explain the mechanics? What will you use to change the "pitch"?

Sure - or at least how I see it. A motor turns a shaft connected to the blade housing. The shaft runs right through the center of a conventional driver magnet and voice coil. On a section of the shaft that is grooved length wise (I call it a slip shaft), it is connected to a grooved flange. The shaft needs to rotate the flange, yet the flange needs to be able to move forward and back. The interlocking grooves allow for this. The flange will therefore rotate, and it is pushed forward and back by the voice coil. The voice coil does not rotate (it can, but then you have to purchase expensive rotary electrical connections), so it is connected to the flange with a bearing. The bolts extending from the flange are connected to the blade arms via a socket joint, or something similar.

The voice coil is fed a standard amplified audio signal and operates just like it does in a standard driver. When it moves forward, it causes the bolts on the flange to twist the blade arms, changing blade pitch. The modulation of pitch is the frequency, and the amount of pitch is the amplitude. The speed of the fan is fixed during operation, but it would be adjusted at first to set a baseline amplitude level, so you kind of have two gain controls. The blades are perfectly flat and have no pitch at resting state (no bass signal). The blades move inward when the voice coil extends, and outward when the voice coil retracts.

A member at AVS just purchased one and has shared a lot of pictures. Additionally, Soho seems to have already understood the operation of this device. I combined the two and fiured out what the rest of the components would need to be to operate in such a manner. The attached picture I made will break it down to make more sense.

 

What will you use for the bearing Steve. Is there potential for the bearing to create its own noise. I wonder if it would be possible to create some kind of non contact floating bearing using magnets, to make it completely noise free. It might be an issue considering the VC being close etc, and it simply might not have enough control.

What do you think? How can you ensure something like a normal bearing is quiet enough?

 

Moonfly, the drive and motor will be 0.5hp, 120V, 3 phase. The drive is single phase input, 3 phase output. You set the speed as a baseline amplitude. You need to separate the front of the fan from the back with a wall so as to not cancel the output. The guy at AVS killed two birds with one stone by building a 4' x 4' x 8' enclosure in his basement as an intermediary stage, mounted the fan at the back of it, had the front of it feed into the listening room (opening covered with acoustically transparent cloth), and lined the enclosure with batting. This intermediary enclosure lined with batting significantly attenuates the self noise of the fan and is a much better implementation that what I have seen from other owners.

 

The bearing won't exactly be load bearing :heehee:, as the shaft is connected to the flange. The bearing is just there to prevent the transfer of rotation to the voice coil. The bearing should be silent.

It will just be a standard ball bearing like this, although I'll probably get a sealed one. With all the air flow, you don't want dust to accumulate in the grease of the bearing and gum it up.

 

Cheers Steve :T

If this project works out, this is going to be amazing. You could see the birth of a new area of DIY, with potential for huge savings for DIY'ers.

One other question, where are you going to source the VC from?

 

I'm hoping FiCar can help me out considering they do some customization. I need a high Bl driver with a large diameter pole vent.

 

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

 

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:

 

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?

 

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.

 

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?

 

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.