AVS Forum banner

261 - 280 of 318 Posts

·
Registered
Joined
·
29 Posts
Hey guys, longtime lurker but I finally registered since I'm going to start undertaking some initial testing of a rotary subwoofer based on Chris' concept.

I've ordered basically the same flybarless rotor assembly and will be buying a cheap 12" woofer with an 8.5mm Xmax (it's really cheap so I won't mind getting another better one later if everything works out).

I will be doing initial mounting, fixturing, and balancing setup with an ~1/3hp single phase motor, then if everything goes well I have a DC brushless (48v/70A! :eek:) (~4.5 hp) motor I'm considering pulling from an electric scooter depending on whether I can find a good way to power it. I may still end up just purchasing a cheap 3 phase and VFD.

I have no basement or attic to mount it in, so I'll probably be building a baffle that locks into the garage door that opens into my theater area, but we'll tackle that once we have a prototype.

I'm not trying to design the optimal system up front, I'm trying to make a functional prototype that I can then test and incrementally upgrade.

Information on the system that it will eventually integrate with:

Marantz SR6010 (with Audyssey XT32) AV receiver running a 5.4.4 Dolby Atmos system. Definitive Technology Bipolar speakers up front and front overheads, Klipsch in the back and rear overheads for now.

MiniDSP 2x4 Balanced controlling:
Klipsch SW110
DIY High Xmax 10"
Two 10" powered subs in my Definitive Technology Towers (as one channel)
An array of 8 Dayton Audio Tactile Transducer pucks in the main listening position (a loveseat)

As it stands I have good response right down to about 19 Hz, but of course I want more and I can't afford (nor do I have the ability to mount) 8 IB 18"s. Hence the Rotary idea. It's not critical for me to have earth shaking response at 3Hz, but If I can get a solid 9 or 10Hz with good integration to my existing system, I'll be a veeery happy camper.

Anyway, I'll try to keep you all in the loop as I progress. It'll probably be a slow project, my band is doing some touring over the next month so we'll see how fast I can proceed.

Zettlesm, any progress on your end?

Kody
 

·
Registered
Joined
·
49 Posts
Hey guys, longtime lurker but I finally registered since I'm going to start undertaking some initial testing of a rotary subwoofer based on Chris' concept.

I've ordered basically the same flybarless rotor assembly and will be buying a cheap 12" woofer with an 8.5mm Xmax (it's really cheap so I won't mind getting another better one later if everything works out).

I will be doing initial mounting, fixturing, and balancing setup with an ~1/3hp single phase motor, then if everything goes well I have a DC brushless (48v/70A! :eek:) (~4.5 hp) motor I'm considering pulling from an electric scooter depending on whether I can find a good way to power it. I may still end up just purchasing a cheap 3 phase and VFD.

I have no basement or attic to mount it in, so I'll probably be building a baffle that locks into the garage door that opens into my theater area, but we'll tackle that once we have a prototype.

I'm not trying to design the optimal system up front, I'm trying to make a functional prototype that I can then test and incrementally upgrade.

Information on the system that it will eventually integrate with:

Marantz SR6010 (with Audyssey XT32) AV receiver running a 5.4.4 Dolby Atmos system. Definitive Technology Bipolar speakers up front and front overheads, Klipsch in the back and rear overheads for now.

MiniDSP 2x4 Balanced controlling:
Klipsch SW110
DIY High Xmax 10"
Two 10" powered subs in my Definitive Technology Towers (as one channel)
An array of 8 Dayton Audio Tactile Transducer pucks in the main listening position (a loveseat)

As it stands I have good response right down to about 19 Hz, but of course I want more and I can't afford (nor do I have the ability to mount) 8 IB 18"s. Hence the Rotary idea. It's not critical for me to have earth shaking response at 3Hz, but If I can get a solid 9 or 10Hz with good integration to my existing system, I'll be a veeery happy camper.

Anyway, I'll try to keep you all in the loop as I progress. It'll probably be a slow project, my band is doing some touring over the next month so we'll see how fast I can proceed.

Zettlesm, any progress on your end?

Kody
Kodyaudette, thanks for the post.

No new progress to report since my last post. Summer in Montana is a busy time for me and my projects do best when the weather starts getting cold.

That said maybe I needed your goad to get back in gear.

It sounds like you are on the right track. If you go the 4.5HP DC motor route you'll be able to push some serious air! You could use even bigger fan blades than my plan if the speaker motor will swing them. How do you propose to provide speed control for your DC motor? Important to provide a load on a DC motor when starting otherwise it can overspeed and draw serious amps. The advantage I see for bigger blades is lower rpm which will make your out-of-band fan noise that much less, though rpm also sets some upper limits on response. . .

I'll try to make some progress this coming week -- I've been walking past the parts of the rotary sub every day and it's been making me feel guilty that I've sidelined my project for a while.

Steve
 

·
Registered
Joined
·
29 Posts
Kodyaudette, thanks for the post.

No new progress to report since my last post. Summer in Montana is a busy time for me and my projects do best when the weather starts getting cold.

That said maybe I needed your goad to get back in gear.

It sounds like you are on the right track. If you go the 4.5HP DC motor route you'll be able to push some serious air! You could use even bigger fan blades than my plan if the speaker motor will swing them. How do you propose to provide speed control for your DC motor? Important to provide a load on a DC motor when starting otherwise it can overspeed and draw serious amps. The advantage I see for bigger blades is lower rpm which will make your out-of-band fan noise that much less, though rpm also sets some upper limits on response. . .

I'll try to make some progress this coming week -- I've been walking past the parts of the rotary sub every day and it's been making me feel guilty that I've sidelined my project for a while.

Steve
Well, the DC motor is currently integrated into an electric scooter which means it already has a fairly sophisticated speed control system built in (perhaps too sophisticated, we'll see if it likes the load I'm going to be giving it) which I would probably just remove and repurpose as is (might as well take advantage of the engineering someone else has already done!). I'm not sure whether the throttle control input to the speed controller is as simple as a potentiometer, but if so I'll just remove that and mount it in a recessed box or something so that it's not easily mess-with-able but can still be adjusted if I feel like it needs to be. The biggest hurdle with using the DC motor is powering it, it's designed to use four 12v car batteries and pull up to 70A at 48v- which is a pretty stiff requirement for most DC power supplies (or even 120v/15A circuits, haha). Now, in use, this will be able to have its own circuit and breaker and I'm certain I don't need anything like that much actual power for my application, but at this stage its a little hard to figure out how much I would realistically need. If anyone has a good source or idea about cheap 48v DC power supplies, I'm listening. 1 horsepower (which I would assume is plenty of power) is about 745 watts, so at 48v, that should draw around 15A. Factoring in some overhead, maybe a 48v/20A DC power supply would work?

I do like the idea of larger blades for reduced noise, etc. But I'm thinking I may try to optimize for solid high frequency extension for integration with my existing subs. They do pretty well down to 20hz at reasonable volumes, but I honestly haven't tried pushing them to reference that low and I'm not certain they will keep up. If I can shift some of the sub 30-35hz duties to a rotary, I think it would really let them shine. So I'm considering ways to ensure my high frequency (for a rotary sub) region is solid and I may make some low frequency tradeoffs in the interest of getting that integration really solid since I don't expect a ton of situations where I'll be dealing with sub 3hz content, etc. I fully expect to make several different blade designs in different sizes and profiles and balance that with motor speed to get the best results. I plan to take measurements at each stage, hopefully to provide some useful information for everyone interested. I'm not certain how reliable my measurement equipment is down that low, but I'll get what I can.

Anyone have thoughts on ways to increase high frequency extension? My understanding is that smaller, lighter blades that are still stiff helps with that. Would increasing the motor rpm and decreasing the Voice coil movement/blade pitch displacement to achieve an equivalent spl help with that, or is that six of one half dozen of the other as far as forces being applied to the blade? Any thoughts or ideas are certainly welcome. I don't have a ton of money to put into this project, but I'm going to try to get as much performance as I can from it and hopefully provide some knowledge for anyone else trying in the future.

Sorry for goading you back into a project you don't have time for! Haha. If you do get a chance to work on it, I look forward to seeing what you manage to get together!

Kody
 

·
Registered
Joined
·
29 Posts
Rotor assembly is on its way, and I just ordered a GRS-12SW-4 woofer from Parts Express (can't post links yet, not enough posts)

Deciding factors:

Super cheap!
8.5mm Xmax - should be enough to get some reasonably good extension, especially once the cone is off.
Flat dust cap for easier mounting to swashplate
Central Voice coil vent for through mounting of the rotating shaft.
120wrms/240w max power rating. Should be enough to get some decent movement out of the coil.

I expect I may want to buy a better driver eventually, but if I'm going to learn some lessons (ie make some mistakes), I'd rather do it on a cheaper driver, then do it right the first time with a more expensive one.

Now I'm trying to determine what material to use for my initial blade iterations. Eventually I'm thinking some aluminum would be reasonably light and rigid, but the process of cutting multiple blades that are similar enough to be balanced may be trickier. It may be worthwhile to decide on a blade profile with an easier/cheaper material first, then make a final version of the blades in a superior material. I have access to standard power tools, drill press, jigsaw, sawzall, circular saw, etc. but nothing fancy as far as machining custom parts, etc. Anyone thought of making a fiberglass mold for blades? I've never worked with the stuff, but perhaps it would be suitable.

Kody
 

·
Registered
Joined
·
49 Posts
Anyone have thoughts on ways to increase high frequency extension? My understanding is that smaller, lighter blades that are still stiff helps with that. Would increasing the motor rpm and decreasing the Voice coil movement/blade pitch displacement to achieve an equivalent spl help with that, or is that six of one half dozen of the other as far as forces being applied to the blade? Any thoughts or ideas are certainly welcome. I don't have a ton of money to put into this project, but I'm going to try to get as much performance as I can from it and hopefully provide some knowledge for anyone else trying in the future.

Sorry for goading you back into a project you don't have time for! Haha. If you do get a chance to work on it, I look forward to seeing what you manage to get together!

Kody
Smaller, lighter blades will reduce the load on the subwoofer motor by reducing the air-load on the blades and by reducing moving mass (very minor in comparison to the air-mass being moved by the blades). This in itself isn't going to give you higher frequencies, but it's probably going to be a necessary step because to achieve higher frequencies your blades are going to be pitching back and forth at a higher cyclical rate relative to a lower frequency upper limit.

The closer you can get the pivot axis of the blades to the blades' longitudinal centerline the the air-load on one half the blade will more closely match the air-load on the opposite side of the other half the blade. A bit more area on the trailing half of the blade will provide some self-centering force. Symmetrical blades makes it easier to accomplish this, but if you've got some calculus skills the area on each half of an assymmetrical blade can be calculated.

As far as cutting out and balancing a number of blades why not rough-cut the blades, bolt or clamp them together, then finish the contour of the edges all at once for all blades. You can can use a hand file, Sawzall or grinder (make sure to use a blade or wheel for non-ferrous cutting). Flat aluminum ought to work just fine.

Goads are always appreciated!

Steve Z
 

·
Registered
Joined
·
29 Posts
I think you're on the right track as far as making matching blades by rough cutting, then clamping them together and finishing them to the same shape and profile. I was looking at some of the iterations of the RW-17's blades. They used to have a noticeable trailing edge, presumably to help with self-centering. They seem to have trimmed that down to something more like this: (_( with just a slight backwards curve. May try a couple different profiles. Probably a simple symmetrical one and one that's more asymmetrical. I'll have to shake the rust off of my calculus, but it's still there ;)

So high frequency extension is in essence limited by the voice coil's ability to overcome air resistance forces that increase with rpm.

So:

Blade flexing increases distortion and lowers output efficiency.

Lighter blades are less load that the VC must move for each cycle.

Smaller blades will be lighter and easier to cycle at higher hz, but will move less air and be significantly less effective at lower frequencies.

So smaller very light, very rigid blades with a very strong magnet are probably just about the only concessions to be made as far as high frequency extension goes. Does that seem to agree with your understanding as well?

I'm going to have to do a little more research into the forces involved when comparing motor rpm with blade pitch. That may be something I just need to objectively test and measure once I have a prototype working.
 

·
Registered
Joined
·
29 Posts
Initial blade mock ups I made from some corrugated plastic. I'm certain these won't be the final design, but they are light and rigid, probably much too thick and the open corrugations will be awful for drag, etc. First blade design is 1" wide at the rotor head and 3" wide at its widest point. These are very rough, but I will stack and sand them to the same size eventually, depending on whether or not I find another suitable material before I get around to refining them. I think my initial tests will all be at a fairly low rpm, basically just to ensure I have a decent mounting concept.
 

Attachments

·
Registered
Joined
·
49 Posts
I think you're on the right track as far as making matching blades by rough cutting, then clamping them together and finishing them to the same shape and profile. I was looking at some of the iterations of the RW-17's blades. They used to have a noticeable trailing edge, presumably to help with self-centering. They seem to have trimmed that down to something more like this: (_( with just a slight backwards curve. May try a couple different profiles. Probably a simple symmetrical one and one that's more asymmetrical. I'll have to shake the rust off of my calculus, but it's still there ;)

So high frequency extension is in essence limited by the voice coil's ability to overcome air resistance forces that increase with rpm.

So:

Blade flexing increases distortion and lowers output efficiency.

Lighter blades are less load that the VC must move for each cycle.

Smaller blades will be lighter and easier to cycle at higher hz, but will move less air and be significantly less effective at lower frequencies.

So smaller very light, very rigid blades with a very strong magnet are probably just about the only concessions to be made as far as high frequency extension goes. Does that seem to agree with your understanding as well?

I'm going to have to do a little more research into the forces involved when comparing motor rpm with blade pitch. That may be something I just need to objectively test and measure once I have a prototype working.
I agree with everything you've written -- a strong VC motor will allow pitching the blades with a higher load. Increasing motor rpm will produce more SPL than lower rpm, all other variables held constant. But of course, higher rpm also increases the load on the blades the VC has to handle. This load can be mitigated by ensuring close to equal areas in front and behind the pivot axis of the blades, with a little more trailing area for self-centering effect.

So there's a trade off or sweet spot to be had with blade size, motor rpm and desired high frequency extension. From Bruce Thigpen's rotarywoofer.com website it appears that the upper frequency specification for current TRW-17 rotary woofers is 30 hz. From what I gather from links on his website as well as posts from some of his customers, rpm is tuned to the particular space to provide enough SPL to integrate with the rest of system response, but is somewhere in the range of 680 to 900 rpm. Someone on one of the DIY rotary subwoofer threads scaled the size of the blades from photographs of the more recent Thigpen iterations with a 17" blade circle diameter and came up with something on the order of 5 1/2" tall and 3 3/4" wide in a (_( shaped blade, but I can't find the particular post to verify those dimensions now. However, these figures are good enough to get in the ballpark.

The nice thing about DIY versus simply assembling a kit is that it allows for experimentation and optimization as long as the variables don't become too complex.

Looks like you're making great progress!

Steve Z
 

Attachments

·
Registered
Joined
·
49 Posts
Initial blade mock ups I made from some corrugated plastic. I'm certain these won't be the final design, but they are light and rigid, probably much too thick and the open corrugations will be awful for drag, etc. First blade design is 1" wide at the rotor head and 3" wide at its widest point. These are very rough, but I will stack and sand them to the same size eventually, depending on whether or not I find another suitable material before I get around to refining them. I think my initial tests will all be at a fairly low rpm, basically just to ensure I have a decent mounting concept.
The corroplast blades will give you very good stiffness vs. their light weight and additionally won't require nearly as much filler material at the rotorhead blade holders to grip the blades securely. With the flat aluminum blades I have, I need to use half-round stock on each side of the blades to fill up the blade grip -- though I admit the quick and dirty way would be to use several washers stacked up on each side of the blades. My thought is the half-round can be extended up the length of the blade to add additonal stiffness, though at the cost of more rotating weight.

The thick leading and trailing edges of the corroplast might be more noisy and cause more load at rpm, but your motor is more than strong enough to deal with that. There will be more wind resistance trying to rotate the blade backward in the blade holder around the mounting bolt, but if that is actually a problem you could drill a second mounting hole in the blade holder and use two bolts per blade.

The shorter and wider the blades, the less flexing relative to longer, narrower blades.

Steve Z
 

·
Registered
Joined
·
29 Posts
Here's the stripped out VC. I ran some tests with 2-15hz sine waves yesterday, getting solid extension so far. Also I found some 3mm thick abs plastic with a ridge that is 2mm (for a total of 5mm thickness) at the base, then tapers down to almost zero at the edges. These blades are a bit heavier, but I can taper and profile them easily enough. 5 1/8" long and 3 1/8" wide at the widest part of the blade. 1 1/8" at the rotor. The ridge not only holds them into the rotor perfectly, but also provides stiffness. Initial rotational tests (putting the assembly in a drill and manipulating the swash plate while spinning) seem much more effective than the corrugated plastic. Now I just need to profile the edges a bit, then find a longer 5mm diameter central shaft that can extend through the vc to mount to a motor. On dealerselectric.com I found a .5 hp motor with VFD for $159. I may end up just going that route for the motor as it would probably be easier than trying to make the high power DC motor work.
 

Attachments

·
Registered
Joined
·
49 Posts
Here's the stripped out VC. I ran some tests with 2-15hz sine waves yesterday, getting solid extension so far. Also I found some 3mm thick abs plastic with a ridge that is 2mm (for a total of 5mm thickness) at the base, then tapers down to almost zero at the edges. These blades are a bit heavier, but I can taper and profile them easily enough. 5 1/8" long and 3 1/8" wide at the widest part of the blade. 1 1/8" at the rotor. The ridge not only holds them into the rotor perfectly, but also provides stiffness. Initial rotational tests (putting the assembly in a drill and manipulating the swash plate while spinning) seem much more effective than the corrugated plastic. Now I just need to profile the edges a bit, then find a longer 5mm diameter central shaft that can extend through the vc to mount to a motor. On dealerselectric.com I found a .5 hp motor with VFD for $159. I may end up just going that route for the motor as it would probably be easier than trying to make the high power DC motor work.
Dealerselectric.com is where I bought my 1/2HP three-phase motor and VFD. Good service and fast shipping, although the shipping was pricey for the heavy motor. The quality of the VFD and motor, while not military or heavy industrial spec, looks more than adequate for intermittent (non 24/7) use.

Good looking blades and very nice that they fit perfectly into your rotorhead blade holders.

You're way past me now; I look forward to reading about a successful test!

Steve Z
 

Attachments

·
Registered
Joined
·
29 Posts
Blades have been flattened, rough sanded to match, and edges have been beveled. Next is to use some finer grit sandpaper to smooth everything down. I may round over the edges of the central support as well. Then it's just a matter of getting a motor solution, determining how to couple the motor shaft to the rotor shaft, then fabricating some way to mount everything together reliably. My VC doesn't have the handy mounting holes that Chris' did, so I'll probably have to build some sort of standoff/strap solution for that. The hole through my VC is 1" diameter, I am considering finding doing the following for shaft coupling/mounting. Drilling a 5mm diameter hole in the center of the motor shaft to mount the rotor shaft into, drill a hole for a small screw through both the motor shaft and the rotor shaft to transfer rotational force. Drill out the internal hole in the VC to 1 1/8" so I can mount a 5/8" ID 1 1/8" OD bearing onto the motor shaft and insert it into the VC cavity. This should ensure that everything is aligned properly and that it stays there. That's assuming I can figure out a way to drill out a little material from the rear of the VC.
 

Attachments

·
Registered
Joined
·
29 Posts
Ouch, I was just about to order that motor from dealerselectric, but the $65 shipping cost puts it a bit outside what I was looking to spend. I seem to remember Chris finding a reasonably priced VFD. Maybe I can find out what he ordered, get that, then find a local source for a motor. Maybe its time to hit the surplus stores.
 

·
Registered
Joined
·
29 Posts
Well, I stopped by the Surplus Store yesterday and I found a single phase 1/3 hp motor that runs at 1050rpm. It's not totally ideal, but it only cost $30, so if I need to buckle down and get something better, I can. It definitely moves some air. I have the hole drilled through the VC cone, I got the rotor shaft mounted to the motor shaft and mocked everything up. Everything fits, I just need to figure out how I'm going to mount everything, check alignment, then epoxy the swash plate to the VC cone. Depending on how much time I have, I may have a fully functioning prototype this weekend.
 

·
Registered
Joined
·
49 Posts
Well, I stopped by the Surplus Store yesterday and I found a single phase 1/3 hp motor that runs at 1050rpm. It's not totally ideal, but it only cost $30, so if I need to buckle down and get something better, I can. It definitely moves some air. I have the hole drilled through the VC cone, I got the rotor shaft mounted to the motor shaft and mocked everything up. Everything fits, I just need to figure out how I'm going to mount everything, check alignment, then epoxy the swash plate to the VC cone. Depending on how much time I have, I may have a fully functioning prototype this weekend.
Way to go -- you've definitely not let any grass grow under your feet on this project!

Best of luck for the weekend.

Steve Z
 

Attachments

·
Registered
Joined
·
29 Posts
Haha, thanks. Well, as much as I love the process of figuring a problem out and assembling, the point is to have a functioning assembly, right? Having half assembled parts around doesn't help me terrify the neighbors or feel the explosions in movies. Haha.

Got things about halfway lined up yesterday, basically mounted the VC on the edge of a 2x6 with the remaining pieces of the basket bent down to provide a mounting point and small wood blocks around the magnet, then wooden rails extending back to keep the motor aligned with it. I need to get some metal hanger strap or something to lock them down to the base, but overall it's looking pretty good. Should be able to buy the last parts I need for the rotary subwoofer assembly itself today. Then I can worry about mounting it in a way that works for my theater. Because I don't have a basement or attic, I'm probably going to buy a solid wood door to replace the door between my theater and my garage for the time being, and initially try mounting the rotary woofer directly in the door. We'll see how much undesirable noise there is with that setup (honestly I'm pretty sure it will be less than my air conditioner already makes). Then if necessary I can modify to include some sort of baffle to filter high frequency noise.

We'll see, I'll keep you in the loop as it progresses!
 

·
Registered
Joined
·
29 Posts
Well, I'm getting some wobble in the rotor shaft. It looks like the pocket i drilled in the motor shaft is perfectly centered, but it seems like the diameter is just big enough that the rotor shaft has a little room to play so I'm trying to figure out how to shim it in place adequately. Once that is ironed out, I should be good to go.
 

·
Premium Member
Joined
·
24,654 Posts
I'm not quite clear, but if it's a radial clearance you could use some low or medium strength Loctite mixed with glass microspheres.

The latter come in .005 dia and are mixed with epoxy to create optimum bond thickness.

Not sure if they come in different diameters; if not maybe you could fine wire with the desired strand dia to use instead of the glass.
 
261 - 280 of 318 Posts
Top