Way cool demo!
A quick point on a valve-type driver:
--As you scale this up, keep in mind that it can be done with a single fan, because one side blows and the other side sucks. If you build ducts from the front and back out to the valve, you can use just one fan.
So how fast can you swing it back and forth?
The video looked like it was going at about 6-8HZ, but even if it were a good bit faster, I don't think I could tell from looking.
How much air can those fans put out? Is there a rating in cfm or something stamped on the backs?
Cool.
A quick point on a valve-type driver:
--As you scale this up, keep in mind that it can be done with a single fan, because one side blows and the other side sucks. If you build ducts from the front and back out to the valve, you can use just one fan.
So how fast can you swing it back and forth?
The video looked like it was going at about 6-8HZ, but even if it were a good bit faster, I don't think I could tell from looking.
How much air can those fans put out? Is there a rating in cfm or something stamped on the backs?
Cool.
I know this can be done with a single fan, but I wanted to keep this model simple, and since I had 2 of the same fans, I used them. There is no rating on them other than 12v, 80mA, 1W. They are about 1.5 inches square. In the vid, the hard drive head was receiving a 5 hz sine wave. With the paper tunnel on it, I got it past 20 hz, but at that point, the paper was flexing way too much. The arm can do 30-40 hz easily (with the paper). I don't know how it would handle a heavier and more rigid tunnel. That's an experiment for another day.
rotary transformer
Needed,
two toroids would not couple together side by side. However, I have built some fun rotating led light show items using the two halves of a pot core as a rotary transformer. The two have to be very close but you can get a very efficient transfer doing this. The rotating head in vcrs uses the same setup. I am intrigued by this fan item and will be following the thread. I have tons of parts including head actuators from large hard drives. You would be suprized at the amount of force they can generate.
Best regards, gearheaddruid.
Needed,
two toroids would not couple together side by side. However, I have built some fun rotating led light show items using the two halves of a pot core as a rotary transformer. The two have to be very close but you can get a very efficient transfer doing this. The rotating head in vcrs uses the same setup. I am intrigued by this fan item and will be following the thread. I have tons of parts including head actuators from large hard drives. You would be suprized at the amount of force they can generate.
Best regards, gearheaddruid.
Automotive alternator slip-rings.
good for 50 amps right up to 130 amps. cheap to replace when they wear out. Commonly available. once they bed in low noise too. pre-made brush blocks with correct spring pressure, good for millions of revolutions, what more could you want?
good for 50 amps right up to 130 amps. cheap to replace when they wear out. Commonly available. once they bed in low noise too. pre-made brush blocks with correct spring pressure, good for millions of revolutions, what more could you want?
Re: rotary transformer
We just need the signal to jump fom one ring to the other, without a big loss of power.
So what do the windings look like if we have a 1:1 transformer and hold the primary side still and spin the secondary windings?
This sounds good, but would it be noisy? Not physical noise, but would it add noise to the signal?
"Rotary Transformer" is what I was going for here. Two separated toroids wouldn't work because they wouldn't have a common core, right?gearheaddruid said:
We just need the signal to jump fom one ring to the other, without a big loss of power.
So what do the windings look like if we have a 1:1 transformer and hold the primary side still and spin the secondary windings?
OzMikeH said:
This sounds good, but would it be noisy? Not physical noise, but would it add noise to the signal?
rotary transformer
How I did it was to use a bobin from a half height core set and put one in each cup. The windings were one to one so I had ten turns on each bobbin.
One cup is stationary while the other rotates. The cups should be of the ungapped sets. The rotating cup is centered with respect to the stationary and within .010 inches face to face. I did not detect any noise introduction but then I was using pwm square wave that drove banks of leds.
I will try to find a picture of this and post it. With the right core material you could pass most of the audio spectrum.
The signal could be a mixed bag and provide both power and info to the control winding.
My big concern is how to make the vanes swivel without any looseness that whould generate noise.
I would bet that the vanes on the production unit have a rotating permanent magnet and the hub has a winding in it. A spring or torsion rod would act to center the vanes. A voltage on the coil would either push or pull on the magnet and articulate the vanes. This would eliminate all problems with electrical to the rotating assembly.
Just my thoughts on the idea. All friendly rebuttals welcome
How I did it was to use a bobin from a half height core set and put one in each cup. The windings were one to one so I had ten turns on each bobbin.
One cup is stationary while the other rotates. The cups should be of the ungapped sets. The rotating cup is centered with respect to the stationary and within .010 inches face to face. I did not detect any noise introduction but then I was using pwm square wave that drove banks of leds.
I will try to find a picture of this and post it. With the right core material you could pass most of the audio spectrum.
The signal could be a mixed bag and provide both power and info to the control winding.
My big concern is how to make the vanes swivel without any looseness that whould generate noise.
I would bet that the vanes on the production unit have a rotating permanent magnet and the hub has a winding in it. A spring or torsion rod would act to center the vanes. A voltage on the coil would either push or pull on the magnet and articulate the vanes. This would eliminate all problems with electrical to the rotating assembly.
Just my thoughts on the idea. All friendly rebuttals welcome
Thought you guys might enjoy seeing this... anyone know if it ever went any further than this?
http://www.betteraudio.com/geolemon/servoproject/
http://www.betteraudio.com/geolemon/servoproject/
First, this is my first post on diyAudio, so 'Howdy'. Second, obviously, I'm new here, so if I'm stepping on any toes by posting to this old thread with this idea, sorry.
Ok, so I've been fascinated by this fan sub idea for a couple of years, but haven't really tried anything yet. Last night I started building a rough model based on one simple idea. I took a little 3 inch 5W driver and epoxied a post onto the dust cap, and four bearings on the mounting flanges. Into the bearings I installed four rods with fan blades and a short arm to be used to rotate the rod. I then brought four wire pushrods down from that center post to rotation arms. When the cone moves the rods rotate, adjusting the angle of the blades. Very simple, and noisy. To use as a fan sub I'd mount the whole thing onto a motor and couple the signal in on a couple of slip rings (with a simple low-pass filter to remove any brush noise).
While this works, it doesn't seem to me that even a large version of it will work for my AV room, which is very large (approx 28x25x10) with an open 12x12 stairwell. I'd have to use a pretty heavy speaker, and even then it's going to be a relatively inefficient and noisy fan. I think that even a real fan of the size I could fabricate would have some trouble making an appreciable sub-20Hz pressure change in this big leaky room.
So I started thinking about other ways I could accomplish the same task. In the past I've explored the idea of using a simple fan and moving ducts, similar to the setup in the video someone posted a page or two back, but I've always dismissed it because of the difficulty in scaling it up. Moving a large duct at 20Hz would be noisy and require a heck of a driver system.
The simplicity of the idea has great appeal though, so I kept sketching. I want the system to be something I can build with mostly off-the-shelf and salvage parts, to keep costs down. It occured to me that I could probably get old HVAC equipment pretty easily, and that it is designed to move lots of air with a minimum of noise. By arranging ducts properly I could valve the air such that it could move either direction, no direction, or anything in between.
Obviously large swinging valves or ducts won't work, and I'm not going to fabricate a louvered grill (too much work, likely to be noisy). The only option is a sliding grating. This is two overlapping sheets of material with vertical slots. When the slots in the two sheets are aligned, air flows through, when one sheet slides sideways the slots line up with the bars in the other sheet and no air flows.
The gratings can be arranged such that two sets of gratings can valve air in either direction. When left at the midpoint with both gratings half open the air circulates in the valve rather than going to or from the listening room.
The moving portion must be light so that it can move rapidly, and strong so that the air pressure does not bend it. I think it could be made of foam (blue sheet construction foam) . The gratings can be set up as inch-wide slots such that the foam only has to move about 1 inch peak-to-peak. Thats doable with a typical sub driver or with a servo system.
Since the grating would have a large area there would be a substantial pressure acting on it, it would have to ride on bearings to allow it to slide easily. These can be mounted in the stationary grating. The clearance gap between the moving grating and the grill behind it will leak, but with a large HVAC squirrel cage fan pushing the air, some leakage will be ok.
To drive the moving grating the simplest option is to use speakers. The gratings can be attached to rods that extend outside the ducts and connect to a rocker arm. This ensures that the two gratings remain in sync and provides a lever so that the speakers can be given a mechanical advantage that allows them to fully open and close the gratings.
It's late, so I'm sure this description probably doesn't completely make sense, so I drew a picture to help explain.
(direct link in case the embedded image doesn't work: <http://picasaweb.google.com/codesuidae/FilePile/photo?authkey=iddYZc2GDTQ#5123711693373308914> )
Air flows up into the right central chamber and down from the left central chamber. When the bottom moving grating is fully left the slots on the left side line up with the slots in the stationary grating, allow air to flow from the duct at the bottom left of the image down into the fan. The slots at the right of the bottom grating line up with the bars in the stationary grating, blocking air flow there. The top grating is opposite the left side blocks flow, the right side is open, so air flows out of the fan into the left duct and out the main duct at the top right. When the speakers switch direction everything reverses and air flows the other way.
When the speakers are in the neutral position, all the gratings are half-open, so air circulates around without going out either duct. Note that the duct cross section is always the same, so the load on the fan is fairly constant.
The output from the system should go through a muffler of some sort before it hits the listening room, maybe a pipe and stuffing arrangement like a car muffler (two parallel pipes with holes in a sealed box full of an appropriate quantity of stuffing).
Thoughts?
Ok, so I've been fascinated by this fan sub idea for a couple of years, but haven't really tried anything yet. Last night I started building a rough model based on one simple idea. I took a little 3 inch 5W driver and epoxied a post onto the dust cap, and four bearings on the mounting flanges. Into the bearings I installed four rods with fan blades and a short arm to be used to rotate the rod. I then brought four wire pushrods down from that center post to rotation arms. When the cone moves the rods rotate, adjusting the angle of the blades. Very simple, and noisy. To use as a fan sub I'd mount the whole thing onto a motor and couple the signal in on a couple of slip rings (with a simple low-pass filter to remove any brush noise).
While this works, it doesn't seem to me that even a large version of it will work for my AV room, which is very large (approx 28x25x10) with an open 12x12 stairwell. I'd have to use a pretty heavy speaker, and even then it's going to be a relatively inefficient and noisy fan. I think that even a real fan of the size I could fabricate would have some trouble making an appreciable sub-20Hz pressure change in this big leaky room.
So I started thinking about other ways I could accomplish the same task. In the past I've explored the idea of using a simple fan and moving ducts, similar to the setup in the video someone posted a page or two back, but I've always dismissed it because of the difficulty in scaling it up. Moving a large duct at 20Hz would be noisy and require a heck of a driver system.
The simplicity of the idea has great appeal though, so I kept sketching. I want the system to be something I can build with mostly off-the-shelf and salvage parts, to keep costs down. It occured to me that I could probably get old HVAC equipment pretty easily, and that it is designed to move lots of air with a minimum of noise. By arranging ducts properly I could valve the air such that it could move either direction, no direction, or anything in between.
Obviously large swinging valves or ducts won't work, and I'm not going to fabricate a louvered grill (too much work, likely to be noisy). The only option is a sliding grating. This is two overlapping sheets of material with vertical slots. When the slots in the two sheets are aligned, air flows through, when one sheet slides sideways the slots line up with the bars in the other sheet and no air flows.
The gratings can be arranged such that two sets of gratings can valve air in either direction. When left at the midpoint with both gratings half open the air circulates in the valve rather than going to or from the listening room.
The moving portion must be light so that it can move rapidly, and strong so that the air pressure does not bend it. I think it could be made of foam (blue sheet construction foam) . The gratings can be set up as inch-wide slots such that the foam only has to move about 1 inch peak-to-peak. Thats doable with a typical sub driver or with a servo system.
Since the grating would have a large area there would be a substantial pressure acting on it, it would have to ride on bearings to allow it to slide easily. These can be mounted in the stationary grating. The clearance gap between the moving grating and the grill behind it will leak, but with a large HVAC squirrel cage fan pushing the air, some leakage will be ok.
To drive the moving grating the simplest option is to use speakers. The gratings can be attached to rods that extend outside the ducts and connect to a rocker arm. This ensures that the two gratings remain in sync and provides a lever so that the speakers can be given a mechanical advantage that allows them to fully open and close the gratings.
It's late, so I'm sure this description probably doesn't completely make sense, so I drew a picture to help explain.
An externally hosted image should be here but it was not working when we last tested it.
(direct link in case the embedded image doesn't work: <http://picasaweb.google.com/codesuidae/FilePile/photo?authkey=iddYZc2GDTQ#5123711693373308914> )
Air flows up into the right central chamber and down from the left central chamber. When the bottom moving grating is fully left the slots on the left side line up with the slots in the stationary grating, allow air to flow from the duct at the bottom left of the image down into the fan. The slots at the right of the bottom grating line up with the bars in the stationary grating, blocking air flow there. The top grating is opposite the left side blocks flow, the right side is open, so air flows out of the fan into the left duct and out the main duct at the top right. When the speakers switch direction everything reverses and air flows the other way.
When the speakers are in the neutral position, all the gratings are half-open, so air circulates around without going out either duct. Note that the duct cross section is always the same, so the load on the fan is fairly constant.
The output from the system should go through a muffler of some sort before it hits the listening room, maybe a pipe and stuffing arrangement like a car muffler (two parallel pipes with holes in a sealed box full of an appropriate quantity of stuffing).
Thoughts?
for that much money, one might be able to rig a hydraulic circuit to drive a very large diaphragm, or just shake the crap out of your floor 😉. You could use an LVDT for position sensing negative feedback. You would probably want to place the pump in the basement, though 😉
Maybe you could use it to find the resonant frequency of your house and break stuff 😉
Maybe you could use it to find the resonant frequency of your house and break stuff 😉
The cost should be fairly low, provided one uses salvaged parts.
Not low as in 'look at the sub I built with these Radio Shack drivers and chipboard', but low for the amount of air it could move at sub-audio frequencies.
The most expensive part would be the drivers for the actuator. If the moving grills were set up on bearings it doesn't seem that it would take too much power to move them.
I'd think that getting in-room response to something approximating linear would be the hardest part. One cheap way to address this would be with progressive valves. By tapering the slots in the grills you could adjust the response to compensate for some of the combined non-linearity in the rest of the system.
Isn't that similar to the Tom Danley design that's in the SBG (Sonic Boom Generator)? As I recall, each enclosure (6 in total) of the SBG contained a 5 HP fan and a rotary 'servo valve' that opened and closed the air passageways to create a "Class AB" device for airflow modulation.
Look into the types of axial blowers that are used in grain drying.
Look into the types of axial blowers that are used in grain drying.
CodeSuidae, I think you'll need a blower that's capable of creating higher pressures. A thought is high-performance PC case fans, placed end-to-end in series to create the high pressures that you'll need. I'm talking about the 4-inch square ones, but be sure you can supply them with enough power. And not just any of the 4-inch ones, either - check into these:
http://www.sidewindercomputers.com/de12tfexhisp.html
Specifications: 120x120x38mm, 220.01 CFM @ 4600 RPM, 65.0 dBA, 29.40 watts, 2.45 amps, Maximum Air Pressure 26.44 mm H²O, 12v DC, 3 or 3+4 pin Molex header connector(s).
Specifically, look at the air pressure rating. By placing these fans in series (in a block of four or six), you could conceivably create the air source for a tiny servo valve loudspeaker. The single fan pressure rating of 26.44mm H20 equates to only 0.03 psi, but six in series could generate up to 0.225 psi. The pressure rating comes into play when you need to do something like driving a horn, or reproducing higher frequencies, but these might be an option to consider as an air source. You would then use something like 3" or 4" PVC drain pipe to form your ducts. With these fans, though, you'd need proper muffling, as you'd essentially have a vacuum cleaner motor in terms of noise output.
If you need higher-pressure air, look at regenerative blowers. The high-frequency noise could be more easily muffled, I think. These can run about $250 on ebay.
A flat servo valve could also be designed using circular sections divided up into 90-degree 'cells', with a flat rotor that could be cut from a sheet of woven carbon fiber. I can provide more details about this if necessary.
What has always troubled me about the 'fan subwoofer' is that it's such an inefficient design. There could be large gaps between the edges of the fan blades and the hole in the wall, or between the leading edge of one fan blade and the trailing edge of another that allow air to leak back across the fan in the other direction. With a servo valve design, the internal gaps can be far smaller, and as a result the efficiency of the design will be higher.
I wonder, what about a servo-valve corner horn?
http://www.sidewindercomputers.com/de12tfexhisp.html
Specifications: 120x120x38mm, 220.01 CFM @ 4600 RPM, 65.0 dBA, 29.40 watts, 2.45 amps, Maximum Air Pressure 26.44 mm H²O, 12v DC, 3 or 3+4 pin Molex header connector(s).
Specifically, look at the air pressure rating. By placing these fans in series (in a block of four or six), you could conceivably create the air source for a tiny servo valve loudspeaker. The single fan pressure rating of 26.44mm H20 equates to only 0.03 psi, but six in series could generate up to 0.225 psi. The pressure rating comes into play when you need to do something like driving a horn, or reproducing higher frequencies, but these might be an option to consider as an air source. You would then use something like 3" or 4" PVC drain pipe to form your ducts. With these fans, though, you'd need proper muffling, as you'd essentially have a vacuum cleaner motor in terms of noise output.
If you need higher-pressure air, look at regenerative blowers. The high-frequency noise could be more easily muffled, I think. These can run about $250 on ebay.
A flat servo valve could also be designed using circular sections divided up into 90-degree 'cells', with a flat rotor that could be cut from a sheet of woven carbon fiber. I can provide more details about this if necessary.
What has always troubled me about the 'fan subwoofer' is that it's such an inefficient design. There could be large gaps between the edges of the fan blades and the hole in the wall, or between the leading edge of one fan blade and the trailing edge of another that allow air to leak back across the fan in the other direction. With a servo valve design, the internal gaps can be far smaller, and as a result the efficiency of the design will be higher.
I wonder, what about a servo-valve corner horn?
Someone check my math on this, but...
( 26.44 mmH2O / 25.4 mm/in. ) = 1.0409 in. H20
( 1.0409 in. H2O * 249.1 Pa/in. H2O ) = 259.299 Pa
Ok, so that's the pressure generated by one fan.
From Wikipedia, I found the following equation for SPL in decibels.
Lp = 10 log ( prms^2 / p0^2 )
Lp = 10 log [( (259.3 Pa/1.414) / (20*10^-5 Pa) )^2]
Lp = 119.24 dB
So, disregarding the fluid dynamic efficiency of the fan itself and of the valve, it seems like based upon its static pressure figure, a single fan should be able to generate a SPL of 122dB at the exit of a servo valve loudspeaker.
Let's combine four fans (the pressures of each fan are additive):
Lp = 10 log [( ((259.3 Pa * 4)/1.414) / (20*10^-5 Pa) )^2]
Lp = 131.3 dB
Then with six of the aforementioned fans:
Lp = 134.8 dB
I wonder if it would truly be feasible to try a DIY SVL based on these results. It would, however, need to be an infinite-baffle type of design, since I can't explain how to load a SVL acoustic source to a horn. Only Tom might know that.
( 26.44 mmH2O / 25.4 mm/in. ) = 1.0409 in. H20
( 1.0409 in. H2O * 249.1 Pa/in. H2O ) = 259.299 Pa
Ok, so that's the pressure generated by one fan.
From Wikipedia, I found the following equation for SPL in decibels.
Lp = 10 log ( prms^2 / p0^2 )
Lp = 10 log [( (259.3 Pa/1.414) / (20*10^-5 Pa) )^2]
Lp = 119.24 dB
So, disregarding the fluid dynamic efficiency of the fan itself and of the valve, it seems like based upon its static pressure figure, a single fan should be able to generate a SPL of 122dB at the exit of a servo valve loudspeaker.
Let's combine four fans (the pressures of each fan are additive):
Lp = 10 log [( ((259.3 Pa * 4)/1.414) / (20*10^-5 Pa) )^2]
Lp = 131.3 dB
Then with six of the aforementioned fans:
Lp = 134.8 dB
I wonder if it would truly be feasible to try a DIY SVL based on these results. It would, however, need to be an infinite-baffle type of design, since I can't explain how to load a SVL acoustic source to a horn. Only Tom might know that.
Taterworks said:
Sounds interesting, anyone have a link? I couldn't find much about it.
Hm, all the grain dryers I've ever been around were huge 3 phase beasts that sounded about like a small airplane. No doubt it would generate more than enough pressure to make convincing sub-bass (and difficulty closing doors for that matter), but I'm hoping for something a little quieter, and that won't require getting the utility company out here to put in 3 phase. 🙂
Taterworks said:
I think you are correct about the pressure.
For the full size that I've diagrammed I was thinking more on the order of a standard 120vac 1/2HP 2000 cfm furnace blower. These are designed to run fairly slow (1000rpm) to keep them quiet, and can push better than 1400 cfm with 0.5"H2O. I've only seen specs for up to 0.8" on single-family home type blowers, but I don't know what the maximum practical operating pressure would be.
I haven't any idea what the necessary pressure difference for good performance would be, I suppose it goes up with the frequency.
I had considered rotary designs, but it seems much simpler and cheaper to use regular drivers as linear actuators than to come up with something to drive a rotary actuator. It could be done with a servo motor, but I don't think the cost-benefit analysis bears out there, unless you happen to have a contrabass kit lying about. And, in that case, you'd be working on the wrong project.
You can certainly improve the efficiency with more precise engineering, but one of my design goals is to keep it cheap. If I have to go from a 1/2 HP blower to 3/4 HP blower, or add a second 1/2 HP blower in series, that isn't too big of a deal, provided they are coming surplus from the HVAC guys.
For me, a central theme is to keep it cheap and mechanically simple. Given my time constraints, this necessarily means that it's going to be pretty rough. If I make the design complicated or expensive it'll either never get done or never work. Once it's functional, I can think about making it fancier to improve performance, if necessary.
As it stands, it's just some ductwork, some carefully cut up foam, bearings, and old speaker drivers. With some poking around in the right places it can all be had for the price of the gasoline to go pick it up.
For my scale model, do you have any ideas of how the symptoms of poor pressure capability from the fan would manifest themselves? I'm thinking I'd see good air volume at, say, 1Hz, with a progressive reduction at increasing frequencies.
Taterworks said:
I don't think it works that way. The maximum static pressure is the maximum pressure that will develop if the fan is attached to a sealed duct. So you get 0 CFM at that pressure. Likewise, at max CFM you get near zero pressure.
Like the power from an electric motor, the maxium power output will probably be somewhere in the middle. Where that point is will vary depending on the fan. Axial fans are designed for high volume, low pressure. Centrifugal blowers of the type I'm considering are typically moderate pressure, high volume. You can get multistage or vaneaxial fans for high pressure, high volume use, but I'm at a loss as to where I'd get such equipment cheaply.
Let's hope so, since that's what I'm planning. One duct will go to the listening room, the other to the garage. With any luck I won't end up blowing ALL the air conditioned air outside. 🙄
CodeSuidae said:
If you have enough pressure, you'll be able to open and close doors with your remote control - just make a CD track for "open" and another for "close." I am guessing that a nice loud 1/4 wave in one direction at about 1/2 Hz would do the trick. Remember, we have infinite excursion and don't need to return to center. You could just blow the doors shut, and suck 'em open.
neededandwanted said:
If you have enough pressure, you'll be able to open and close doors with your remote control - just make a CD track for "open" and another for "close." I am guessing that a nice loud 1/4 wave in one direction at about 1/2 Hz would do the trick. Remember, we have infinite excursion and don't need to return to center. You could just blow the doors shut, and suck 'em open.
Actually, I was wrong on that. You are only blowing air into the room, not sucking it back out using a gated fan approach.
It's sort of a big DC bias applied to the signal, I guess.
Need to work out something else to un-slam the doors...
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