noah katz said:
Thanks for the link. I started reading it (again?) and didn't make it past the first 2 pages. It looks like a cat fight, and I gave up.
Does it get better, if I keep reading?
One very valid point I found there is that sub-20Hz content tends to not be very accurate. I would imagine that the bottom "inaudible" octaves tend to not get the full mastering treatment, as that sound would get overlooked in most of the mastering process. Certainly a foley artist marching in a box of rocks and snapping celery to to make kung fu fighting noises isn't paying a bunch of intentional attention to the realism of his artificial sounds in the 5-20Hz range.
Once the technology for the reproduction of the sub-20Hz sound starts to become more mainstream, then they will need to put more attention on it. For now, I think they are arbitrarily boosting and cutting this sound to control effects in the audible range.
Effects are nearly always artificial re-creations of sounds, since the visuals are usually artificial as well. Movie makers don't spend their budget blowing up real helicopters, and I don't know where you could find real stock recordings of alien battleships shooting each other or an asteroid being blown up by nuclear weapons...
xplod1236 said:
I understand that part, they look like the picture of the rod ends you posted 🙂 The arms trace out a semi-circle as they move. The cross piece moves straight up and down. In order for the two to work together with the ball joint on the arms, the cross piece must also rotate slightly, right?
noah katz said:
Thanks for the link, that's a long thread, I'm not sure if I've read the entire thing before, but I'll review it later when I have more time.
I'm thinking about testing a few blade designs before finishing the actuator. In particular I'm interested in determining the torque on the blade for each angle and blade configuration.
I'm figuring on doing a flat rectangle. I also want to do a tapered blade where the root is wider than the tip such that the air volume is constant from root to tip. I'll do one that is 'swept' similar to the one in the TRW design. I'd like to do a cupped design as well, but I don't want to take the time to fabricate the blades. I'm thinking that perhaps I can cut some blades from cardboard sonotube to make something reasonable. Does anyone have any other blade configurations they'd like me to test?
Also, if anyone has any particularly clever designs for a simple strain gauge, feel free to post it. I'm figuring a simple setup with an RC servo setting the blade angle should do it. The power the servo requires to hold a position can be related directly to the torque. This is easy to set up as it only takes a couple of brushes for measuring the current. It's also handy as I can adjust the blade pitch while the fan runs via the RC remote control, which makes it easy to check all the angles quickly.
neededandwanted said:
Good point. This is probably a good thing for DIY equipment.
I've got a high quality audio recording of both of those events actually. Curiously, the RMS power is exactly zero. For the best effect you really need to listen to it in a very quite room 😀
CodeSuidae said:
You are correct. This design allows the cross piece to move up and down as well as rotate back and forth. I took all that into consideration. I also considered that as the blades rotate to 45 deg (max) and the cross piece rotates, the rod ends will be pulled towards the center by about 0.01 inch or 0.3 mm. Since this will not be a super precise contraption, I should have enough play to not even worry about the rod end displacement.
I also started reading the thread, but found nothing interesting in the first few pages.
To address the accuracy issue in sub-20 Hz content, I don't think we need to worry about that. In both of the fan designs, the translation from linear movement of the voice coil to rotation of the blades is not exactly linear. Plus, we can't hear distortion at such low frequencies anyway.
xplod1236 said:
I don't think the problem is distortion in the lower octaves but artificial levels set by distortions in the synthesis/recording, mixing, mastering processes.
Someone artificially boosts and cuts volume in the bottom octaves but can't really hear what he is doing well, so the levels are artificial.
When someone complains that shoes walking on a floor in a movie are sending out thunderous 12 Hz thumps, that's possibly where it's coming from.
Compare it to waking up one morning and being able to see in infra-red. Many things that used to be the same color no longer would be and several things might look unrealistic.
In early TV production sets were painted lots of vibrant colors that ended up being displayed in black and white on home TV. Sometimes they didn't convert well.
When movies first started adding soundtracks, many famous actors with weak voices didn't get to work again.
All I'm saying is that we are now planning on actually doing something with the ultralow frequency content, don't be surprised if it was neglected during the production and post-production phases and some movies or portions sound crappy and artificial.
Likely every improvement in fidelity of reproduction has exposed problems in the source materials. ("Gonna have to buy the White Album again...")
CodeSuidae said:
My listening room was a nearly complete vacuum, to replicate the recording space.
I had a recording of the alien battleship stuff, but my wife accidently erased it. 😉
xplod1236 said:
At first, I thought I understood your picture 100%. Now I am having doubts.
Couldn't the two bearing sets be replaced by one? Many bearings can take load forces in several directions, and by stacking yours, you are providing an axial load on the thrust bearing, and a thrust load on the axial bearing, as far as I can make out from the image.
[I may be getting the terms wrong here. What I am calling axial is like a skateboard wheel bearing, and what I am calling thrust is like a turntable or lazy susan]
How would you connect the sleeve to the voice coil (and the big honkin' magnet) ? It looks like your sleeve could almost be the voice coil former-- just wrap in Kapton and wind on the magnet wire.
For that matter, if the shaft is magnetized...
Let me try again. The black bearing (rulon) has a flange on the far end. That will move the cross piece towards the magnet/VC. The tan thrust bearing (nylon) will push the cross piece back. The sleeve will be press fitted onto the rulon bearing, squeezing the cross piece between the flange and the nylon thrust bearing. So I'm trying to create a 2-sided thrust bearing.
I could just use the rulon bearing, and sandwich its flange between the cross piece and a cover plate, but then I would not have a way to eliminate any play. Using the 2 bearings seems like the easiest way to do this, and it will allow for clearance adjustment. Hope this clears things up.
The sleeve would be glued to the voice coil former. I could make my own voice coil, but for now, I'll try to keep it simple and use an existing motor assembly.
I could just use the rulon bearing, and sandwich its flange between the cross piece and a cover plate, but then I would not have a way to eliminate any play. Using the 2 bearings seems like the easiest way to do this, and it will allow for clearance adjustment. Hope this clears things up.
The sleeve would be glued to the voice coil former. I could make my own voice coil, but for now, I'll try to keep it simple and use an existing motor assembly.
neededandwanted said:
If you were designing to manufacture them you would might turn the speaker design inside out. Mount a light but powerful magnet on the rotating disk, then place the large driving coil around the magnet, but fixed to the stationary parts.
For a one-off design the closest you might want to get to that (without spending more time and money that it is worth) would be to remove the spider and connect the VC to the rotating part, with brushes to bring out the connections. Then, with very careful alignment, you could allow the voice coil to rotate inside magnetic gap of the magnet structure. The mechanics would need to be fairly precise to keep the VC axial alignment while allowing normal excursion, but it would simplify the motion transfer.
"Does it get better, if I keep reading?"
Yes, search the thread for posts by Bruce Thigpen, the designer, though I can't remember if that's his posting name.
He addressed some questions of mine, so you should be ble to find him by searching for my name.
IIRC he got the idea when contempalting the blade mechanism on a model helicopter.
Yes, search the thread for posts by Bruce Thigpen, the designer, though I can't remember if that's his posting name.
He addressed some questions of mine, so you should be ble to find him by searching for my name.
IIRC he got the idea when contempalting the blade mechanism on a model helicopter.
Higher Frequency Version?
OK. Stupid question:
What would be the considerations / constraints / dependencies on making higher frequency version of the fan sub?
I had actually wondered if Thigpen had first set out to make a fan version of a conventional sub (or just a conventional woofer), and then found out how bad it was at reproducing higher LF, but how it got better as frequency went lower.
So, that's what I am now wondering: If I am willing to forego moving large amounts of air below 20Hz, but really want to address the band from 20-60Hz, can a fan woofer be easily built to address this?
Smaller fan diameter and/or smaller blade pitch. Higher fan rpm. Lower blade mass. Lower moving mass in general. (More blades, maybe?)
I see no immediate reason why this wouldn't work.
The output would be closer to the out of band noise, so external physical filtering (bandpass "muffler") would need to be more accurate, but this doesn't see like a severe obstacle.
Seriously, would a smaller fansub, spinning faster, be practical?
OK. Stupid question:
What would be the considerations / constraints / dependencies on making higher frequency version of the fan sub?
I had actually wondered if Thigpen had first set out to make a fan version of a conventional sub (or just a conventional woofer), and then found out how bad it was at reproducing higher LF, but how it got better as frequency went lower.
So, that's what I am now wondering: If I am willing to forego moving large amounts of air below 20Hz, but really want to address the band from 20-60Hz, can a fan woofer be easily built to address this?
Smaller fan diameter and/or smaller blade pitch. Higher fan rpm. Lower blade mass. Lower moving mass in general. (More blades, maybe?)
I see no immediate reason why this wouldn't work.
The output would be closer to the out of band noise, so external physical filtering (bandpass "muffler") would need to be more accurate, but this doesn't see like a severe obstacle.
Seriously, would a smaller fansub, spinning faster, be practical?
Maybe.
I think the inertia of the fan blades play an important role in freq response.
Personally i think the fan sub produces too much noise to be a practical speaker.
We got motor noise, bearing noise and air noise from the gaps between the fan blades. It´s made for beeing put in the attic as chamber so you cannot hear it on idle.
I think the inertia of the fan blades play an important role in freq response.
Personally i think the fan sub produces too much noise to be a practical speaker.
We got motor noise, bearing noise and air noise from the gaps between the fan blades. It´s made for beeing put in the attic as chamber so you cannot hear it on idle.
Tekko said:
Exactly. If a big fan with heavier blades works well at 20Hz, we should be able to get smaller blades on a smaller fan to work well two octaves higher.
It would still be able to go lower -- all the way down to DC for any fan size--but the displacement would not be adequate for the lowest frequencies to be audible. Four times the displacement is required for each halving of frequency.
This leads me to believe that this could be workable for higher low frequencies.
I certainly would not try and mount this on the baffle board of a conventional loudspeaker enclosure. It could likely go into a 4th order bandpass box, or simply have an acoustic filter in front of it. If we roll off everything above 60 or 90Hz using a muffler (tuned hipass or bandpass conduit) we should eliminate all of the HF noise.
I certainly think it would be easier to make a smaller fan driver. While it would do the deep infrasonics, it might be a really powerful woofer for the bottom 2 octaves of the "audible" range.
The fan sub is a peculiar beast designed to solve a particular problem, I'm not sure why you'd want to apply it outside the range of situations it was designed to handle.
Consider how the blades move the air. They move through the air deflecting it into the room. If the fan is about 1ft in diameter and runs at 1750 RPM, the blade tip travels about 90 feet per second. At 20Hz the blade moves 4.5 linear feet per cycle, or a bit over 2 feet zero to zero. For some of that travel it is at a low pitch, providing little power to accelerate the air. At 60Hz you're down to about 9 inches of linear travel.
To get sufficient linear travel to deflect enough air while the blade is pitched up, you'll have to spin the fan faster. The faster it is, the more noise it's going to produce.
Now compare the output of the fan sub to a diaphragm sub. Starting at the zero crossing point of a sine wave input signal and starting up the slope toward the peak the fan sub has a low blade pitch with a high rate of pitch change. The low pitch means it's putting minimal power into moving air. As the rate of pitch change decreases toward the peak of the input signal, the fan sub is putting a lot of energy into moving air, at peak is the maximum power output where the most air is being compressed into the room. As the input signal starts back down toward zero the blade pitch starts to decrease, reducing the amount of energy used to compress air into the room until it reaches zero.
Now consider the diaphragm sub. As the input signal starts up the slope where the rate of change is fast, it is immediately pushing hard on the air, compressing air into the room, this is maximum power. As it reaches the peak where the rate of change is slow, the diaphragm is slowing down and stopping at the peak, this is minimum power. As the input signal starts back down toward zero the diaphragm is moving backwards, decompressing the air in the room.
The two do completely different things with the input signal. For one the rate of change of the signal corresponds to the output power (high rate of change, high audio power output), for the other, the absolute value of the signal indicates power output (high signal level, high power output).
If you were to compare the output sound of both types operating in the same frequency range you'd get very different sounds. When we're talking about sub-20Hz signals this probably isn't significant, the sensation is probably mostly tactile rather than auditory, and so less precise. When you're getting up into low audio frequencies at 40 and 60Hz I'm betting your really going to notice the difference. Those low frequencies are often components of waves that are also being handled by your midrange drivers and the mismatch isn't going to reproduce the original full-range sound.
It would probably sound terrible.
If you wanted to use a fan sub at higher frequencies you'd probably need a signal processor to take the input signal and convert it to make the fan sub emulate a diaphragm sub for higher pitches. I suppose you'd overlay the audio frequency signal over slower sub-audio frequencies that require the fan-like pressurization.
Much easier to let each component do what it was designed to do, IMO.
Consider how the blades move the air. They move through the air deflecting it into the room. If the fan is about 1ft in diameter and runs at 1750 RPM, the blade tip travels about 90 feet per second. At 20Hz the blade moves 4.5 linear feet per cycle, or a bit over 2 feet zero to zero. For some of that travel it is at a low pitch, providing little power to accelerate the air. At 60Hz you're down to about 9 inches of linear travel.
To get sufficient linear travel to deflect enough air while the blade is pitched up, you'll have to spin the fan faster. The faster it is, the more noise it's going to produce.
Now compare the output of the fan sub to a diaphragm sub. Starting at the zero crossing point of a sine wave input signal and starting up the slope toward the peak the fan sub has a low blade pitch with a high rate of pitch change. The low pitch means it's putting minimal power into moving air. As the rate of pitch change decreases toward the peak of the input signal, the fan sub is putting a lot of energy into moving air, at peak is the maximum power output where the most air is being compressed into the room. As the input signal starts back down toward zero the blade pitch starts to decrease, reducing the amount of energy used to compress air into the room until it reaches zero.
Now consider the diaphragm sub. As the input signal starts up the slope where the rate of change is fast, it is immediately pushing hard on the air, compressing air into the room, this is maximum power. As it reaches the peak where the rate of change is slow, the diaphragm is slowing down and stopping at the peak, this is minimum power. As the input signal starts back down toward zero the diaphragm is moving backwards, decompressing the air in the room.
The two do completely different things with the input signal. For one the rate of change of the signal corresponds to the output power (high rate of change, high audio power output), for the other, the absolute value of the signal indicates power output (high signal level, high power output).
If you were to compare the output sound of both types operating in the same frequency range you'd get very different sounds. When we're talking about sub-20Hz signals this probably isn't significant, the sensation is probably mostly tactile rather than auditory, and so less precise. When you're getting up into low audio frequencies at 40 and 60Hz I'm betting your really going to notice the difference. Those low frequencies are often components of waves that are also being handled by your midrange drivers and the mismatch isn't going to reproduce the original full-range sound.
It would probably sound terrible.
If you wanted to use a fan sub at higher frequencies you'd probably need a signal processor to take the input signal and convert it to make the fan sub emulate a diaphragm sub for higher pitches. I suppose you'd overlay the audio frequency signal over slower sub-audio frequencies that require the fan-like pressurization.
Much easier to let each component do what it was designed to do, IMO.
CodeSuidae said:
I'm not gonna flat-out agree with this right away. If I bought into the "use it for what it was made for" I would have never gotten to listen to drivers driven effectively below resonance, horn-loaded ribbons, horizontal line source arrays, tapped horns, or lots of other treats.
I actually don't know if the fan sub was even designed to work only below 20Hz originally. It apparently excels in this area, and I am assuming that Thigpen considered it more applicable (or marketable) in this realm, but possibly mostly because no one was really addressing high power down to DC.
I think I am gonna have to disagree with you here.
Just like a conventional pistonic driver, the acceleration is actually at a peak thru the flat blade transition. The air velocity goes from high in one direction (at pitch peak) to zero (at zero pitch) to high in the other direction (at the other pitch peak), but this change in velocity is the result of a strong acceleration.
To look at it another way, imagine the blades staying at full pitch--here is would be emanating constant velocity air, meaning no acceleration.
As the blades sweep thru the "flat" area, they are rapidly decelerating the air. They had been blowing air in one direction, but are now strongly stopping the air. This midpoint IS the high energy area. It is where the driver is rapidly changing the direction of the moving air.
If the blades stop at any position, there is no acceleration, even though large amounts of air are being moved. It's just a breeze with no signal. It is the change in pressure, not the pressure that we are spending our signal on. A pressurized room, or a room with a breeze isn't loud. A room with periodic changes in barometric pressure is can be. It's the alternation of compression and rarefaction that takes signal energy (not fan-spinning energy) to create.
This, like many other aspects of a fan sub are counter-intuitive, since there are multiple movements. You can be moving a ton of driver mass (spinning blades) and a ton of air with no signal. All of the signal power goes into changing the rate of movement, not the movement itself. This would be similar to a pistonic driver with infinite displacement. If it just keeps coming at you, it isn't a signal.
It seems like a fan with with a fixed pitch on the blades IS accelerating the air, but in a closed system, it is just overcoming friction as it continues a circulation pattern. If the fan were removed, the air would continue to circulate in the room until slowed and stopped by friction, not really by lack of inertia.
For me this confusion certainly carries over into the nomenclature: the "moving mass" isn't the spinning blades, but the axial rotation of each blade.
I'd love to see a response curve to see if your statments about the non-linearity shows up in the existing fan sub. You may be very right about this, and a fan sub might need something close to a square wave to produce a clean sine wave.
I guess the thing to do at that point would be a signal processor as you mentioned - ideally something like finite impulse response correction. I would imagine that some of Thigpen's electronics are doing skewing corrections like this.
The only thing that would seem to contradict this, is that any non-linearity (sine wave in, doesn't make a sine wave out) would tend to show up as higher frequency harmonics.
Possibly, this is occuring and is currently beneficial, even if inaccurate. If I send a 10Hz signal to the sub and it generates harmonics at 20 and 40 Hz, it is probably not unpalatable, and would tend to be coherent with the rest of the sound.
Totally agreed. I think that turbulance causes most of the noise. The noise increases as the square of the speed, so this is likely the show-stopper. If I need to spin the fan 4 times as fast, I will end up with 16 times as much noise.
The only way I can see of doing this without adding additional noise would be to keep a large outside radius but use lots of small blades with a large inner radius, like more of a turbine or something. Basically, a large center hub (pole piece?) with small blades around the outside. In this way, we could have very low mass blades and still have large amounts of air being moved.
Since we would be dealing with higher frequencies, we would have to move much less air, as the required displacement is inversly proportional to the square of the frequency. If I want to get the same SPL levels, I only need to move 1/16th of the air required two octaves lower.
I must be missing something here, but don't yet see it.
I think it still merits some experimenting, but not having accomplished it the "right" way, I have to use as a baseline.
OK. I know I am letting my confusions slop over into my explanation here.
Approaching it another way...
Imagine putting a fan sub in the middle of a tube. At a blade pitch peak, it is sucking air into one end of the tube and blowing it out the other end.
Now, cut the power to the voice coil (but not the fan motor). The air has inertia and is still moving in the same direction. The blades will not close (to zero pitch) but will freewheel in the same position (unless there is some restoring force applied). It apparently takes no energy to keep the blades at any position, but it would take power to close the blades. The voice coil must be driven hard to close the blades.
I believe the power/acceleration closely approximates that of a standard pistonic driver for this reason.
Another way to look at it:
Imagine the tube had two fan subs in series (isobaric?). Spin them both, but only drive the voice coil of one of them. The second one (passive radiator?) will mimick the blade pitches of the driven one. To make it do otherwise will require power. To drive the blades of the second one thru the zero pitch region will require the most power.
In a sense, a fan driver is really an amplifier. It takes a small signal and leverages it into the modulation of a larger power source. The voice coil signal is modulating the power of the fan's rotation. Potentially, the voice coil could be driven with a few watts, and the fan's current draw will rise and fall as it does more and less work (and the inertia of the fan's overall spinning mass tends to level this out). This includes the effort expended in speeding up air, and slowing it down.
I don't know if that made it any clearer or not.
I could also be totally wrong about this.
Approaching it another way...
Imagine putting a fan sub in the middle of a tube. At a blade pitch peak, it is sucking air into one end of the tube and blowing it out the other end.
Now, cut the power to the voice coil (but not the fan motor). The air has inertia and is still moving in the same direction. The blades will not close (to zero pitch) but will freewheel in the same position (unless there is some restoring force applied). It apparently takes no energy to keep the blades at any position, but it would take power to close the blades. The voice coil must be driven hard to close the blades.
I believe the power/acceleration closely approximates that of a standard pistonic driver for this reason.
Another way to look at it:
Imagine the tube had two fan subs in series (isobaric?). Spin them both, but only drive the voice coil of one of them. The second one (passive radiator?) will mimick the blade pitches of the driven one. To make it do otherwise will require power. To drive the blades of the second one thru the zero pitch region will require the most power.
In a sense, a fan driver is really an amplifier. It takes a small signal and leverages it into the modulation of a larger power source. The voice coil signal is modulating the power of the fan's rotation. Potentially, the voice coil could be driven with a few watts, and the fan's current draw will rise and fall as it does more and less work (and the inertia of the fan's overall spinning mass tends to level this out). This includes the effort expended in speeding up air, and slowing it down.
I don't know if that made it any clearer or not.
I could also be totally wrong about this.
"Seriously, would a smaller fansub, spinning faster, be practical?"
No.
Plenty of reasons have been given, but to sum it up, it's the wrong tool for the job.
You could use it anyway, but compared to a conventional driver it would be more complicated, bigger, more expensive, and not sound as good even after taking measures to limit the side effects of its construction and operating principle, said measures not even being necessary for the conventional driver.
It sounds like you haven't read what the designer said.
No.
Plenty of reasons have been given, but to sum it up, it's the wrong tool for the job.
You could use it anyway, but compared to a conventional driver it would be more complicated, bigger, more expensive, and not sound as good even after taking measures to limit the side effects of its construction and operating principle, said measures not even being necessary for the conventional driver.
It sounds like you haven't read what the designer said.
noah katz said:
OK, now we are getting somewhere. It looks like I am missing vital documentation, and it is showing through.
I don't want to be asking stupid questions, or making incorrect assumptions or unfounded assertions.
Where are the documents?
neededandwanted said:
I think that would depend very much on the blade design. Picture the blade configuration of a ceiling fan; it's easy to see why going to zero pitch won't do much to decelerate the air moving past the blades (or hold pressure in the room).
Some ventilation fans are designed with variable pitch blades and to create a strong baffle effect at zero pitch, allowing them to act like a closed duct. I haven't seen the blade design on these so I'm not sure what features you'd need to get that effect. Blades that form a nearly complete disk at zero pitch would probably be effective and might be the simplest solution.
I don't disagree. However, I do not believe that an audio signal used to adjust the pitch of the blades on a fan sub will produce the same (or even close to the same) sound that you would get by feeding that same signal to a VC-driven diaphragm.
For this reason I think that to use a fan sub in the audio range where it must work in conjunction with other drivers to reproduce a given sound would require signal processing (or extensive blade/fan engineering) to be at all workable.
This might be complicated by the room response. Any given fan has a maximum static pressure at which it will produce no air movement. If we presume that the fan is moving air into an effectively sealed listening room then the pressure in the room will only increase up to whatever the maximum static pressure of the fan is under it's current operating conditions.
I'm not sure if that would be a factor for anything except extremely low (like low single-digit) frequencies. I think it must have some effect but I have no way to guess how much of a factor it would be.
That would be a design issue, but I think it would be difficult to design blades that would idle wherever you left them. Either way though, I don't think it's a very good idea to not maintain positive control over the pitch at all times.
I think you may have a contradiction here. If you have a blade profile that will stay put at whatever pitch you leave it (if that is even possible), then in follows that blowing air past it will not cause the pitch to change.
In the case where the blades adjust in reaction to the air driven past them by the first fan, the second fan would only adjust pitch enough to minimize the blade profile to the oncoming air (presumably a pitch somewhat less than the first fan). I don't think it could be designed to add to the movement of the air, as that would effect the parameter that causes it's blades to move. It would enter a feedback loop and go to a pitch that was flat relative to the air stream.
I agree, although I think you're going to have to spend more power controlling the blade pitch.
For modeling purposes it might be useful to ignore the mechanism by which the driver moves air and just think of it in terms of what it would do in the ideal case. I believe that would be moving a quantity of air sufficient to cause a pressure change in the listening environment proportional to the input signal.
From there you can look at the behavior of the driver mechanism to see to what extent it can do that.
My intuition is that if the blade pitch tracks the input signal exactly the resulting sound won't look much like the input signal (but you would be able to generate a DC offset).
I imagine that the blade pitch required to reproduce a given signal depends to some degree on the fan design, frequency and room characteristics. I have no idea how much these parameters would actually contribute though, I think it would require accurate modeling or testing.
I think I'd want to try the derivative of the input signal. I think that this would more closely model a really high displacement diaphragm driver. If you consider a sine wave, when the input signal is crossing zero the diaphragm is moving quickly past it's zero point, pushing lots of air. As the signal reaches max the diaphragm is stopping and changing direction, 'pulling' back on the air.
If you use the derivative of the input signal to control the blade pitch on a fan you get the same behavior. When the signal is at the zero crossing the derivative is at a maximum, so the blade is pitched all the way up, pushing lots of air. As the signal comes to a peak the blades pitch down to flat, pushing no air (and hopefully providing something of an air dam effect), then as the signal starts back down the blades reverse pitch and begin pulling the air back the other way, just as the diaphragm would.
If the signal is zero then the derivative is also zero and the pitch is flat. If the signal is constant at a non-zero point I suppose that ideally the blades would and should be flat, because the ideal behavior is to not increase or decrease pressure.
In practice you'd need some blade pitch to prevent leakage past the imperfect air dam formed by the blades, but I can't see that being a major issue in the real world.
One factor may be in the power required to twist the blades at a higher rate. As the frequency goes up the blades have to rotate more quickly which increases the amount of power required to move them.
Also, if the blades are flapping at audio frequencies they'll act a bit like a diaphragm, making sound independent of the fan action. I suppose that is going to be heavily effected by the rotation, but it's something to check.
Hmm isn't the derivative of a sine function a cosine function? So I'm expecting a phase shift with the fan sub when compared to its pistonic counter part. Perhaps something to keep in mind when you actually use the fan sub. Perhaps a fan speaker is more like a current source (signal changes current, in this case, signal changes flow) and a pistonic speaker is more like a voltage source (signal changes voltage, signal changes pressure)
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