Hello
I have seen a fan woofer once over the net, it produced sound by modulating the angle of the blades.
Now came an idea:
how about modulating rpm speed?
I know there are problems with high inertia and low friction so that it normally produces only "DC pressure", but if one used rpm output feedback I guess some acoustic frequencies could be obtained. 😀
Any input?
I have seen a fan woofer once over the net, it produced sound by modulating the angle of the blades.
Now came an idea:
how about modulating rpm speed?
I know there are problems with high inertia and low friction so that it normally produces only "DC pressure", but if one used rpm output feedback I guess some acoustic frequencies could be obtained. 😀
Any input?
I think your reference to high inertia hits the nail on the head. Starting and stopping (or slowing and speeding up) fan blades at audio frequencies sounds like a trickier proposition than changing their blade angles. Just a gut reaction.
Few
Few
not only that but:
what if the instrument has a different waveform than the fan will always produce, or,
2 intruments are playing different frequencies (a drum and a bass for instance). or,
There is a noise rumble (explosion noise) which is not at any fixed frequency.
it might make a cool instrument, but not an accurate sound reproduction device.
what if the instrument has a different waveform than the fan will always produce, or,
2 intruments are playing different frequencies (a drum and a bass for instance). or,
There is a noise rumble (explosion noise) which is not at any fixed frequency.
it might make a cool instrument, but not an accurate sound reproduction device.
Thanks for replies.
Note that I would like to use RPM feedback loop so that RPM could be linearly proportional to input signal.
Adam
Note that I would like to use RPM feedback loop so that RPM could be linearly proportional to input signal.
Adam
darkfenriz said:
Hi Adam,
i suppose you already know this one:
http://www.rotarywoofer.com/
In changing RPM i see the problem of inertia too.
The rotary woofer avoids changing RPM. Maybe you can keep
your feedback loop to hold RPM constant, when changing the
angle of the blades ...
Cheers
When talking about weird ideas:
Since the movable blades are complex, maybe
some kind of variable aperture in front of a (very strong) fan
could work. Maybe using an enclosure like the pressure source
of an organ ("Windkasten") would help between Fan and aperture.
Misusing conventional drivers as some kind of valve ?
Since the movable blades are complex, maybe
some kind of variable aperture in front of a (very strong) fan
could work. Maybe using an enclosure like the pressure source
of an organ ("Windkasten") would help between Fan and aperture.
Misusing conventional drivers as some kind of valve ?
Modulating air flow
many moons ago ......we're talking about mid 70's ....an inventive pal of mine who later went on to be Australian technical manager of a large multinational company, came up with the idea of modulating the air flow out of an apperture as a type of speaker. Something like a constant flow out of the end of a pipe with a variable size opening that was modulated with the sound signal.
However I remember that he dropped the idea when was thinking of patenting it as he found that the idea had already been used in WWll .....aparently as PA airfields. Lousy quality but very efficient and very high SPLs.
many moons ago ......we're talking about mid 70's ....an inventive pal of mine who later went on to be Australian technical manager of a large multinational company, came up with the idea of modulating the air flow out of an apperture as a type of speaker. Something like a constant flow out of the end of a pipe with a variable size opening that was modulated with the sound signal.
However I remember that he dropped the idea when was thinking of patenting it as he found that the idea had already been used in WWll .....aparently as PA airfields. Lousy quality but very efficient and very high SPLs.
Modulating air flow is a well known technique.
Our human speech organs are based on that principle e.g.
High pressure and modulated air flow by special
control valves is used in high power sonic transducers.
With horns attached to the outlet of the control valves
the sound radiated from rocket engines can be simulated, to
test the structures which are exposed to that sound.
Sound pressures are so high, that exposed structures can
be damaged.
Our human speech organs are based on that principle e.g.
High pressure and modulated air flow by special
control valves is used in high power sonic transducers.
With horns attached to the outlet of the control valves
the sound radiated from rocket engines can be simulated, to
test the structures which are exposed to that sound.
Sound pressures are so high, that exposed structures can
be damaged.
LineArray said:
I think I saw something like this on TV a long time ago. I just remember this big industrial pipe looking thingy with stuff attached to it in a closed room. It involved moving air in this fashion. If I remember correctly, they had a brick wall placed in the room and made it fall over with the massive SPL from the device. I think it fell over on the floor and busted to pieces.
R U talking about soemthing like that ?
http://www.diyaudio.com/forums/showthread.php?postid=616188#post616188
Or do you want to have a fan blowing in the same direction all the time and modulate RPM form zero to full as a function of the input signal, where half the max RPM means zero input signal ?
Things like that have already been done but they used a constant air-pressure "source" that was modulated by some controllable valve.
Regards
Charles
http://www.diyaudio.com/forums/showthread.php?postid=616188#post616188
Or do you want to have a fan blowing in the same direction all the time and modulate RPM form zero to full as a function of the input signal, where half the max RPM means zero input signal ?
Things like that have already been done but they used a constant air-pressure "source" that was modulated by some controllable valve.
Regards
Charles
Hi Charles
I know solutions with a shaft, there are some patents on it. What I exactly want is to modulate RPM of a constantly blowing fan, or probably two push-pull fans modulated out of phase.
Everything in the feedback loop so that RPM follows audio input.
Do you think it could be made practical?
Regards,
Adam
I know solutions with a shaft, there are some patents on it. What I exactly want is to modulate RPM of a constantly blowing fan, or probably two push-pull fans modulated out of phase.
Everything in the feedback loop so that RPM follows audio input.
Do you think it could be made practical?
Regards,
Adam
Look up the formula for the moment of inertia for a disc, which is close enough to a fan. I bet you'll find that the power required to modulate anything over an inch or so in diameter, at even low audio frequencies and useful amplitude, will be mind boggling.
When you have a look at the cyclone woofer (with its seemingly high inertia) one has the impression that it is also feasible but I assume only for very low frequencies.
But you might possibly have troubles to find powerful fans equipped with DC motors.
Regards
Charles
But you might possibly have troubles to find powerful fans equipped with DC motors.
Regards
Charles
Conrad Hoffman said:
OK, I can calculate torgue and "delta PRM ", but how about the amplitude? Seems like a fan is enourmously efficient transducer compared to woofers.
So where is the enargy loss going to happen?
The energy loss will happen because you have to slow this flywheel down- even with a 100% efficient motor you'll have to dump the energy in your amplifier. Then you have to do the reverse and lose about the same amount of energy to speed it up. 150 times a second. No different than a woofer, but a woofer is designed for reasonably low mass and isn't trying to be a flywheel. The only way I can see this having a chance is with an "ironless rotor" type motor, which has very low inertia. Then you might want to construct the motor on the perimeter of the fan, not the shaft. The question then is whether there's enough torque and good enough coupling to get any bandwidth, which I doubt. The reason the variable pitch unit works is because it's so much easier to change the pitch, than the overall speed. Too bad the cost is so high.
I worked a bit with motor control for stage scenery back the the early 90s. Don't' remember too much about it, but here are a few things I learned.
Inertia is the enemy. We always overshot the target, so I had to build in all sorts of tricks to overcome the problem. The mass was very, very high tho. But speeds were low compared to waht you want to do.
We used 3 phase synchronous motors. The 3 phases allowed direction change, but you don't need that, single phase will do you.
Speed was controlled NOT with voltage, but with frequency. The control boards took 220V 50Hz in, rectified it, then generated a 3 phase sine wave of variable frequency. Just a fancy switching supply that ran at 20Khz.
Motor frequency went from DC to about 400Hz, IIRC. DC was used for braking. Voltage was always 220V out, only the frequency changed. The synchronous motors tracked the frequency pretty well and always had full power.
To do something similar, you'd need a VCO to convert your bass voltage signal into a variable frequency to control fan speed.
The advantage of the synchronous motor is that it runs at full power all the time. That helps a lot to control the mass you need to.
Just a thought!
Inertia is the enemy. We always overshot the target, so I had to build in all sorts of tricks to overcome the problem. The mass was very, very high tho. But speeds were low compared to waht you want to do.
We used 3 phase synchronous motors. The 3 phases allowed direction change, but you don't need that, single phase will do you.
Speed was controlled NOT with voltage, but with frequency. The control boards took 220V 50Hz in, rectified it, then generated a 3 phase sine wave of variable frequency. Just a fancy switching supply that ran at 20Khz.
Motor frequency went from DC to about 400Hz, IIRC. DC was used for braking. Voltage was always 220V out, only the frequency changed. The synchronous motors tracked the frequency pretty well and always had full power.
To do something similar, you'd need a VCO to convert your bass voltage signal into a variable frequency to control fan speed.
The advantage of the synchronous motor is that it runs at full power all the time. That helps a lot to control the mass you need to.
Just a thought!
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