Modulate the rpm of the fan to get acoustic output?

[darkfenriz]

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?

[Few]

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

[neutron7]

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.

[darkfenriz]

Thanks for replies.

Note that I would like to use RPM feedback loop so that RPM could be linearly proportional to input signal.

Adam

[LineArray]

Originally posted by darkfenriz
Thanks for replies.

Note that I would like to use RPM feedback loop so that RPM could be linearly proportional to input signal.

Adam

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

[wakibaki]

Originally posted by darkfenriz
how about modulating rpm speed?

What, you mean, like, just for the hell of it?

You need to get out more...

w

[LineArray]

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 ?

[kimbo]

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.

[LineArray]

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.

[BHTX]

Originally posted by LineArray
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.

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.