I wanted to make a small (e.g., three-inch diameter) but powerful loudspeaker for low frequencies, and thought one might be able to modulate the movement of a lot of air by varying the pitch of a propeller.

The idea is that you'd make a propeller shape from a moderately flexible material (e.g., HDPE, such as you could cut from a plastic milk jug), glue thin bar magnets to the blades, mount the propeller on a motor shaft, locate it inside a solenoid, and vary the solenoid current to change the propeller pitch. If you make the propeller spin fast enough, and drive the solenoid with an audio waveform, you should be able to hear the audio. (Certainly you'd also get regular old propeller noise. Bummer. Maybe that could be baffled.)

Of course, the air the propeller is driving must come from somewhere, and you don't want sound from back of the propeller to cancel sound from the front, so you'd need to put the whole thing in a duct.

I actually started trying to build one of these things when I was in college, but I never finished it, and I wouldn't tell anyone what I was trying to make, because I didn't want to be laughed at. One very nice thing about getting older is that I don't care about that anymore.

(Looks like I need to learn how to search better. My idea is similar to Ultimate Subwoofer (see link), but in that idea, a fan is modulated by a valve.)
-- colorclocks, Jan 19 2009

Ultimate Subwoofer Ultimate_20Subwoofer
fan + valve [colorclocks, Jan 19 2009]

Auxetophone video http://www.youtube....watch?v=J7SV65DFNy8
Mechanical compressed air audio amplification. [Spacecoyote, Jan 19 2009]

Auxetophone article http://www.douglas-...ne/auxetoph.htm#aux
How it works, and more. [Spacecoyote, Jan 19 2009, last modified Dec 09 2015]

Thigpen Rotary Subwoofer http://www.eminent-....com/RWbrochure.htm
Prior art [csea, Feb 03 2009]

Videos of the Rotary SW http://bassment.wordpress.com/
as above [csea, Feb 03 2009]

Reminds me of those Tesla coil speakers.

And of course, the Auxetophone [link].
-- Spacecoyote, Jan 19 2009


That Auxetophone is awesome! Someone was having way too much fun.
-- colorclocks, Jan 19 2009


I imagine this design could be fairly efficient, as the motor would be doing little work except when it is actually moving air.

The amplified instrument in the Auxetophone article is clearly a double bass, not a cello as described.
-- spidermother, Jan 26 2009


//I wouldn't tell anyone what I was trying to make, because I didn't want to be laughed at.//

<with open arms>
Welcome to the half-bakery!
</woa>
-- pertinax, Jan 26 2009


Bruce Thigpen has been working on this for some time; see [links].
-- csea, Feb 03 2009


Presuming there is a way to make a tuneable pulse-jet, that would certainly fit the bill decibel-wise.
-- not_morrison_rm, Jun 29 2013


I have never heard of a tunable pulse jet, and anyway thee would surely still be a fairly narrow frequency range within which it would pulse, and also it would not be very responsive int he stopping and starting sense?

But you could have an explosive speaker if you had a carousel of explosive charges. The charges would explode when they reached a high-speed detonator mounted at the entrance of the amplifying horn; on the exit side of the horn would be a mechanism to clean the carousel and insert a new charge. The speed of the carousel would be modulated for the pitch required.
-- pocmloc, Jun 29 2013


At what point on the amplitude/frequency/power map does sound become weather?
-- MaxwellBuchanan, Jun 29 2013


//anyway thee would surely still be a fairly

anno bun for use of "thee" (well, if there was a way to bun annos, I'll keep it until it is possible) nothing like a bit of King James bible English on a Sunday morning.

Well, I was thinking some kind of trombone-like pulse-jet. Just remember not trying to blow into it when it's working.

//a carousel of explosive charges.

Carruthers "stop the armoured car!"

Driver "What is it?"

Carruthers "look carefully, under that mound...."

Driver "Yes, good lord it's an improvised musical explosive device!"

Carruthers "Exactly! And I'm never listening to "Copacabana" at 120 decibels ever again. Call the sapper.."
-- not_morrison_rm, Jun 30 2013



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