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Old 15th April 2003, 02:46 AM   #1
phonon is offline phonon  United States
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Default 0 Hz capable subwoofer (crazy idea...)

This is a somewhat insane idea that I do not have the ability or intention of actually implementing myself, but that may be interesting to others...

Wouldn't it be nice to have a subwoofer with frequency response to indefinitely low frequencies such as, say, 0 Hz? A "DC" 0 Hz output from a speaker would mean a continuous flow of air into the listening area. Even with resonance problems not considered, this is impossible with traditional moving cone speakers, which can only move a volume of air up to the product of their surface area and maximum excursion. To get 0 Hz response, such a speaker would have to have infinite area or excursion, both of which pose manufacturing difficulties.

Here is an alternative method that would allow indefinitely low frequency response:
In a separate room from the listening room, have two air pumps (vacuum cleaners, air compressors, whatever). Lengths of tubing run from these into the listening room; one tube attached to the "output" of an air compressor and one to the "input," so that air is flowing in opposite directions in the two tubes. In the listening room, both of these tubes connect into an enclosure with an opening to the room. Under normal conditions, airflow is simply out one tube and into the other, circulating through the tubing, with none escaping into the room.

Now place an obstruction across the air inflow or outflow tube. The air can no longer circulate; there is either a flow being pumped into or sucked out of the room. Finally, make this obstruction variable, and place an identical one on the other tube. For example, use a paddle that can be rotated in or out of the airflow by a motor. Drive the obstructions 180 out-of-phase with each other with the desired sound signal... airflow into the room will be modulated by the signal... and there you have the ultimate subwoofer! The device can go down to 0 Hz, continuously blowing air into the room. The upper frequency limit is determined by the speed at which the blocking motor can move; with a high-torque drive inserting a lightweight plastic or aluminum paddle into the airflow, 40 Hz or better should be possible. The max. output level is determined by the power of the air-pumps being used.

In fact, this is just like a push-pull amplifier with air currents instead of electricity!

If anyone could build such a device, it would certainly be interesting.

-Michael
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Old 15th April 2003, 03:04 AM   #2
Duo is offline Duo  Canada
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That's an interesting concept, however, you'll have a great trouble producing waveforms on that thing. Only the audio frequency will be reproduced but the bass pipe will always have the same sound, no matter what instrument drives it.
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Old 15th April 2003, 03:12 AM   #3
Kanga is offline Kanga  Australia
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I have just one question: Why?

Nice idea BTW. Sometimes these crazy ideas can lead to totally new ways of doing things. Maybe a voice coil or other traditional loudspeaker technology could be used to control the "valve", so that the power came from the pump and the voice coil was just modulating the amplitude.

Mick
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Old 15th April 2003, 03:25 AM   #4
phonon is offline phonon  United States
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Quote:
Originally posted by Kanga
I have just one question: Why?
Why not ?

It might be easier than building house-sized subfloor bass horns or 100 ft. tall El-Pipe-O's for similar effect. And you'd be one step closer to a speaker system that can really reproduce a square-wave (for those of you who listen to squarewaves instead of music).
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Old 15th April 2003, 03:48 AM   #5
Kanga is offline Kanga  Australia
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Yes but why 0 Hz? You can't hear below 20 Hz, and there are usually no frequencies recorded below about 16Hz.

Also - I thought reproducing square waves required good high frequency response rather than low frequency response.

On a positive note, without having any idea of the theory here, maybe this idea has the potential for high efficiency bass without needing subfloor horns etc. A fan can push air with fairly high efficiency, so if you could modulate the air velocity the possibility is there.

Mick
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Old 15th April 2003, 04:19 AM   #6
CarMan is offline CarMan  United States
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What's the pont of listening to a square wave?

Anyways, the sub is a very good idea.

1: How do you plan to get a fan that will compress air this quickly?

How would you create an air compressor worthy of sub 15hz tones? It should be cheaper to get 15hz tones by conventional methods. It requires tremendous pressures for sub 15hz, much more easlily obtained by a seal (speaker) rather than a fan.. The compressor would have to be very fast and separated by at least 1 chamber to regulate pressures of this level.

2: It *might* be cheaper if you have: a small room: a high (VERY HIGH) powered air compressor 3: sufficient insulation from the air compressor 4: A sufficient air regulator. Doubtful that paddles would work, only variable disc type valves.

Variable disc valves are valves that are of variable circular diameter, The variation would have to be very fast to create a significant frequency response.


Perhaps your idea is nice for subsonic reproduction.
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Old 15th April 2003, 04:29 AM   #7
CarMan is offline CarMan  United States
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Oh... Close to 0Hz response is obtainable in a small room (enclosure.) Use the lowest Fs driver possible and make an EBS box.

If you calculate or measure properly, room gain should make up for the driver rolloff.

This is most easily obtainable if you use half of the listening space for the enclosure. Then response should be equal on both sides and a regular box program should give a good estimate.

You have a very good idea, but if cost is involved it is a very bad idea to even try this.

VERY high quality air compressors and valves would be needed, almost certainly exceeding the cost of speakers capable of the same thing; Except for levels unobtainable by traditional loudspeaers. VERY EXPENSIVE; ossibly viable in the future.
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Old 15th April 2003, 06:39 AM   #8
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My take on this idea:

An air compressor will literally run out of breath very quickly. Sound may be pressure waves, but it's a relatively small change in pressure we're talking, not a shockwave (which even then is higher volume than pressure, I think). I'd go with a turbocharger, these can supply high volume at up to a few atmospheres of pressure per stage. It can be either a fan or squirrel-cage type.

The compressor could be well removed from the load with a pipe, and a local holding tank near the valve, the equivalent of bypassing at the load, would be a nice touch.

The transducer could be a round/rectangular/triangular/whatever-cross-section-allows-linear-response pipe, with a seperator attached to a voice coil (this will probably require a lot of drive power, depending on arrangement, pressure and bearing quality) so that the restriction is variable.

The room will have to be completely sealed, with the only vent being a fixed and calibrated bleed. When the drive vent opens fully, pressure fills the room, and if it had an open door it would just go phut right out the door. Restricting it would allow the actual pressure level to vary.

Tim

P.S. How come you guys say here that response below 20Hz is unnecessary, yet you build your amplifiers for 10Hz or perhaps even less?
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Old 15th April 2003, 09:48 AM   #9
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I say use an entire wall[or two!] to create a huge piston
and use air in the form of pneumatic rams to push the huge piston back and forth ,it's just a matter of horse power
it will work like a big diaphragm compressor then

Just don't try hanging any pictures though


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Old 15th April 2003, 10:50 AM   #10
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Quote:
Originally posted by CarMan
What's the pont of listening to a square wave?

Anyways, the sub is a very good idea.

1: How do you plan to get a fan that will compress air this quickly?

How would you create an air compressor worthy of sub 15hz tones? It should be cheaper to get 15hz tones by conventional methods. It requires tremendous pressures for sub 15hz, much more easlily obtained by a seal (speaker) rather than a fan.. The compressor would have to be very fast and separated by at least 1 chamber to regulate pressures of this level.

2: It *might* be cheaper if you have: a small room: a high (VERY HIGH) powered air compressor 3: sufficient insulation from the air compressor 4: A sufficient air regulator. Doubtful that paddles would work, only variable disc type valves.

Variable disc valves are valves that are of variable circular diameter, The variation would have to be very fast to create a significant frequency response.


Perhaps your idea is nice for subsonic reproduction.
This is a pretty smart idea. I remember that years ago NASA had a similar problem. They wanted to test the structural integrity of the Space Shuttle on its launch pad against the very high power, very low frequency vibrations of the booster engines at launch.
They built the ultimate subwoofer to vibrate the Shuttle assembly. I dont remember all details, but it seems it reached pain levels up to 1 km away....
They used something similar to this idea, but with an electromagnetically operated valve in the high pressure line from a compressed air tank, leading to a standard lf horn with massive dimensions. But it was relative easy for them as they didn't need a nice sinewave output.
There still may be something on the web from this.

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