This unit would have to operate into a sealed box - the 'listening' room, otherwise the air pressure would leak out. Also, the SPL wouldn't be anywhere near high enough in my opinion.
Therefore, I reckon it would be better to construct a large (rubber?) diaphram, maybe 3 or 4 feet across, driven at the centre by a push-rod, itself operated electromagnetically in a similar manner to a conventional speaker, but with the voice coil being static and surrounding the pushrod.
This large diaphram would then be mounted semi-permanently into a doorway or wall, the idea being to pressurise or rarify the air in the room, creating that infrasound effect.
The big issue with both of these ideas is that no sound wave is actually launched. All that is happening is that the internal pressure of the room is being varied, and I'm not sure how realistic the end results would be. I certainly can't see a radial fan generating anything like enough pressure difference to be noticable.
Interesting idea though - thanks for showing us!
Andy.
Therefore, I reckon it would be better to construct a large (rubber?) diaphram, maybe 3 or 4 feet across, driven at the centre by a push-rod, itself operated electromagnetically in a similar manner to a conventional speaker, but with the voice coil being static and surrounding the pushrod.
This large diaphram would then be mounted semi-permanently into a doorway or wall, the idea being to pressurise or rarify the air in the room, creating that infrasound effect.
The big issue with both of these ideas is that no sound wave is actually launched. All that is happening is that the internal pressure of the room is being varied, and I'm not sure how realistic the end results would be. I certainly can't see a radial fan generating anything like enough pressure difference to be noticable.
Interesting idea though - thanks for showing us!
Andy.
I have a couple of the original phoenix gold cyclone subwoofers, fixed vane coupled to dc servo motor devices that are supposed to play down to 11hz. Let me know if somehow they can help you with your project.
Hans
Hans
No, they are a fixed vane servo sub. The cylinder housing has a "screw"/ramp inside, so the air-motion is transfered forward.
Hans
Hans
I’m working on a mini setup right now of this fan sub with the blades I’ll let you know if it works its almost finished.
I reckon this would work 2 subwoofers with massive excursion e.g. jlaudio w7, mtx thunder9500 or the RFL subwoofer would work great with like 4 inch travel, 2 of those subs forcing the air out of a very small port that’s like have the pressure of the fan sub.
I reckon this would work 2 subwoofers with massive excursion e.g. jlaudio w7, mtx thunder9500 or the RFL subwoofer would work great with like 4 inch travel, 2 of those subs forcing the air out of a very small port that’s like have the pressure of the fan sub.
An externally hosted image should be here but it was not working when we last tested it.
the fan sub's claim to fame is its large displacement not high pressure. It can move a huge amount of air at low frequencies compared to any reasonable number of conventional subwoofers. What Hayden seems to have come up with is a compression driver in that it would produce high pressure, but not the necessary volume at say five hertz. Horn load it, then we are talking!
What? Are you talking volume as in the amount of air (pressure) moved?
What’s the difference in horn loading it?
What’s the difference in horn loading it?
Yes, that is what i mean by volume. Volume in the physical sense. If you put dc to a fan subwoofer it could move enough air to fill up a hot air balloon if given enough time. If you gave dc to a conventional subwoofer the cones would move to their xmax and stop. now, most of the time we are nowhere near dc, however the volume of air which needs to be moved at five hz is pretty much unachieveable by conventional means. This is where a fan subwoofer comes in. Horns are a different beast altogether which i dont understand so I will recite the doctrine: horns convert high pressure into high volume like a transformer. I think your diagram is very interesting because you have drawn a horn compression driver without knowing what one looks like!
The small sealed chamber behind the woofers limits their excursion and their functionality to moving small amounts of air at high pressures. Then the air must exit through a small mouth which again reduces the excursion and increases the pressure. In a proper compression driver what all this equates to is a lightweight diaphram which doesnt have to move much. This allows one to concentrate a strong magnetic force on a short voice coil capable of generating large amounts of force. In order to reap this advantage a horn must be added to convet the high pressure into high volume. There is always a tradeoff, especially down low, where a horn capable of acting at five hz would have to have to have a mouth of a quarter the wavelength of sound in air at five hz.
The small sealed chamber behind the woofers limits their excursion and their functionality to moving small amounts of air at high pressures. Then the air must exit through a small mouth which again reduces the excursion and increases the pressure. In a proper compression driver what all this equates to is a lightweight diaphram which doesnt have to move much. This allows one to concentrate a strong magnetic force on a short voice coil capable of generating large amounts of force. In order to reap this advantage a horn must be added to convet the high pressure into high volume. There is always a tradeoff, especially down low, where a horn capable of acting at five hz would have to have to have a mouth of a quarter the wavelength of sound in air at five hz.
The surrounding air (ie. the air in your room) has a low pressure, so a high pressure build-up like in front of a driver is inefficient in transporting it's energy too the surrounding air.
A horn transforms the high pressure (/low velocity) in front of the driver into low pressure (/high velocity). This way it's much more efficient in the energy transport to the surrounding air, resulting in higher SPL (sound pressure level).
To direct the sound, the length and moutharea of a horn need to be in proportion to the size of the wavelength, that the loudspeaker is going to reproduce.
A 5 Hz wavelength is ~ 67 meter (~220 ft.). To keep the horn small, the horn length is ussually the same as the lowest 1/4 wavelength it represents, so 67/4 meter (still very long).
Below the 1/4 wavelength resonance the horn frequency response drops of quickly. If the energy at the rear of the driver is utilized (like with a closed box) these properties can be optimised for the desired parameters.
The fan would likely still need a IB as rearchamber if at all.
Wkr Johan
Correct me i I'm wrong,
A horn is an acoustical transformer. If pressure decreases from high to low, something else must increase proportionally. Like an electrical transformer where a (transformed) rise in current equals a drop in voltage (all else equal). Afaik velocity (edit- perhaps I'm using the wrong word) is connected with pressure in this way.
Because of this, in the beginning of a horn you can get away with a sharp bend. Due to the increase near the mouth conventional horndesign incoörperate only moderate bends in the horn path near the mouth.
Wkr Johan
A horn is an acoustical transformer. If pressure decreases from high to low, something else must increase proportionally. Like an electrical transformer where a (transformed) rise in current equals a drop in voltage (all else equal). Afaik velocity (edit- perhaps I'm using the wrong word) is connected with pressure in this way.
Because of this, in the beginning of a horn you can get away with a sharp bend. Due to the increase near the mouth conventional horndesign incoörperate only moderate bends in the horn path near the mouth.
Wkr Johan
nice explanation!
I think air velocity is constant and the speed of sound. so if the pressure is decreasing the volume of swept air must be increasing.
groovy
Can we treat the mouth area of a horn as the area of a driver where polar response is concerned?
I think air velocity is constant and the speed of sound. so if the pressure is decreasing the volume of swept air must be increasing.
groovy
Can we treat the mouth area of a horn as the area of a driver where polar response is concerned?
Funny that everyone talks about helicopter blades.
When I 1st saw this thing I thought "hey, it's a variable pitch prop!" Like on an airplane. All blades have the same pitch. Airplane props use oil pressure to change the pitch, but I don't know what the mechanics inside look like. Same idea as the helicopter, just simpler.
But how about another idea? Variable speed fans.
My electronic catalogs always have small 12V plastic fans on sale. Computer type fans. How about a large block of those?
Since they are small, the mass is low - so they may be controlable - you just use a lot of them to get the volume you want. Best idea might be a DC bias to keep the fans moving all the time, just vary the speed. No need to change direction, the fans probably can't do that very fast - like 30Hz. But they should be able to change speed fast enough, if they are spinning all the time. Remember, low mass.
One problem might be dynamic range. What's the difference in flow between lowest and highest speed?
Could be fun to try. You'd just need a big (very big) DC blocking cap and a DC supply to keep the fans running. Add an AC signal from your amp and voila! Cheap Fan Sub. 😀
When I 1st saw this thing I thought "hey, it's a variable pitch prop!" Like on an airplane. All blades have the same pitch. Airplane props use oil pressure to change the pitch, but I don't know what the mechanics inside look like. Same idea as the helicopter, just simpler.
But how about another idea? Variable speed fans.
My electronic catalogs always have small 12V plastic fans on sale. Computer type fans. How about a large block of those?
Since they are small, the mass is low - so they may be controlable - you just use a lot of them to get the volume you want. Best idea might be a DC bias to keep the fans moving all the time, just vary the speed. No need to change direction, the fans probably can't do that very fast - like 30Hz. But they should be able to change speed fast enough, if they are spinning all the time. Remember, low mass.
One problem might be dynamic range. What's the difference in flow between lowest and highest speed?
Could be fun to try. You'd just need a big (very big) DC blocking cap and a DC supply to keep the fans running. Add an AC signal from your amp and voila! Cheap Fan Sub. 😀
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