Possibly a silly question but here goes anyway.
What would happen if you were to take an acoustic suspension enclosure, make it airtight, and then evacuate the air inside to create a partial vacuum? What would happen to the rear wave since it would have no medium through which to propagate?
Would this reduce cabinet colourations?
What would happen if you were to take an acoustic suspension enclosure, make it airtight, and then evacuate the air inside to create a partial vacuum? What would happen to the rear wave since it would have no medium through which to propagate?
Would this reduce cabinet colourations?
Here are some recent thoughts...
http://www.diyaudio.com/forums/showthread.php?postid=741374#post741374
-Casey Walsh
http://www.diyaudio.com/forums/showthread.php?postid=741374#post741374
-Casey Walsh
Well, multiply the pistonic surface area of the cone by atmospheric pressure. That number is the force applied to the cone [inward].
I don't think any kind of speaker can handle that kind of ridiculous abuse.
EDIT: However, if you do have a cone/suspension that can handle it, I believe you could achieve a response like that of an infinite baffle (but in sealed enclosure with a volume of virtually zero).
I don't think any kind of speaker can handle that kind of ridiculous abuse.
EDIT: However, if you do have a cone/suspension that can handle it, I believe you could achieve a response like that of an infinite baffle (but in sealed enclosure with a volume of virtually zero).
Actually, it would not necessarily take a lot of force to move it. You would just need a suspension with the right spring constant and a lot of play.
Think of it this way- if you had a really long spring (hundreds of feet), it might have 500 pounds hanging from it, but another ounce of force could move it an inch or more. That's assuming a linear spring constant.
I think it sounds reasonable as far as the physics of it go, but I'm not sure if you could arrange a geometry that would be able to take advantage of it. Perhaps a really funky spring constant could do it; or a servo motor with a DSP controlled displacement/force curve.
I wouldn't call the vacuum woofer a physical impossibility yet, but I doubt it's within the range of the DIY community, nor will it be within the next 10 years.
Edit- change units to metric
Disclaimer- I am a fan of science fiction
Think of it this way- if you had a really long spring (hundreds of feet), it might have 500 pounds hanging from it, but another ounce of force could move it an inch or more. That's assuming a linear spring constant.
I think it sounds reasonable as far as the physics of it go, but I'm not sure if you could arrange a geometry that would be able to take advantage of it. Perhaps a really funky spring constant could do it; or a servo motor with a DSP controlled displacement/force curve.
I wouldn't call the vacuum woofer a physical impossibility yet, but I doubt it's within the range of the DIY community, nor will it be within the next 10 years.
Edit- change units to metric
Disclaimer- I am a fan of science fiction
What if you make loudspeaker box U-shaped, put some elastic foil on it and suck the air away. The foil bend inside the box until it has enough tensile elongation. Pressure is "only" 1/5-1/10 what you have in bicycle wheel. Not anything that the right material won´t stand.
And there you have the "cone". Box can be very thin, very tall and/or wide.
But how to move the "cone" since it´s not stiff enough for a point force?
ESL?
And there you have the "cone". Box can be very thin, very tall and/or wide.
But how to move the "cone" since it´s not stiff enough for a point force?
ESL?
Construction:
1) U-shaped box
2) Put the foil on
3) Suck the air away. Foil bend inside and form "almost" U-shaped surface.
4) Use that suface as a mould. Put on some stuff and build an exact copy of the surface.
5) Build a new box using that "copy". Make it conductive.
6) Buld a grille using that same "copy".
7) Put a foil on the new box.
8) Suck the air away.
9) Insert grille.
10) There you have an vacuum ESL.
But what would be the best foil material?
If should stretch a lot and after that keep the shape even if tensile and elongation are big. These make a kind of "offset spring" to compensate air pressure (DC) and still allowing foil to move a bit (AC) with small force.
1) U-shaped box
2) Put the foil on
3) Suck the air away. Foil bend inside and form "almost" U-shaped surface.
4) Use that suface as a mould. Put on some stuff and build an exact copy of the surface.
5) Build a new box using that "copy". Make it conductive.
6) Buld a grille using that same "copy".
7) Put a foil on the new box.
8) Suck the air away.
9) Insert grille.
10) There you have an vacuum ESL.
But what would be the best foil material?
If should stretch a lot and after that keep the shape even if tensile and elongation are big. These make a kind of "offset spring" to compensate air pressure (DC) and still allowing foil to move a bit (AC) with small force.
Atmosphere is 14.7 pounds per square inch above absolute vacuum.
For the sake of the exercise let's make the driver active surface 6 inches in diameter and flat. This is approximately 28 square inches.
For the driver to work in a conventional manner with reasonable excursion the suspension must be pre-loaded with a force of 411 pounds (187 kilograms) pushing out. (A 12 inch diameter is 755 kilograms, you could lift a compact car with that.)
Down-firing with a weight on the diaphragm to compensate for the atmosphere is somewhat impractical.
The suspension pressure will have to be re-calibrated for the altitude you live in. Severe storms with their low pressures would also require adjustment. A solution would be reducing the pre-load to about 150kg and varying the remaining pressure in the cabinet to maintain the rest position.
A magnetic method of compensation may work, think of an opposing pole rare earth magnet under the dust cap pushing on the pole piece of the magnet. Perhaps a chunk of room temperature superconductor.
I think the foil under tension would sound terrible, look at the inflatable speakers thread that was about recently, the membrane under tension has a tendency to ring. Perhaps the ringing may be damped, ENABL? (Edit, then again electrostatics are under a little bit of tension, it might be fine)
Filling a cabinet with helium would be a very interesting experiment, It would keep the VC cool too. Some military aircraft instruments are actually helium filled (instead of the usual dry nitrogen) to aid heat dissipation. it may reduce box colorations, but the air-spring effect on the cone would be unchanged for a given volume.
For the sake of the exercise let's make the driver active surface 6 inches in diameter and flat. This is approximately 28 square inches.
For the driver to work in a conventional manner with reasonable excursion the suspension must be pre-loaded with a force of 411 pounds (187 kilograms) pushing out. (A 12 inch diameter is 755 kilograms, you could lift a compact car with that.)
Down-firing with a weight on the diaphragm to compensate for the atmosphere is somewhat impractical.
The suspension pressure will have to be re-calibrated for the altitude you live in. Severe storms with their low pressures would also require adjustment. A solution would be reducing the pre-load to about 150kg and varying the remaining pressure in the cabinet to maintain the rest position.
A magnetic method of compensation may work, think of an opposing pole rare earth magnet under the dust cap pushing on the pole piece of the magnet. Perhaps a chunk of room temperature superconductor.
I think the foil under tension would sound terrible, look at the inflatable speakers thread that was about recently, the membrane under tension has a tendency to ring. Perhaps the ringing may be damped, ENABL? (Edit, then again electrostatics are under a little bit of tension, it might be fine)
Filling a cabinet with helium would be a very interesting experiment, It would keep the VC cool too. Some military aircraft instruments are actually helium filled (instead of the usual dry nitrogen) to aid heat dissipation. it may reduce box colorations, but the air-spring effect on the cone would be unchanged for a given volume.
Well, I did get the patent issued (# 7,068,806), but got caught up with raising three very active, inquisitive little boys, so developing something of it has been hit-and-miss. Mostly miss. PBS very nearly took the idea to develop a commercial product through their engineering-based reality show “Everyday Edisons”, but ultimately saw too much risk due to a potentially longer development effort that their one-year cycle for the show. It was kind of cool being interviewed under the big lights with cameras rolling, glossy forehead and all!
Here are some old pictures I found on my web server. I’ll leave them without descriptions for now because I love hearing peoples’ guesses before I explain things.
http://photonlogic.com/PicServe/SubProto003/IMG_2256.JPG
http://photonlogic.com/PicServe/SubProto003/IMG_2294.JPG
http://photonlogic.com/PicServe/SubProto003/IMG_2303.JPG
http://photonlogic.com/PicServe/SubProto003/IMG_2305.JPG
http://photonlogic.com/PicServe/SubProto003/IMG_2307.JPG
http://photonlogic.com/PicServe/SubProto003/IMG_2332b.JPG
I’m presently at work, but I’ll try and pull up some more explanatory pictures or CAD renderings in the next couple days.
Cheers,
Casey Walsh
Here are some old pictures I found on my web server. I’ll leave them without descriptions for now because I love hearing peoples’ guesses before I explain things.
http://photonlogic.com/PicServe/SubProto003/IMG_2256.JPG
http://photonlogic.com/PicServe/SubProto003/IMG_2294.JPG
http://photonlogic.com/PicServe/SubProto003/IMG_2303.JPG
http://photonlogic.com/PicServe/SubProto003/IMG_2305.JPG
http://photonlogic.com/PicServe/SubProto003/IMG_2307.JPG
http://photonlogic.com/PicServe/SubProto003/IMG_2332b.JPG
I’m presently at work, but I’ll try and pull up some more explanatory pictures or CAD renderings in the next couple days.
Cheers,
Casey Walsh
Interesting thought.
In real life a force of the atmospheric pressure multiplied with the driver membrane area would push the cone inwards. For a 4" driver with an area of about 100 cm², the force would be 0.01*100000=1000 N, or roughly the weight of 100 kg. This force would have to be balanced somehow.
Given the unlikely case that this was actually possible, the box would behave as if it had an infinite volume, and there would be no resonances in the cavity. Resonances in the walls would remain, though. Then again, the wall would have to be made thicker in order to be able to take the vacuum inside the box without imploding.
Bottom line here is that it would be very costly to do this in real life. It is a very nice and stimlulating thought, though.
In real life a force of the atmospheric pressure multiplied with the driver membrane area would push the cone inwards. For a 4" driver with an area of about 100 cm², the force would be 0.01*100000=1000 N, or roughly the weight of 100 kg. This force would have to be balanced somehow.
Given the unlikely case that this was actually possible, the box would behave as if it had an infinite volume, and there would be no resonances in the cavity. Resonances in the walls would remain, though. Then again, the wall would have to be made thicker in order to be able to take the vacuum inside the box without imploding.
Bottom line here is that it would be very costly to do this in real life. It is a very nice and stimlulating thought, though.
Extremely clever idea though. That is what I love about the DIY community. Someone posts what if? and next thing you know a guy is posting pictures of the actual device that he has been experimenting with. I'm going to have to keep an eye on this development, but a good Linkwitz transform gets a small enough box for me!
Regards,
David
Regards,
David
I pondered this back when it dawned on me that a big box actually does nothing except to lessen the effect of the air inside, i.e., lower the airspring stiffness the compressing of which is what requires all that power at low freq.
As said, no air would mean no spring to compress, but there's the side effect of the huge external air pressure force to contend with.
What is needed is a low mass, low rate balance spring which can develop a huge preload.
Mechanical springs would be too massive, and a magnetic preload would cost more than the standard way of getting a lot of deep bass from a small box - beefy drivers and lots of power.
Not to mention the cost of a diaphragm that could withstand the pressure.
As a point of interest, I calculated the max air pressure of a pair of Avalanche 18's at max excursion in a 15 cf box to be a few tenths of a psi.
As said, no air would mean no spring to compress, but there's the side effect of the huge external air pressure force to contend with.
What is needed is a low mass, low rate balance spring which can develop a huge preload.
Mechanical springs would be too massive, and a magnetic preload would cost more than the standard way of getting a lot of deep bass from a small box - beefy drivers and lots of power.
Not to mention the cost of a diaphragm that could withstand the pressure.
As a point of interest, I calculated the max air pressure of a pair of Avalanche 18's at max excursion in a 15 cf box to be a few tenths of a psi.
Soulwax said:
Hi.
To go back to the original question, with no air to conduct the soundwave, there would not be a rear wave.
Let's continue by asking the question in a different way.
What can be done to the gas behind the speaker cone, in a sealed enclosure, to reduce the effect of the rear wave to something approaching zero?
Lower pressure.
Reduced / increased mass of gas.
Different gas.
Speaker construction
Liquid fill / hydro-pneumatic
Modified isobaric construction
ABR
Spherical
Just some thoughts
Andy
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