| hilbren |
I was wondering (not that is possible):
You build a sub with helium in it, the weight of helium is a factor 7 lighter then the air. So the speed of sound in helium is much slower. In fact if the speed is also a factor 7 slower then it is in the air then it should do ~47m/Sec.
That means that if you build a TLsub for about 30 Hz it would normally takes a tube of ~2,75meter, but with helium it should only take 0,39meter.
But because its helium and you want to keep the helium inside, you want to build a closed sub. And if you want to build a closed TL you must build it twice as long. But even then you get a sub thats only 0,78meter.
Just a theory... but is it true? |
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| wes-ninja250 |
When you fill your lungs with helium and speak, you speak in a very high voice. Remember that your voice is given by resonant properties in your larynx, sinuses, etc as well as the frequency of vibration of your vocal chords.
This suggests to me, then, that the Fs of the enclosure will go *up* when filled with helium.
Just a guess, though. It's possible that there's some kind of frequency shift going on when the expelled helium meets regular air, I don't know enough physics to know for sure.
Wes |
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| hilbren |
| So what you are saying is that I have to use a gas thats heavier than air to create the effect? |
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| bigwill |
I just looked on google and it turns out the speed of sound in helium is 970 m/s..
In argon it's 319 m/s, neon is 435 m/s. |
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| hilbren |
okay so with helium the sub gets even longer (that can't be good)
I could not find that with google (wich keyword did you use?)
so I have to look for a gas thats heavier...
but still the big question; would this work? |
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| bigwill |
| I searched for 'speed of sound in helium' etc |
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| 454Casull |
| How do you keep it sealed? |
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| Rademakers |
The speed of sound in sulfurhexafluoride would be about half that of the speed of sound in air. Due to it's inertness and price it would probably be the best choice.
Wkr Johan |
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| tade |
| you live with a billionth of a part of your air as sulphure hexafluoride. I bet it has a terribly bad odor! I would go for uranium hexafluoride. Much better smelling. go ahead, smell it. |
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| BassAwdyO |
| If the sub were completely sealed(which it would HAVE to be for this idea to work) then you wouldnt be smelling what was inside. Take for instance a bottle of colonge, if the bottle is COMPLETELY sealed, can you smell colonge through it? Not unless some is on the outside |
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| Etacovda |
Id be more concerned with some kind of ignition (although it wouldnt be likely to happen if it was vacuumed then filled with helium)
You'd have to have a different method of sealing, id imagine - most glues are porous to a point (much like vinyl tubing is, but it doesnt 'leak' persay) so you'd have to have some kind of container inside the unit to stop it from leaking over time (unless you want to regas your sub... bahaha, classic) |
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| jadenlinkletter |
1. the surround of your woofer leaks air, which makes it impossible to do this.
2. if you could stop the surround of your woofer from leaking air, a sort of nozzle sealed into the side of the box that has an adapter for attaching to a canister of compressed gas might be an easy way to put the gas in, but then you could never fill it completely with the desires gas,, just more and more.
also, has anyone thought about what pressurizing the air behind the woofer would do, (it the surround dind't leak air) i have a feeling that it would change something,
thanks for reading |
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| mazeroth |
| You could put two nozzles on it. One for putting the air in and the other for letting the air inside out. Granted, it would take a LONG time to reach even 95% of the kind of gas you want, but it would work. |
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| BassAwdyO |
| pressurizing the air behind the driver would cause the cone to move out and it would kill the driver's performance. |
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| planet10 |
| quote: | Originally posted by Rademakers
sulfurhexafluoride |
It is something like that, that Michael Dayton-Wright used in both his ESLs & his woofer boxes to give the boxes more apparent volume. (somewhere Nelson posted with the gas)
With Helium you'd need to build a much bigger box.
dave |
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| Peter M. |
Well, give it enough pressure and you would have a dome instead of a cone:D
If you pressurize the gas inside, you would have to do something to keep the voicecoil in the gap. Some kind of springloading maybe. It will stiffen the whole system and the resonant frequency goes up, and sensitivity goes down.
What about loosely filling a gastight thin bag with gas inside the box?
Then you could even have a BR or TL without thinking about gas leaks. |
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| jadenlinkletter |
If the speed of sound is 7:1 in helium:air
and if that means the box would have to be seven times bigger with helium in it.
Who can do the calculations on a very practical gas.
who can figure out what the ratio would be for something that makes sound travel slower
imagine the possibilitie's in a car audio invironment, just think.
When designing a driver, you want it to be sensitive, work in a small box, and handle lots of power. im no expert, but i've been told that you cant have all three. something had to give. as if it were some sort of a trade off between the three.
So if you eliminate the need of working in a small box, you would not need to make a tradeoff on box size, you could focus all your drivers potential in the other areas,
Think about combining this design theory in a driver with FEA(finite element analysis), the driver would be the single best driver in production.
and who better to make the driver then Eminence, They could do the whole thing
Just a thought,
Thanks |
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| Rademakers |
Sulfurhexafluoride is oderless and inflammable, so no problems there (could have found that out yourself btw).
It is also a fairly large molecule so it wouldn't leak that much as air would. Some speakers would be unsuitable because of their surround or dustcap, however a rubber surround would be less sensitive for leakage.
| quote: | | you could never fill it completely with the desires gas,, just more and more |
SF6 is much heavier than air, so it would just sink to the bottom with the air floating on top of it. This way it will replace the air much the same as water would.
Even if it would form a mixture the effect would be very noticeable since the speed of sound in SF6 is much lower as the speed of sound in air.
Wkr Johan |
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| BassAwdyO |
| quote: | | When designing a driver, you want it to be sensitive, work in a small box, and handle lots of power. |
I dont think power handling was in there... I think it was low frequency extension that you were thinking of. |
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| jadenlinkletter |
haha,,
i was hoping someone would help me out with that statement, |
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| Tehrasha |
| Aleast some of those heavy boxes would be easier to move. :D |
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| tade |
Sulfurhexafluoride is oderless and inflammable, so no problems there (could have found that out yourself btw)
Thanks for the tip. i didnt know i could learn information by myself. |
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| xplod1236 |
| quote: | Originally posted by planet10
With Helium you'd need to build a much bigger box.
dave |
I'm not sure what kind of box we're talking about, but for a sealed one, you would need a smaller box.
According to the ideal gas law, "One mole of an ideal gas at STP occupies 22.4 liters." That applies for all gasses, so you would have the same amount of helium as air. The pressures inside and outside the box would be the same, but helium would be a lot easier to compress (because of the smaller molecule size) than air, which would be like having a bigger box filled with air.
| quote: | | the weight of helium is a factor 7 lighter then the air. |
So you would get away with using a helium-filled box to get the same response as you would from a box that is 7 times bigger and holds 7 times more air.
This should be correct according to the ideal gas law, but in my mind it just doesn't seem right, and I can't figure out why.
Anybody want to add to the confusion? |
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| mike.e |
A guy online called MuriloP,he had a site with equations proving why in theory,a box of something other than air could prove useful I forget what it was.
| quote: | | What about loosely filling a gastight thin bag with gas inside the box? |
One or two companys have done it commercially. |
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| rcavictim |
| quote: | Originally posted by Rademakers
Sulfurhexafluoride is oderless and inflammable, so no problems there (could have found that out yourself btw).
Wkr Johan |
Due to a quirk in the english language, there are many, both words flamable and inflamable mean that the substance will burn. Non-flamable, or uninflamable mean no fire. I'm not sure if unflamable is a real word. The meaning seems to be clear though if it is. No fire. |
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| ralphs99 |
In the 70's Cerwin Vega made a system with a Nitrogen filled bladder inside the box.
Cheers,
Ralph. |
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| jadenlinkletter |
i'd say this calls for a project,
if anyone has any useful information.
links,
specs,
company,
post your info.
thanks, |
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| Rademakers |
| quote: | | i didnt know i could learn information by myself |
Meditation perhaps would do that for you, however you could have checked with Google first :rolleyes:
| quote: | | Due to a quirk in the english language, there are many, both words flamable and inflamable mean that the substance will burn. Non-flamable, or uninflamable mean no fire. I'm not sure if unflamable is a real word. The meaning seems to be clear though if it is. No fire. |
Thanx for the pointer :)
Non-flam(m)able it is.
Wkr Johan |
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| phase_accurate |
Re pressurizing box:
Apart form causing problems with coil displacement and challenging a driver's mechanical rigidity it would only do one thing: make the air-volume "stiffer". I.e. it would rise the in-box resonance frequency (fc) of the driver which you don't want - I assume. This effect can be had much cheaper by using a smaller box from the beginning !
Re articifially increasing the internal volume:
There was once a closed-box speaker model by Cerwin Vega (late seventies beginning of the eighties) that had a bag inside which was filled with some inert gas to achieve exactly what is discussed here.
Apart from that I once read about using some stuff that easily changes between fluid and gaseous state at room temperature to achieve the same.
Regards
Charles |
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| SY |
| SF6 is indeed non-toxic, inert, non-inflammable, and dense. It does have one disadvantage- any sparks or arcs will cause it to break down into some VERY toxic components. |
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| Tweeker |
| On the bright side SF6 is harder to arc in than most gases. SF6 is also a very potent greenhouse gas. |
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| Rademakers |
| quote: | | SF6 is indeed non-toxic, inert, non-inflammable, and dense. It does have one disadvantage- any sparks or arcs will cause it to break down into some VERY toxic components. |
Are you sure? :xeye:
It's used in electric insulators, even in the breakers of 100kV+ devices.
If it would break down, it could only break down to the elements it consists of. Perhaps my chemical knowledge isn't extended enough but it seems strange that it would break down from a very stable compound into one of the most reactive elements possible (totall 2 of course).
In other words the opposite of the foundation for predicting the behaviour of molecules by means of electronnegativity and sharing electrons.
In electrical insulators by example, it would mean that it wouldn't insulate at all, but would attack and eventually destroy the metals it would be assumed to protect, unless the reaction is irreverseble enough to immidiatly react to the compound ones again.
On basic chemical and physical properties I would put it in the same class as CFK's and Teflon (carbon surrounded by fluorine). All substances that are known and used for there great thermal stability and overall chemical inertness.
Btw, what does 'arc' mean (couldn't find it)?
Wkr Johan |
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| Tweeker |
The elements it consists of are very nasty. The plasma is hot indeed. S-F bonds arent incredibly strong (as F bounds go). The dielectric strength in power applications is sometimes increased by using N2 to pressurize it. SF6 dielectric strength is about three times that of air.
| quote: | | In electrical insulators by example, it would mean that it wouldn't insulate at all, but would attack and eventually destroy the metals it would be assumed to protect, unless the reaction is irreverseble enough to immidiatly react to the compound ones again. |
Its insulates very well, until it doesnt. Only after its been broken will you have nasties about. |
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| Rademakers |
But is the temperature in a closed bag, inside a closed or vented system, high enough to initiate this?
Wkr Johan |
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| jadenlinkletter |
So one guy sais a gas thats lighter then air will make the internal volume seem bigger
another guy sais that a gas tahts heavier then air will make the internal volume of a box seem bigger,
which is it??
if i go to the store and buy 20 baloons filled with helium, is it going to work?
we all know we can realize huge dynamic peaks in bass in massive ported boxes, i want to put theese in a 5 cubic foot enclosure that has a 15 inch pa woofer in it.
if anyone can help me decide if this will work, i'll have results withtin 3 or 4 hours |
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| SteveCallas |
| Maybe I'm just being cynical, but if you would go to all the work of deciding upon the optimal gas, figuring out a way to remove the air in the enclosure and replace it with said gas, seal off the enclosure completely, and be assured of no ill effects to the driver in order to get the enclosure to act like a bigger enclosure.....why not just build a bigger enclosure instead? :xeye: |
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| jadenlinkletter |
good point,
one answer in my oppinion
more bass, less space.
very practical for DJ'ing and car audio |
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| xplod1236 |
| quote: | Originally posted by jadenlinkletter
So one guy sais a gas thats lighter then air will make the internal volume seem bigger
another guy sais that a gas tahts heavier then air will make the internal volume of a box seem bigger,
which is it?? |
I'm saying that helium is gonna make the internal volume seem bigger. I did a lot of thinking about this, and the theory seems to be correct.
I think that the other guy says that in a gas that's heavier than air, sound will travel slower, which would be useful in the port of a ported box and in a tl.
Somebody correct me if I'm wrong |
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| Svante |
The acoustic compliance of a box is
Ca=V/(rho0*c^2)
for helium rho0=0.1785 kg/m3 and c=970 m/s, V is the box volume. This means that the compliance is 0.85 times that of an air filled box, that is it appears 0.85 times as big. In turn this means that the box would have to be 1/0.85=1.18 times larger to give the loudspeaker the same frequency response as the air filled box.
On the other hand, since the velocity of sound is 970/345=2.8 times higher, all the box resonaces would occur at 2.8 times the frequencies of the air filled box. This is good. In the cases where standing waves is a problem, filling the box with helium could shift these frequencies to a region where they are less of a problem.
However, I think that the main reason that helium is never used is that the gas will eventually leak out of the case, and then the dimensioning of the box will become completely wrong.
And bass-reflex would be out of the question. ;) |
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| SmarmyDog |
It would be nice if we could choose a gas for a sealed subwoofer that would yield much higher compliance by offering a lower 'k' (spring constant – Hooke’s law) for the air-spring that is helping to control and restore the diaphragm. Unfortunately, in a sealed subwoofer system where the captive gas is nominally at ambient atmospheric pressure it will make very little difference which gas we choose since the ideal gas law states PV=nRT (P = pressure, V = volume of gas, n = number of molecules, R = universal gas constant, T = temperature), and n & R are constant. When the diaphragm is displaced it obviously changes V. This slightly alters T but mainly changes P to yield a cone restoring force. Increasing the cabinet size increases the total volume which decreases the change in V for a given cone displacement, thus lowering the restoring P and lowering the resonant frequency of the system. Big box = lower bass.
A note about T: Depending on the thermal coupling of the captive gas with the enclosure, bracing and stuffing materials, T will change somewhere between zero and ‘a tiny amount’ for a given dV/dt.. In other words the system is operating somewhere between adiabatic and isothermic. We add stuffing to a sealed enclosure to give the gas molecules a lot of surface area to thermally couple to, thus biasing the system more toward isothermal operation than adiabatic operation. Isothermal is better since P changes in direct-inverse proportion to V. In adiabatic operation P changes a bit more since the gas is heating during a compression cycle and cooling during a rarefaction ("rarification") cycle – exactly the opposite of what we what since this works to increase the 'k' of the air-spring.
Another option for lowering the frequency response of a sealed subwoofer is to remove the gas altogether and replace it with a low-k alternative to provide the cone restoring force and the force of the atmosphere pushing on the front of the cone. The atmospheric force would be over 1000 pounds for a 12-inch driver. This would obviously be a custom driver and would likely have a honeycomb-core laminated diaphragm and the surrounds could be half-rounds like the ones found in gas-shocks.
-Casey Walsh |
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| SmarmyDog |
Goodness, I didn't mean to run everyone off... :xeye:
-Casey |
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| xplod1236 |
| Anybody have any ideas as to which gas will give us a lower k (spring constant) than air? |
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| phase_accurate |
How about Argon ? I don't know it's rho0 value but it is definitely easy to get, is non toxic and non flammable. It could be enclosed in a bag like CV did. I don't know how much it would leak out of a bag but definitely less than Helium I assume.
Regards
Charles |
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| cyclotronguy |
Just to add a bit more sillyness to our audio hobby. Helium has a habit of escaping out of darn near everything you try to contain it with. In industry this makes H2 very handy for leak checking high vacuum devices.
So for a leaky speaker it would be prudent to design in a constant supply of He, with associated valves, regulators and etc. Lots of spiffy stainless steel plumbing ought to be available surplus thanks to the semiconductor industry.
Concerning ignition (not an issue with He of course). I've worked on quite a few large turbo generators that are filled with H2 as the heat exchange fluid. As long as O2 doesn't get into the mix such that stoichoimetric limits are satisfied, then all that arcing and sparking in a H2 enviornment is of no real concern.
With that in mind we could use any gas we might wish if we keep the speaker enclosure well supplied with gas and the listening area adequately ventilated.;)
Cyclotronguy |
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| hilbren |
| but we already decided that helium would not work. in fact you need a heavier gas to virtually extend the "box".... |
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| EWorkshop1708 |
I was thinking water could be used instead if you can insulate the electrical parts, and have a sub that's sealed.
You could have the sub top-firing from the water filled chamber, and it fires into a 2nd chamber filled with air, that's ported or vented, like a bandpass. |
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| xplod1236 |
| How would that work? You can't compress water:( |
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| Bill F. |
Fun Thread.
I like the idea of bending/breaking things with cheeky, self-important names like "Hoffman's Iron Law." ;)
Exotic enclosure atmospheres aren't a new idea, of course. The earliest patent I'm aware of is from 1957.
Heavy gasses are fine, but they demand a sealed system. The only problem with a sealed system is the fact that we live in a world of fluctuating atmospheric pressure. If your sealed system is a rigid box with a moveable piston, the piston's rest position will change with the weather.
Some have tried to get around this by filling the enclosure somewhat loosely with a sealed bag. In this example, they combined it with a vapor/liquid equilibrium system. Here's another one.
All these are rather complex ways of enhancing compliance. A simpler method, and my favorite, is outlined here. It works by ADsorbing air into a carbon matrix (coconut-husk granular activated charcoal). The only touchy thing about it is GAC's uptake of water. You can allow a pinhole in the sealed enclosure to equalize pressures, but it's recommended to run it through a dessicant canister. |
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| Bill F. |
| And, if you try this, don't worry about KEF coming after you for patent infringment. They're just rehashing U.S. patent # 4,657,108. |
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| SmarmyDog |
Bill F.: As always, you present an incredible amount of useful, pertinent information in very little space. I always look forward to your posts!
I have a pending patent for the use of a full or partial vacuum on the enclosed side of the diaphragm and am always quite interested to see how others have attacked the 'self-important' :) law.
In my vacuum subwoofer prototypes I use half-roll surrounds comprised of a layer of rip-stop nylon (for strength) and a layer of Mylar (to stop in- and out-gassing). It forms an extremely compliant, low loss surround with no limits on excursion. The diaphragm is ˝ inch thick honeycomb-core aluminum laminate.
I love this speaker-building stuff!!! :spin:
-Casey Walsh |
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| Bill F. |
Interesting! I can't see your drawings for some reason, (could you attach one to a post?) but it sounds a lot like something I was chewing on a couple years ago. Here's an early rough drawing from my sketch archives. This one uses a DC servo motor.
Good luck! |
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| SmarmyDog |
| quote: | | I can't see your drawings for some reason |
FWIW, If you're using the AlternaTiff plugin, I cant get it to work in FireFox, only in IE. |
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