I'm asking this more out of a theoretical stance rather than a 'planning to build, please advise' stance, but practical limitations are also interesting and valid concerns.
I know the reason why many sealed boxed get stuffed with polyfil or other similar fibrous materials is that these fibres slow down the speed of sound inside the box, essentially making the box behave larger. Typically, however, this only gains ~20% space change in the physics of the box construction - hardly something an average consumer would take notice of. I had a brainstorm today as I studied for my chemistry final that possibly one could fill a subwoofer box with a different gas than air to achieve the same result but with more efficiency and my mind immediately started contemplating the physics changes that would result to the driver's behaviour. Essentially at this point I decided more knowledgeable input would be wise. Once again, I don't anticipate this happening any time soon but really am still interested if it would be a feasible option to speaker builders.
So I went researching. Speed of Sound suggests that speed of sound is directly affected by the molar mass of a gas and it's specific heat ratio (or adiabatic constant) - temperature for this discussion we can assume to be in the ~293K area since most stereos are indoors - and goes on to suggest that Carbon Dioxide offers a ~22% reduction in sound speed over plain air. I of course went digging to find some safe gas that could possibly increase this size a much larger amount, and sure enough the go-to gas for slowing down the speed of sound in most 'safe' science demonstrations is Sulfur Hexafluoride - with a weight of 146.06 g/mol and a specific adiabatic ratio of 1.0984, this results in an sound velocity of ~135m/s (a reduction of 60% over air). It also has a compressibility factor of 0.98733 (1.013 bar and 15 °C (59 °F)) which is very close to air at 0.999 - though this comparison I'm not terribly familiar with how much difference that entails.
I found a variety of unsuitable gasses that also have a low sound speed, but I'd rather not pump a speaker full of highly flammable or excessively poisonous gas and see what happens...
So would filling a box with sulfur hexafluoride achieve the sonic change of building a larger box and putting the same speaker inside? Will the different density of the gas do weird things to the loudspeaker linearity? What happens if there's a combination of polyfil and sulfur hexafluoride added to the box?
For now, let's keep the discussion concerned with a fully sealed box - tightly sealed - but if there's reason why a partially filled box with a vent at the top would be more appropriate (possibly maintaining barometric pressure constant) please explain.
Part of me really wants to go try this out and measure the differences.
What are your thoughts?
Some of the practical reasons one might want this could include aesthetics, size constraints of the environment such as car audio installs, etc... We could even claim that it's a 'green initiative' 😀 since wikipedia states Sulfur Hexafluoride is the most potent greenhouse gas, with a global warming potential of 23,900http://en.wikipedia.org/wiki/Sulfur_hexafluoride#cite_note-18 times that of CO2 - hence we should sequester it in our speaker boxes.
I know the reason why many sealed boxed get stuffed with polyfil or other similar fibrous materials is that these fibres slow down the speed of sound inside the box, essentially making the box behave larger. Typically, however, this only gains ~20% space change in the physics of the box construction - hardly something an average consumer would take notice of. I had a brainstorm today as I studied for my chemistry final that possibly one could fill a subwoofer box with a different gas than air to achieve the same result but with more efficiency and my mind immediately started contemplating the physics changes that would result to the driver's behaviour. Essentially at this point I decided more knowledgeable input would be wise. Once again, I don't anticipate this happening any time soon but really am still interested if it would be a feasible option to speaker builders.
So I went researching. Speed of Sound suggests that speed of sound is directly affected by the molar mass of a gas and it's specific heat ratio (or adiabatic constant) - temperature for this discussion we can assume to be in the ~293K area since most stereos are indoors - and goes on to suggest that Carbon Dioxide offers a ~22% reduction in sound speed over plain air. I of course went digging to find some safe gas that could possibly increase this size a much larger amount, and sure enough the go-to gas for slowing down the speed of sound in most 'safe' science demonstrations is Sulfur Hexafluoride - with a weight of 146.06 g/mol and a specific adiabatic ratio of 1.0984, this results in an sound velocity of ~135m/s (a reduction of 60% over air). It also has a compressibility factor of 0.98733 (1.013 bar and 15 °C (59 °F)) which is very close to air at 0.999 - though this comparison I'm not terribly familiar with how much difference that entails.
I found a variety of unsuitable gasses that also have a low sound speed, but I'd rather not pump a speaker full of highly flammable or excessively poisonous gas and see what happens...
So would filling a box with sulfur hexafluoride achieve the sonic change of building a larger box and putting the same speaker inside? Will the different density of the gas do weird things to the loudspeaker linearity? What happens if there's a combination of polyfil and sulfur hexafluoride added to the box?
For now, let's keep the discussion concerned with a fully sealed box - tightly sealed - but if there's reason why a partially filled box with a vent at the top would be more appropriate (possibly maintaining barometric pressure constant) please explain.
Part of me really wants to go try this out and measure the differences.
What are your thoughts?
Some of the practical reasons one might want this could include aesthetics, size constraints of the environment such as car audio installs, etc... We could even claim that it's a 'green initiative' 😀 since wikipedia states Sulfur Hexafluoride is the most potent greenhouse gas, with a global warming potential of 23,900http://en.wikipedia.org/wiki/Sulfur_hexafluoride#cite_note-18 times that of CO2 - hence we should sequester it in our speaker boxes.
I would think that the gas would eventually (quickly) leak out through the spider and the voice gap and out the dust cap and/or Surround.
The only reason the gas was used in ESL's was to be able to use higher voltages, This increases the efficiency.
The side effect was a lower resonance peak caused by the higher density of the gas which helped to increase the low end bandwidth.
FWIW
jer 🙂
The only reason the gas was used in ESL's was to be able to use higher voltages, This increases the efficiency.
The side effect was a lower resonance peak caused by the higher density of the gas which helped to increase the low end bandwidth.
FWIW
jer 🙂
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I would think many subwoofers have rubber surrounds, coated cones, and solid plastic dust caps so it may take some time for the gas to permeate these materials. SF6 is also much larger in molecule size than air. That being said, the woofer could easily be placed as an up-firing design so that even if there is some or even readily available permeation of gas through the speaker the weight of it would mean that most stays contained inside the box - still resulting in the hopefully desired effect?
The gas would easily permeate into the motor though, hence why we'd need a non-combustible gas.
The gas would easily permeate into the motor though, hence why we'd need a non-combustible gas.
Here is a very cool video about SF6!!
Oops almost forget to post the link!! 😉
https://www.youtube.com/watch?v=u19QfJWI1oQ
jer 🙂
Oops almost forget to post the link!! 😉
https://www.youtube.com/watch?v=u19QfJWI1oQ
jer 🙂
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Zenon could work too but really expensive gas. Interesting idea though. In the same way, one could decrease the apparent volume of a sealed box by using helium. The problem with any gas is that unless the driver was designed to be gas tight, leaks will ruin the long term effects.
I'd love to see my bosses face as I loaded a cylinder of SF6 into the back of my car. "Brett, where the hell are you going with that?" "Just topping up the subwoofers boss".
Really though the question in my mind is would the physics of the loudspeaker work any differently than if it were in a larger box filled with air? Would the two different mediums on either side of the cone cause non-linearities?
United States Patent 4,101,736
Czerwinski July 18, 1978
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Device for increasing the compliance of a speaker enclosure
Abstract
The present invention is a device for use in combination with a loudspeaker system, that includes a speaker enclosure, in order to effectively enlarge the volume of the speaker enclosure. The loudspeaker system also includes a vibratable cone. The device for effectively enlarging the volume of the speaker enclosure includes a gas having a gamma less than 1.4 and the product of its density and the square of the speed of sound therein less than the same product for air and a bag which is formed from a soft, pliable material for enclosing the gas within the speaker enclosure and which is adapted to seal the gas therein. The device also includes an acoustically transparent and porous cocoon which is disposed about the bag so that it surrounds completely the bag and an acoustical padding which is disposed adjacent to the sidewalls of the speaker enclosure and which is adapted to enclose the acoustically transparent and porous cocoon. The device may also include a device for generating the gas by heating a fluid in its liquid phase so that the fluid changes to its gas phase. The device for heating the fluid may either be a resistive electrical element which is disposed within the speaker enclosure or fibrous, sound absorbent material which is disposed within the bag. The device is placed in the speaker enclosure in back of the vibratable cone in order to increase the compliance that the vibratable cone sees.
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Inventors: Czerwinski; Eugene J. (Studio City, CA)
Assignee: Cerwin Vega, Inc. (Los Angeles, CA)
Family ID: 25113322
Appl. No.: 05/844,709
Filed: October 25, 1977
Czerwinski July 18, 1978
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Device for increasing the compliance of a speaker enclosure
Abstract
The present invention is a device for use in combination with a loudspeaker system, that includes a speaker enclosure, in order to effectively enlarge the volume of the speaker enclosure. The loudspeaker system also includes a vibratable cone. The device for effectively enlarging the volume of the speaker enclosure includes a gas having a gamma less than 1.4 and the product of its density and the square of the speed of sound therein less than the same product for air and a bag which is formed from a soft, pliable material for enclosing the gas within the speaker enclosure and which is adapted to seal the gas therein. The device also includes an acoustically transparent and porous cocoon which is disposed about the bag so that it surrounds completely the bag and an acoustical padding which is disposed adjacent to the sidewalls of the speaker enclosure and which is adapted to enclose the acoustically transparent and porous cocoon. The device may also include a device for generating the gas by heating a fluid in its liquid phase so that the fluid changes to its gas phase. The device for heating the fluid may either be a resistive electrical element which is disposed within the speaker enclosure or fibrous, sound absorbent material which is disposed within the bag. The device is placed in the speaker enclosure in back of the vibratable cone in order to increase the compliance that the vibratable cone sees.
--------------------------------------------------------------------------------
Inventors: Czerwinski; Eugene J. (Studio City, CA)
Assignee: Cerwin Vega, Inc. (Los Angeles, CA)
Family ID: 25113322
Appl. No.: 05/844,709
Filed: October 25, 1977
Reduces chance of leakage. Built a small sealed two way back in '87 using argon and a thermally sealed mylar bag. Works!
Over time gas leaked, especially the day when power went out in the heat of South Carolina on a 100+ day. The before and after was drastic. Got bored with it over the next year and built something a bit more formal size wise.
Can see it affecting a TL type design greatly. BR is out, but passive radiator is in.
Lastly here is my take on all this, we are DIY, if the theory is sound, then why the heck not give it a whirl 😉
Sulfur hexafluoride is expensive! One of the heavier weight freons might be useful in this respect, and easier to come by.
I assume only partially filling the bag would help prevent leakage - gives room for the gas to expand. Also, why a mylar bag? Any reason not to use plastic?
Dayton Wright produced the SW -10 with SF 6 bags internally in the 1970/1980s It lowered the system resonance significantly. . I have two of them which need reconing / driver replacement. I'd let them go cheaply if your interested. It also formed the bass section of the Watson Lab model 10
WATSON LABS MODEL 10 SPEAKER
WATSON LABS MODEL 10 SPEAKER
A heavier gas would change the response of ports, and maybe some funny things related to molar mass/ speed sound, but what you really want is to be on a flatter part of the pressure/volume curve, and that either means more volume or less gas in the same volume. You do the latter by drawing a vacuum, which would work really well if you could figure out a very high impedance way of biasing several hundred pounds force on the cone.
most don't appreciate the range of atmospheric pressure variation in your home - you can't really make a perfectly sealed box loudspeker or the cone would spend much time bottomed or flopped out of the gap
and SF6 is about a million (its the combination with its extreme atmospheric lifetime) times the greenhouse effect of CO2 - so the Greens have banned it under RoHS
despite the electrical power generating/switching industry measuring their annual SF6 leakage in kilotons
and SF6 is about a million (its the combination with its extreme atmospheric lifetime) times the greenhouse effect of CO2 - so the Greens have banned it under RoHS
despite the electrical power generating/switching industry measuring their annual SF6 leakage in kilotons
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Prior attempt
Hi, Stochastic:
I remembered reading an account of one team's experiments along these lines. I searched it up again, and you can find it here: SF6 + 16Ov.2 = Subwoofer Experiment
(Have you already seen this?) A quick review of their (2008) report on their efforts leaves me unsure of whether their results were considered encouraging. Perhaps your further research is warranted...😉
Wilf
Hi, Stochastic:
I remembered reading an account of one team's experiments along these lines. I searched it up again, and you can find it here: SF6 + 16Ov.2 = Subwoofer Experiment
(Have you already seen this?) A quick review of their (2008) report on their efforts leaves me unsure of whether their results were considered encouraging. Perhaps your further research is warranted...😉
Wilf
W Oswald,
Thanks for that link. Great experiment and gives us a lot of background info on this whole idea. There was a question posed in that thread if gas density goes up the speed of sound should go up too based on fact that sound travels faster in water and solids. Speed of sound in a gas is equal to the sqrt (ratio of specific heat x universal gas constant x temperature / molecular weight). So it is inversely proportional to the sqrt of the molecular weight - or slower. Thus the improved Vb should be proportional to ratio of sqrt of MW of SF6/Air or sqrt(146/30) or 2.2x increase. Which is the difference in the net effective increase in Vb they measured. Not 27x - not sure where that number came from. So you get an effective doubling of box volume - which may or may not be substantial.
As long as the box is gas tight and driver cone is not paper and rubber surrounds are used, and airtight foam rubber gaskets or rubber o-rings are used at driver bezel gasket seal - the SF6 should be well contained. Being a heavy gas with a large molecule size it will not leak as easily as air through small spaces between seal surfaces.
It looks like combining polyfill with SF6 provides additive benefits.
Thanks for that link. Great experiment and gives us a lot of background info on this whole idea. There was a question posed in that thread if gas density goes up the speed of sound should go up too based on fact that sound travels faster in water and solids. Speed of sound in a gas is equal to the sqrt (ratio of specific heat x universal gas constant x temperature / molecular weight). So it is inversely proportional to the sqrt of the molecular weight - or slower. Thus the improved Vb should be proportional to ratio of sqrt of MW of SF6/Air or sqrt(146/30) or 2.2x increase. Which is the difference in the net effective increase in Vb they measured. Not 27x - not sure where that number came from. So you get an effective doubling of box volume - which may or may not be substantial.
As long as the box is gas tight and driver cone is not paper and rubber surrounds are used, and airtight foam rubber gaskets or rubber o-rings are used at driver bezel gasket seal - the SF6 should be well contained. Being a heavy gas with a large molecule size it will not leak as easily as air through small spaces between seal surfaces.
It looks like combining polyfill with SF6 provides additive benefits.
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