On any LF transducer, the VAS is proportional to efficiency. So if you build a big VAS transducer you get high efficiency that is what you want. BUT the enclosure must be very big for keeping the Q equally sensible. That is bad.
So what does the enclosure volume mean to the transducer? It represents a kind of air compression stiffness / impedance for the moving cone. Other than that, there is NO need to have big enclosures. That is true at least for closed designs. Why does the typical double 18 sub consume half a cubic meter? Not for the driver size but for the air compliance to be resonable.
If that was not the case, this double 18 sub could be 70 liters in volume as well. Take a closed box and remember that for SPL it does not matter what air volume the transducer is working into AS LONG AS THE CONE PEAK TO PEAK EXCURSION IS EQUAL. If I could build a duoble 18 with only 50 liters where I get a certain cone displacement over frequency function that is equal to that of a 500 liter sub's then they both would SOUND EQUAL, with the former being cheaper and better portable. This is easily possible by using a 10 x lower VAS transducer plus 10 times the electrical power. Think 15000 W instead of 1500. But then - it will quickly overheat the coil. Bad again. And not efficient. That is why dozens of big 100kg speakers are dragged to every concert stage.
If I can modify the behavior of the air volume of the small volume to behave like that of the big one, the transducer then would "think" it is in a big enclosure and perform alike.
Now think of a unpoisonuous non flammable liquid that will boil slightly lower than the operating temp of the sub. ( 25°C for ex.), fill the sub 2 - 3 cm with this and get it to the boiling point. top side, a very small overpressure hose will zero the inside DC pressure even during boiling. On the bottom use a sensor electronic circuit that can measure the liquid and push in some more of it if it gets too low through a small hose like 2mm in diameter. The liquid will always work at the boiling temperature, therefore the dV / dP is almost infinite. The inside will become effectively looking like 1 cubic kilometer of air to the transducer. Now build a transducer with a VAS of about a million or a billion cubic meters or whatever and put it in. The efficiency is maximized while the required volume / weigth is minimized. The subs can even be filled from one common big tank after being brought to venue. No need to carry that liquid around.
So istn t that great?
So what does the enclosure volume mean to the transducer? It represents a kind of air compression stiffness / impedance for the moving cone. Other than that, there is NO need to have big enclosures. That is true at least for closed designs. Why does the typical double 18 sub consume half a cubic meter? Not for the driver size but for the air compliance to be resonable.
If that was not the case, this double 18 sub could be 70 liters in volume as well. Take a closed box and remember that for SPL it does not matter what air volume the transducer is working into AS LONG AS THE CONE PEAK TO PEAK EXCURSION IS EQUAL. If I could build a duoble 18 with only 50 liters where I get a certain cone displacement over frequency function that is equal to that of a 500 liter sub's then they both would SOUND EQUAL, with the former being cheaper and better portable. This is easily possible by using a 10 x lower VAS transducer plus 10 times the electrical power. Think 15000 W instead of 1500. But then - it will quickly overheat the coil. Bad again. And not efficient. That is why dozens of big 100kg speakers are dragged to every concert stage.
If I can modify the behavior of the air volume of the small volume to behave like that of the big one, the transducer then would "think" it is in a big enclosure and perform alike.
Now think of a unpoisonuous non flammable liquid that will boil slightly lower than the operating temp of the sub. ( 25°C for ex.), fill the sub 2 - 3 cm with this and get it to the boiling point. top side, a very small overpressure hose will zero the inside DC pressure even during boiling. On the bottom use a sensor electronic circuit that can measure the liquid and push in some more of it if it gets too low through a small hose like 2mm in diameter. The liquid will always work at the boiling temperature, therefore the dV / dP is almost infinite. The inside will become effectively looking like 1 cubic kilometer of air to the transducer. Now build a transducer with a VAS of about a million or a billion cubic meters or whatever and put it in. The efficiency is maximized while the required volume / weigth is minimized. The subs can even be filled from one common big tank after being brought to venue. No need to carry that liquid around.
So istn t that great?
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Once a molecule is a gas it behaves under the idea gas law approximation for all intents and purposes of a speaker. Just because it just boiled off makes no difference. What can make a difference is the molecular weight which affects speed of sound. There are threads where people put a heavy gas in a sealed sub to give it an effective volume that is bigger. Like xenon gas but effect is maybe 40% or so based on ratio of square root of molecular weight.
If a liquid and its vapor is kept at the boiling point, the volume change (dV) per pressure change (dP) is almost infinite. Does everybody agree?
Where on thermodynamics do you see this? Once a gas molecule has been liberated off the surface of a liquid it is a gas. There is no infinite dV/dP because something is at boiling point. The heat of vaporization for a liquid is very high compared to specific sensible heat. But that is different property.
Maybe this is what you are thinking of because you say it is a property of the liquid at boiling point. For example it takes 100 calories to heat 1 gram of water from 0 to 100C. But it take a whole lot more (540 cal/gram) to heat that 100C water to all 100C water vapor due to amount of to vaporize that gram of water. That is the property that makes water a great heat transfer medium in steam power plants.
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for a sealed box, if input Z can be shifted around such as with capacitor input then there can be an apparent "boost" with ~constant voltage amplifiers (from lower Z around resonance) with tradeoff below the LF corner and save some cabinet volume. "Thoriated" IIRC was trying activated charcoal - maybe he'll see this thread and comment. I don't know if the old Cerwin Vega gas bag approach did much - their S1 6th order assisted reflex was fun.
http://www.diyaudio.com/forums/mult...h-maximizing-bass-output-iron-lawbreaker.html
http://www.diyaudio.com/forums/mult...h-maximizing-bass-output-iron-lawbreaker.html
Hi,
The gas must be lightweight for it to appear to have higher Vas.
Hydrogen or helium bags have been used in loudspeakers.
(if you suspect a big con, and long term dissapointment you would be right.)
Vas and Fs are related by the cone mass, the latter determines
efficiency (~), not Vas. You can build a driver with a meaninglessly
high Vas and low Fs without affecting efficiency at all in a sealed box.
The liquid idea is just wrong, equilibrium will take far longer than
the frequencies of interest, and by definition any liquid that boils
at 25 degrees C will have a vapour far heavier than air, reducing
the effective Vas. (Boiling points are complicated *).
Also compression raises gas temperatures, rarification lowers
temperatures, which will have the opposite effect required.
So utterly no chance DV/DP is infinite, or in fact useful.
Water as usual is way out on its own for its properties,
boiling point versus molecular mass is very high,
compare to NH3 at -33.3 degrees C.
Difference in molecular mass, NH3 17 to H20 18.
They don't compare to N2 at -196C and 02 at -227C.
Even though the molecule masses come in at 28 and 32.
* Why this so I'll leave you to contemplate.
rgds, sreten.
The gas must be lightweight for it to appear to have higher Vas.
Hydrogen or helium bags have been used in loudspeakers.
(if you suspect a big con, and long term dissapointment you would be right.)
Vas and Fs are related by the cone mass, the latter determines
efficiency (~), not Vas. You can build a driver with a meaninglessly
high Vas and low Fs without affecting efficiency at all in a sealed box.
The liquid idea is just wrong, equilibrium will take far longer than
the frequencies of interest, and by definition any liquid that boils
at 25 degrees C will have a vapour far heavier than air, reducing
the effective Vas. (Boiling points are complicated *).
Also compression raises gas temperatures, rarification lowers
temperatures, which will have the opposite effect required.
So utterly no chance DV/DP is infinite, or in fact useful.
Water as usual is way out on its own for its properties,
boiling point versus molecular mass is very high,
compare to NH3 at -33.3 degrees C.
Difference in molecular mass, NH3 17 to H20 18.
They don't compare to N2 at -196C and 02 at -227C.
Even though the molecule masses come in at 28 and 32.
* Why this so I'll leave you to contemplate.
rgds, sreten.
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"Vas and Fs are related by the cone mass, the latter determines
efficiency (~), not Vas. You can build a driver with a meaninglessly
high Vas and low Fs without affecting efficiency at all in a sealed box."
--> Thiele/Small - Wikipedia, the free encyclopedia
see under "other parameters" the second formula for efficiency: if all other parameters are kept the same, then efficiency is proportional to VAS.
efficiency (~), not Vas. You can build a driver with a meaninglessly
high Vas and low Fs without affecting efficiency at all in a sealed box."
--> Thiele/Small - Wikipedia, the free encyclopedia
see under "other parameters" the second formula for efficiency: if all other parameters are kept the same, then efficiency is proportional to VAS.
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