Hi,
I have a question, the isobaric configuration exists of two drivers mounted against each other with as little airspace between them as possible.
This to "mechanical" couple both drivers, from there my thoughts were: if you would glue a second driver without a cone on the first driver we get the same result because mechanically they are connected?
From there I wondered if there is any difference between two drivers in isobaric configuration and a single driver with two voice coils?
Greetings,
Manuel
I have a question, the isobaric configuration exists of two drivers mounted against each other with as little airspace between them as possible.
This to "mechanical" couple both drivers, from there my thoughts were: if you would glue a second driver without a cone on the first driver we get the same result because mechanically they are connected?
From there I wondered if there is any difference between two drivers in isobaric configuration and a single driver with two voice coils?
Greetings,
Manuel
You would also have to double the mass on the single driver. To my opinion there are almost no reason for using an isobaric sub - you can most always select a single sub that is fit for the purpose.
There are however a few differences between an isobaric solution and your idea, amongst these:
1. The isobaric is most often coupled in a push-pull configuration which at some extent makes more symetric force/stroke curve and spring force/stroke curve.
2. The two separate woofers have separetly cooled voice coils, which could be positive with regards to thermal/power compression.
3. The front diaphragm is not exposed to cabinet pressure.
There are however a few differences between an isobaric solution and your idea, amongst these:
1. The isobaric is most often coupled in a push-pull configuration which at some extent makes more symetric force/stroke curve and spring force/stroke curve.
2. The two separate woofers have separetly cooled voice coils, which could be positive with regards to thermal/power compression.
3. The front diaphragm is not exposed to cabinet pressure.
Very Interesting; I’ll have to email CoDrive.
It would be hard to connect a pair of woofers DIY with a cardboard tube or something similar, but it wouldn’t be a bad idea if it could be done.
Oh btw, if I was looking for a sub for “music” I would not go with an isobaric however for HT, hard to beat the low end of an isobaric at “low” to “normal” volume levels.
I use a very low-cost pair in my HT; they slam, bang, thunder and chug along with the best of them; a very wide range of natural, low end bass.
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Well actually that CoDrive looks a bit like the scenario I was describing, but only to come to my last point, does it not quite look like a DVC driver?
The reason for my experimenting with Isobaric configurations is trying to lower the volume of enclosures.
If you were like ODougbo wrote, connect two drivers with cardboard tube it would be the same as two drivers coupled with air? From there we could say that one of the cones is obsolete? Getting something like the CoDrive, that to me actually looks like a DVC driver...
Thermal/power considerations can easily be dealt with by enlarging the voicecoil diameter .. Theoretically could not one consider the voicecoil of a DVC speaker to be as two cones sticked on each other, the front "cone" then also not being exposed to the rear cabinet pressure?
Thank you all for your input! Just brainstorming a little
The reason for my experimenting with Isobaric configurations is trying to lower the volume of enclosures.
If you were like ODougbo wrote, connect two drivers with cardboard tube it would be the same as two drivers coupled with air? From there we could say that one of the cones is obsolete? Getting something like the CoDrive, that to me actually looks like a DVC driver...
Thermal/power considerations can easily be dealt with by enlarging the voicecoil diameter .. Theoretically could not one consider the voicecoil of a DVC speaker to be as two cones sticked on each other, the front "cone" then also not being exposed to the rear cabinet pressure?
Thank you all for your input! Just brainstorming a little
It is exposed to half of the cabinet pressure. (The pressure between the cones is half of that in the cabinet.)
Codrive, thanks for the link Revboden, first time I saw this patent. It will indeed solve some non-linearity issues of magnetic motors. However, copper shortening rings and clever motor design already go a long way in solving these in a probably more economical way.
On Isobaric, I agree with Tytte71, even on the point where he changed his mind on the basis of Don's post. I don't think it is easy to give a simple answer to the question how much of the cabinet pressure is relayed to each cone. Take the example of a very rigid metal cone. The inner driver would withstand all of the cabinet pressure up to the point of flexing, before it could even start relaying part of that pressure to the cone facing outward. Similarly, the outer cone would have to deal with all of the pressures associated with the acoustic loading. To the extent that there is flex in the cones, the issue becomes more complicated.
As long as the cones don't flex, the air contained in the chamber between the two cones will remain at the same - atmospheric -pressure.
vac
Only to the extent that the inner cone will be able to
On Isobaric, I agree with Tytte71, even on the point where he changed his mind on the basis of Don's post. I don't think it is easy to give a simple answer to the question how much of the cabinet pressure is relayed to each cone. Take the example of a very rigid metal cone. The inner driver would withstand all of the cabinet pressure up to the point of flexing, before it could even start relaying part of that pressure to the cone facing outward. Similarly, the outer cone would have to deal with all of the pressures associated with the acoustic loading. To the extent that there is flex in the cones, the issue becomes more complicated.
As long as the cones don't flex, the air contained in the chamber between the two cones will remain at the same - atmospheric -pressure.
vac
Only to the extent that the inner cone will be able to
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Vac,
I was also thinking the way you describe here, but Don's post did me rethink through the problem. Maybe I was a little quick in accepting his statement of half the pressure, but there is something to it.
Isobaric configuration is equal to doubling the mass of a driver and it must therefore be an interacting force between the two driver diaphragms. The only force (I see) that can interact between the two drivers would be though the air in the isobaric cavity.
The drivers "helps each other" in compressing/decompressing the sealed cabinet air volume. The cavity pressure should therefore be something like constant (not atmospheric) with exeption of the equalization of nonlinear driver behavoiur caused by asymetric BL(x) and k(x) when mounted push/pull. Also, due to the inertia of the air, the isobaric cavity volume should be kept as small (stiff) as possible.
Have I completely lost it?
The advantage of isobaric configuration is that you can build a box half the size of that required for one woofer. You will not get the 3db gain from the second woofer but you will get the space saving from it.
Example: You have a 1 cu ft box and a woofer. You add a second woofer and another 1 cu ft box. You have increased the space by 2 and increased the output by 3db.
If you isobarically mount the woofers you will not get the 3db gain but you will only need a box of .5 cu ft. which is half the space of one box or 25% the space of the two boxes. This is best used in car stereo where space is at a premium.
If you run them face to face then you reverse the polarity of one so they still move in the same direction.
Example: You have a 1 cu ft box and a woofer. You add a second woofer and another 1 cu ft box. You have increased the space by 2 and increased the output by 3db.
If you isobarically mount the woofers you will not get the 3db gain but you will only need a box of .5 cu ft. which is half the space of one box or 25% the space of the two boxes. This is best used in car stereo where space is at a premium.
If you run them face to face then you reverse the polarity of one so they still move in the same direction.
All true.
And the .5 cu ft. isobaric pair now requires double the power (+3dB) to equal the single driver in 1 cu ft box.
Given the same power, two drivers each in 1 cu ft box would have 6 dB more output than the isobaric pair in the .5 cubic foot box.
At very low frequencies, 6 dB more output sounds twice as loud.
Vac,
I was also thinking the way you describe here, but Don's post did me rethink through the problem. Maybe I was a little quick in accepting his statement of half the pressure, but there is something to it.
Isobaric configuration is equal to doubling the mass of a driver and it must therefore be an interacting force between the two driver diaphragms. The only force (I see) that can interact between the two drivers would be though the air in the isobaric cavity.
The drivers "helps each other" in compressing/decompressing the sealed cabinet air volume. The cavity pressure should therefore be something like constant (not atmospheric) with exeption of the equalization of nonlinear driver behavoiur caused by asymetric BL(x) and k(x) when mounted push/pull. Also, due to the inertia of the air, the isobaric cavity volume should be kept as small (stiff) as possible.
Have I completely lost it?
Tytte, upon rethinking my train of thoughts that led me to this conclusion, I think I was wrong, so you have't lost it and I have to eat my hat. Which I am doing, right now.
vac
Hi,
Increasing cone mass is not the same as isobaric, as isobaric has two motors.
Having said that, increasing cone mass could suit particular drivers, but the
changes are not so straightforward as the isobaric arrangement *.
However its always possible to make a single driver with the same parameters
as an isobaric driver pair, if you have that design flexibility. If you don't then
isobaric is a good way of effectively changing the driver Vas parameter**,
and very useful for some cheap higher Q drivers to reduce the volume needed.
Low Q drivers simply don't benefit from isobaric typically.
rgds, sreten.
* play with changing cone mass in WinISDpro's driver definition window.
** Halving Vas, and leaving Fs and Qts the same.
Increasing cone mass is not the same as isobaric, as isobaric has two motors.
Having said that, increasing cone mass could suit particular drivers, but the
changes are not so straightforward as the isobaric arrangement *.
However its always possible to make a single driver with the same parameters
as an isobaric driver pair, if you have that design flexibility. If you don't then
isobaric is a good way of effectively changing the driver Vas parameter**,
and very useful for some cheap higher Q drivers to reduce the volume needed.
Low Q drivers simply don't benefit from isobaric typically.
rgds, sreten.
* play with changing cone mass in WinISDpro's driver definition window.
** Halving Vas, and leaving Fs and Qts the same.
Please, you don't have to. I ate mine after Don's post and it tastes shi..Tytte, upon rethinking my train of thoughts that led me to this conclusion, I think I was wrong, so you have't lost it and I have to eat my hat. Which I am doing, right now.
vac
Haha
So drivers with a high Q have the advantage that they need a lower enclosure volume?
If so then mounting two drivers in isobaric configuration does the same? Besides the higher cone mass, I understand that connecting both woofers with the airspace makes that both drivers have only half the airload to deal with?
So if you have a DVC, the two coils wit a certain Q, driving them both will add both Q's together for the whole driver?
I have also heard "stories" of shorting one of the DVC coils giving a better bass response?
I'm just trying to understand the physics behind the "lowering of the enclosure volume", thanks for your efforts!
So drivers with a high Q have the advantage that they need a lower enclosure volume?
If so then mounting two drivers in isobaric configuration does the same? Besides the higher cone mass, I understand that connecting both woofers with the airspace makes that both drivers have only half the airload to deal with?
So if you have a DVC, the two coils wit a certain Q, driving them both will add both Q's together for the whole driver?
I have also heard "stories" of shorting one of the DVC coils giving a better bass response?
I'm just trying to understand the physics behind the "lowering of the enclosure volume", thanks for your efforts!
Mmmm, mushrooms...
I found it easier to visualise as a rigidly coupled system. Imagine placing two drivers face to face and gluing a cardboard tube between the centres of their cones, so that they must move together. This is a true "isobaric" driver. In fact, the term "isobaric" is misleading - like the term "tapped horn". In a true isobaric setup, the rear driver would move independently to keep the air pressure between the drivers constant - as if the enclosure size was infinite. In real life "isobaric" configurations the pressure between the drivers does vary, so it is not truly isobaric.
To show that the pressure between the drivers is half that in the enclosure, imagine connecting the rear driver to a battery so that it pushes into the enclosure with a force of (say) 1 newton, raising the enclosure air pressure by (say) 1 kPa. Now connect the "front" driver to the battery so that it also moves into the enclosure.
If the drivers are rigidly coupled, the front driver will apply 1 newton of force to the rear driver. This will add to the 1 newton of force being generated by the rear driver, making a total of 2 newtons, raising the enclosure air pressure by 2 kPa.
Next, replace the rigid coupling with a spring. It can be a real coil spring or an air spring. The front driver will apply 1 newton to the spring, compressing it - the front cone will move closer to the rear cone. If it is an air spring, the pressure between the cones will rise by 1 kPa.
Things get messy when the air volume between the drivers becomes significant compared with the enclosure volume. The drivers will not move in unison. For homework, try and work out what the cone movement behaviour will be if the volume between the drivers equals the volume of the enclosure...
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Interesting, so actually each driver in a isobaric configuration uses half the power of one single driver in normal configuration to build up the same pressure in the enclosure...
Witch actually means that the increased powerhandling (double the drivers) of an isobaric configuration will make up for the 3dB loss of efficiency?
The air load on the rear and the front of the driver pair is actually just divided over both "pistons", so two "pistons" in series... would this also count for more then 2 drivers in "series"?
Witch actually means that the increased powerhandling (double the drivers) of an isobaric configuration will make up for the 3dB loss of efficiency?
The air load on the rear and the front of the driver pair is actually just divided over both "pistons", so two "pistons" in series... would this also count for more then 2 drivers in "series"?
It makes up for it exactly. There is no increase in output.
The driver cone has to move a certain distance (say 1 cm) to generate a certain SPL (say 90 dB SPL) at a given frequency (say 40 Hz).
Doubling the drivers in isobaric configuration does not change the cone travel required to reach the same SPL. But you also halve the enclosure volume, which means that the enclosure pressure change will double for a given SPL. The same 1 cm movement will generate twice the pressure in the enclosure. This will require twice the total force from the drivers, which will require twice the power from the amplifier.
1 driver, 1 watt, 10 litre enclosure, 1 mm travel, 1 kPa pressure change = xx dB SPL.
Halve the enclosure size. 1 mm of travel will now generate 2 kPa pressure in the enclosure, which will require twice the force from the driver (twice the power). Now add a second driver in series (isobaric) and share the power between them. So for a given SPL in isobaric configuration you require twice the total power to compensate for the "half sized" box. There is thus no increase in overall power handling, because each driver still requires 1 watt.
In theory, yes. You reach a point of diminishing returns because the total of the volumes between the drivers starts to become significant compared with the enclosure volume.
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