Question: Isobaric vs. DVC

[manuelbauwens]

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

[Tytte71]

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.

[revboden]

do you mean something like this: http://www.codrive.com/tech.htm

[ODougbo]

Originally Posted by revboden

do you mean something like this: http://www.codrive.com/tech.htm

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.

[manuelbauwens]

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 From there we could say that one of the cones is obsolete?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

[Don Hills]

Originally Posted by Tytte71

...
3. The front diaphragm is not exposed to cabinet pressure.

It is exposed to half of the cabinet pressure. (The pressure between the cones is half of that in the cabinet.)

[Tytte71]

Originally Posted by Don Hills

It is exposed to half of the cabinet pressure. (The pressure between the cones is half of that in the cabinet.)

Sorry You're right.

[vacuphile]

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

[Tytte71]

Originally Posted by vacuphile

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

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?

[Brian Oshman]

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.