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Magnet-to-magnet isobaric design question
Magnet-to-magnet isobaric design question
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Old 18th February 2019, 08:12 PM   #1
jimmyjazz is offline jimmyjazz  United States
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Default Magnet-to-magnet isobaric design question

If one were contemplating a magnet-to-magnet configuration in an isobaric subwoofer, what adverse effects might arise if a custom "hourglass" isobaric chamber were used? It could be cast or molded or even 3D printed, depending on material. In design cartoons one typically sees a straight cylinder spanning the distance between the mounting rings of the two drivers, but it seems that the entrained volume of air could be reduced below what it is with that concept, tapering in and back out in close proximity to the drivers, thus reducing air compliance as well as freeing up internal volume in the main box.

Would there be issues with the air that sloshes around in the isobaric chamber, perhaps related to higher velocity?

For what it's worth, I am aware of the general disdain for isobaric designs given modern driver performance standards, and in particular for any isobaric layout that's not a cone-to-cone ("clamshell") configuration. Indulge me.
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Old 18th February 2019, 08:15 PM   #2
adason is offline adason  United States
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best way to minimize volume between woofers in isobaric is clamp
front to front
off course one is switched in polarity
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Old 18th February 2019, 08:17 PM   #3
jimmyjazz is offline jimmyjazz  United States
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I understand that. I have reasons for not considering such a layout.
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Old 18th February 2019, 09:36 PM   #4
weltersys is offline weltersys  United States
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Quote:
Originally Posted by jimmyjazz View Post
If one were contemplating a magnet-to-magnet configuration in an isobaric subwoofer, what adverse effects might arise if a custom "hourglass" isobaric chamber were used?
Would there be issues with the air that sloshes around in the isobaric chamber, perhaps related to higher velocity?
A custom "hourglass" isobaric chamber would reduce the air volume between the drivers compared to a cylinder of the same depth, which is a positive feature. The reduction in the air volume between the drivers will increase voice coil heating, a negative feature especially if the drivers are used for heavily compressed low frequency program material.

The air velocity would be reduced with a custom "hourglass" isobaric chamber compared to a cylinder of the same depth, less "sloshing".

Curious as to why you don't want to use a cone-to-cone isobaric chamber?
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Old 18th February 2019, 09:42 PM   #5
jimmyjazz is offline jimmyjazz  United States
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Thanks. Why would you say the air velocity would be reduced? It would have the same volume displacement (= 2*Vd of each driver) but that air would be moving around in a smaller entrained volume. Maybe I'm thinking about it wrong.

I'm not hard sold against a clamshell arrangement, but I've been led to believe that the back side of a driver tends to be noisier than the front side -- chuffing noises, wires moving around, etc. Plus, I would need this to get up into at least the low midrange (maybe 300 Hz) and I'm worried about diffraction around the basket and magnet structure. Maybe that's not an issue.
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Old 18th February 2019, 10:22 PM   #6
weltersys is offline weltersys  United States
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Originally Posted by jimmyjazz View Post
Thanks. Why would you say the air velocity would be reduced?

I'm not hard sold against a clamshell arrangement, but I've been led to believe that the back side of a driver tends to be noisier than the front side -- chuffing noises, wires moving around, etc. Plus, I would need this to get up into at least the low midrange (maybe 300 Hz) and I'm worried about diffraction around the basket and magnet structure. Maybe that's not an issue.
Assuming the air is all moving with the drivers, there is no relative velocity. The larger the enclosed volume, the more pressure variation potential between the two drivers. In either case, the volume is enclosed, and won't be creating chuffing noises.
Chuffing noises can be a problem with the magnet side on some vented pole piece or spider designs, but easy to determine that with a free-air test.

The wavelength of 300 Hz is over a meter long, wouldn't worry about diffraction around the basket and magnet structure as an issue to not use a cone to cone arrangement.
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Old 18th February 2019, 10:29 PM   #7
jimmyjazz is offline jimmyjazz  United States
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Well, there may be no relative velocity between the air and the cones, but there will be between the air and the fixed structures (magnets, baskets, chamber walls, etc.). I don't know what might occur if local velocities get high (decoupling of the cones, etc.?).

Just to be clear, when I referred to "chuffing" I was speaking only about noise emanating from the exposed magnet side of the "heard" driver in a cone-to-cone isobaric pair. I'm not too worried about such noise within the isobaric chamber.
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Old 18th February 2019, 10:58 PM   #8
GM is offline GM  United States
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Hmm, shouldn't since the goal is a small enough coupling chamber to ensure a ~uniform particle density, so no velocity and a bad plan for cooling the motors except through the dust caps.

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Old 18th February 2019, 11:01 PM   #9
scottjoplin is offline scottjoplin  Wales
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Quote:
Originally Posted by jimmyjazz View Post
For what it's worth, I am aware of the general disdain for isobaric designs given modern driver performance standards, and in particular for any isobaric layout that's not a cone-to-cone ("clamshell") configuration. Indulge me.
As you say, pointless exercise these days?
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Old 18th February 2019, 11:51 PM   #10
jimmyjazz is offline jimmyjazz  United States
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As you say, pointless exercise these days?
I'm not sure it's pointless if a small box is paramount. Thoughts?
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