Monopole and dipole subwoofers

[SE-Raider]

I have been considering subwoofers for my office/studio and one thing I have been wondering about is what would be the relative advantages of building a sealed enclosure with a single 12" driver (monopole) versus a sealed enclosure with a 10" driver on one face and a 10" driver on the opposite face. I like the idea that the opposing drivers' motion will cancel each other and prevent the enclosure from "walking". The surface area of the two tens will be about equal to that of the twelve so output should be about the same (put aside the increased cost of the two tens for a moment). I have wondered what if any benefit sonically there would be from the dipole arrangement.

I have also been looking at dipoles lately just to learn, since my room and placement won't allow them. And I wondered if there would be an advantage to applying this type of subwoofer arrangement to a dipole speaker. I would think that if the sub enclosure front to back could be kept as thin as possible, it might help to preserve the dipole relationship with the mid and tweeter, while possibly gaining an increase in the lowest bass due to the sealed enclosure, over a sub driver on a baffle in free air. Similarly would such an application benefit an electrostatic (i.e Maggies) or ribbon speaker (B&G) in the same way?

Or more generally, what would be the advantages of a dipole subwoofer implementation, if any?

[ThomasW]

What you're describing is monopole vs bipole (not dipole). So there are no analogies to planar dipole speakers.

And the only difference is paying for double the number of drivers, having the increased Vd, and the sub not walking. Note that 10" drivers in general don't move enough air for the box to walk, unless it's poorly made.

Sub boxes should be deep not thin...

[SE-Raider]

ThomasW: Not trying to be argumentative (except in the true forensics lets-debate-the-issues-and-go-have-a-beer kinda way); just trying to learn and understand your comments:

Now that I stand (or sit actually) corrected, what is the difference between bipole and dipole? Dipole radiates in two directions, front and rear, from the same cone diaphragm; the subwoofer described, if the drivers and enclosure are treated as a single assembly, would be similar in function.

What difference does the shape of the sealed enclosure make at sub frequencies which are much longer than the internal dimensions of the enclosure? As you move up in frequency response where the wavelengths (and their harmonics) approach the dimensions of the box I can see this becoming an issue. Non-parallel walls and damping/stuffing could address this effectively.

Wouldn't it be the moving mass of the driver that would cause the walking? I understood the coupling of the cone with the surrounding air is very inefficient anyway, which is a premise for horn design.

Thanks for your response.

[ThomasW]

In a bipole the drivers must be wired in phase otherwise the output from the drivers is canceled.

Dipoles have the front wave out of phase with the rear wave. The effect of this is to create a steep null at the sides of the baffle where the front and rear waves meet.

So as one can see dipoles and bipoles are completely different animals.

Subwoofers should be deep because if they aren't, the pressure from the rear wave bounces off the back wall and comes forward interfering with the operation of the woofer.

[SE-Raider]

[QUOTE=ThomasW]In a bipole the drivers must be wired in phase otherwise the output from the drivers is canceled.

Dipoles have the front wave out of phase with the rear wave. The effect of this is to create a steep null at the sides of the baffle where the front and rear waves meet.

So as one can see dipoles and bipoles are completely different animals.

Now I understand.

If the wavelength is longer than the face of the dipole enclosure do the front wave and rear wave cancel completely as with a driver in free air, or only to the sides as you describe ? Any reference source you know of where I can learn further about this? Thanks again for your response.

As far as the subwoofer cabinet depth I had in mind enough depth to allow two drivers to oppose each other, and allow any necessary voice coil porting etc.

How deep is deep enough?

[Paul H]

... Any reference source you know of where I can learn further about this? ....

Go to Sigfried Linkwitz's site (linkwitzlab.com) for a wealth of information on dipoles.

Paul

[ThomasW]

You'll find quite a bit about dipoles on Linkwtiz's website, and a Google search will turn up thousands of hits.

How deep is deep enough?

One doesn't design for this, just make the cabinet as deep as possible within the constraints of the size of the box.