Metal in proximity to motor

[fbov]

I'm building a pair of NatP's which use the RS180S-8 shielded drivers. I've got an idea for using screw jacks to brace the back of the drivers against existing crossmembers behind the drivers. My question is if the motor cares if there's a 1.8mm thick steel plate against the back of the magnet assembly. If so, how far away would I need to be to mitigate the audible effect?

I'll use a layer of cork at the interface as a minimum, so there's some damping and no metal-to-metal contact. I can add spacers if proximity matters (and space allows), but my gut tells me it doesn't...

Thanks,
Frank

[ThomasW]

The 'cans' on the back of a shielded driver aren't really a structural element. So unlike bracing the magnets, the cans aren't the best thing to brace if the goal is supporting the driver.

That said, a 1.8mm plate isn't going to have too much effect on a shielded motor assembly.

[fbov]

Good point. I'll have to examine the drivers for structurally effective mounting points. Point contact is easy to arrange, as is a thick conforming layer.
Thanks!
Frank

[b_force]

Maybe use aluminium, that has no effect at all!
Nothing to worry about

Besides that, I have the same opinion as Thomas.
I thought that Linkwitz also had some experiments with this type of mounting.

another link:

http://home.wanadoo.nl/dezaire/ultiem/ultimate.htm

[JonP]

In a driver's motor, all the force lines (well, all that are effecting the voice coil) are focused internally across the voice coil gap, so no effects from the outside.

Just looked at one I have... there is a vent hole in the center of the back, and that shouldn't be covered, or it will affect things due to air pressure and resonance effects.

The back cover/shield seems to be pretty solidly attached... Somewhere there's a car audio site that shows how to remove the shield cap and bucking magnet to make the driver thinner. (you now can buy unshielded versions) It might be a flat glue joint to the outer shell, then it might be a very solid to the magnet assembly attachment point. But, go look at the pictures to be sure...

[fbov]

As you note, Jon, the driver has a nice flat back with a vent hole. The screw jack has a matching void, and I'll use pads to space it off a bit and allow peripheral ventilation. I've got some nice rubber ones from another project.

b, I always like reading about extreme projects (I consider any laminate enclosure extreme) to understand that basic principles are being addressed. The link you provide qualifies, but to the current point, both he and Linkwitz (if I read him right) are mounting the driver from the rear in order to reduces transmission of sonic energy into sympathetic radiating areas like a baffle. The fact that the motor is the only other place you can use is fortuitous in its benefits for momentum transfer.

I'm adding a brace that uses the motor and basket to connect to the baffle. This should give me the same momentum transfer benefits of direct mounting, but there's a subtlety in my design that should reduce sympathetic radiation as well. The drivers only connect to the baffle, and it's isolated from the enclosure by a layer of cork. The back of this brace rests on an orthogonal brace crossing, so there are no direct paths to the enclosure walls.

I really need to update my build thread ...

Thanks, as always, for the advice,
Frank