# Optimal spacing between driver and base plate for downfiring Tumult?

Discussion in 'Home Theater Projects' started by Mattak, Apr 16, 2005.

1. ### Mattak Stunt Coordinator

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Is there an optimal spacing between the driver and base plate for a downfiring Tumult? Or should I just try different spacings?

2. ### Seth_L Screenwriter

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You want there to be a larger opening than the Sd of the driver. So, if you have a driver with a 1200cm^2 Sd you want more than 1200cm^2 of opening. The more you have, the better.

So, a Tumult has a Sd of 749cm^2. That means the moving diameter is 30.99cm. So, the circumference is 97cm. 749/97=7.72cm. I'd add in the 34mm of x-max to be safe, so you end up 11.12cm. That's 4.37"

3. ### SammyXP Auditioning

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Am I missing something? What are you calculating when you are dividing the Sd by the circumference (749/97)? Geometry class must have been a long time ago because I'm not seeing it. Seems like Sd/C simply equals 0.5R

I'm asking because I'm building a downfiring sub and the only criteria I've found for spacing on the bottom face is the ensure it is greater than the port diameter. I've got dual 3" ports, so I've been planning on making 3.5" legs, just to be safe.

4. ### Mattak Stunt Coordinator

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Extrapolating from his post, I think what he was getting at was that you want the opening as big as the Sd of the driver, and in this case your opening can be assumed to be a cylinder wall around the edge of the driver (not sure how to make that clear with words). The radius of the cylinder is the radius of driver, but not the entire physical driver, just the cone basically, which we can call R. Since manufactures don't give you R, they give you Sd, you find R:

piR^2 = 749

R = 15.44cm

R is the radius of our cylinder, so now we find the height (distance from driver to baseplate):

cylinder area is height*circumference, H*2piR

Sd = H*2piR
749 = H*2pi(15.44)
H = 7.72cm

My Xmax is 34mm, so at full extension I'd lose 34mm of my cylinder opening, so 34mm is added to H to make up for it.

7.72 + 3.4 = 11.12cm

Basically, you don't want the base plate loading the subwoofer (at least not in my case I didn't).

In the end, though, Sd = piR^2 and H = Sd/2pi, so H = R/2

5. ### SammyXP Auditioning

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Ok, I think I understand now. You're basically looking for the surface area of a cylinder around the perimeter of the effective cone area. Otherwise, the air coming from the cone face would have to compress before exiting the openings underneath the enclosure.

Turns out not to be too big a deal for me since, for a Dayton 15" DVC, R=16.03cm and Xmax is 1.51cm, so I need 3 3/4" of height, which is only 0.25" taller than I was planning on being.

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