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#### SQBubble

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I was wondering, how to calculate how much air a subwoofer moves in L at (x)hz? you have to multiply xmax with SD or vas?

#### mdrake

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Yes, multiply the SD times the total XMAX. In WinISD you can model what the xmax will be at a given frequency.

Matt

#### collo

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287 Posts
Sd times Xmax times two ...... (Xmax is one-way, so double for total excursion)

#### SQBubble

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ok thx, so, they say that a PR has to be able to move 2x to 4x the displacement of an active sub, so, if ;

xxx18", 54xmax, 1210cm2 = 130 680cm3 of air? and
one PR 18" from CSS, 38xmax, 1210cm2 = 91 960cm3 of air? x2 cuz 2 PR = 183 920cm3?

thats 1.5times the xxx18", so would i be safe to run an xxx18" with 2PR?

#### Mike P.

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To find the displacement, multiply by the same units. The xxx18" Xmax is 54 mm, 5.4 cm.
5.4 cm x 1210 cm2 = 6534 cm3 which is 6.534 Liters.
The PR is 3.8 cm x 1210 cm2 = 4598 cm3 which is 4.598 liters.

A PR being able to move 2x to 4x the displacement of a sub is a general rule, the actual results are dependent on the cabinet size, tuning frequency and the amount of power applied to the sub which modeling will show.

#### Ricci

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No. 2 CSS 18" PR's are not enough to keep up with a XXX 18 IMO. Believe me I looked at this stuff. Try modeling it and looking at the PR excursion around tune when you are getting the XXX to do it's thing. I like to have at-least 3 times the displacement available for the PR's over the driver. 4 CSS 18" PR's is about right. :coocoo:

Mike is right it's 6.54L going off of one way displacement, or if you want to play the #'s game and multiply that by 2 like a lot of people do for subs... Just call it 13 liters.

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