What is this wierd piece of thin soft foam that came with sub?

[Unregistered]

In the box with my Infinity Kappa Perfect 12.1D I got a circular piece of black foam with it. The foam piece is about a fourth of an inch thick, has a spiral cut, and on one side it is sticky. What is this thing and where does it go?

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[mykeys 10+ year member]

Its a seal to go between the sub and the box to make it air tight.

Mike