They haven't gone public with it yet, but it's an open secret the Philadelphia Eagles will switch to a 3-4 defense under new head coach Chip Kelly. Their early free agency signings show how that 3-4 defense is taking shape.

Compared with recent seasons, the Eagles have been low-key in free agency. That's certainly a wise policy after being burned by the big spending that produced the so-called "dream team." However, staying below the radar, doesn't means the Eagles haven't been active and Smart.

So far they have added four new pieces to their secondary. Safeties Patrick Chung and Kenny Phillips will compete to start right away. As will new cornerbacks Bradley Fletcher and Cary Williams. However, it is up front where the Eagles have made their best moves and have been most revealing about their choice of defensive scheme.

One of Philly's first deals was to secure former San Francisco 49ers defensive lineman Isaac Sopoaga. His arrival was the first major hint at a move to a 3-4 front. Sopoaga is a hulking nose tackle, the kind of run-stuffing force every 3-4 needs in the middle.

Sopoaga is purely a 3-4 lineman. Even for the 49ers he often only played base defense and usually against teams with a fondness for running the ball. Sopoaga would frequently be removed from nickel packages, because he barely offered a credible pass-rush threat.

Yet as a dominant run-stuffer, Sopoaga knows few equals in the entire NFL. The 6'2", 330-pounder dominates the interior of offensive lines. He is a natural 2-gapper commanding double teams from the center and one of the guards.

With Sopoaga blocking up the middle, the Eagles linebackers will be free to attack the ball. The entire success of a 3-4 depends on the nose tackle and the Eagles smartly secured the best one available.

Their latest acquisition is further proof of the Eagles intent to quickly create a viable 3-4 front. They recently handed a solidly-priced contract to former Houston Texans rush end Connor Barwin.

He switched from defensive end in a 4-3, to outside linebacker in the Texans version of the 3-4. Barwin made a successful transition between the two schemes. During two seasons in a 3-4, he posted 14.5 sacks, including 11.5 in 2011.

Barwin works best attacking the open-side of a formation, away from a tight end. A natural pass-rusher off the edge is another key component of the 3-4. The defense relies on its outside, rush linebackers to create pressure.

The Eagles have equipped themselves with a credible player at this position, who will be able to help others grasp the transition between defenses. Signing Sopoaga and Barwin makes it clear the Eagles are re-shaping their front seven.

After faltering defensively in 2012, refreshing the approach and structure of the defense is a smart move from Kelly, who favored the 3-4 at Oregon. Even if it means the end for a stellar veteran like Trent Cole, who has spent his career thriving in 4-3 schemes.

By acquiring two obvious 3-4 pieces, the Eagles are leaving little doubt about what kind of defense they will play in 2013.

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