Performance Optimized Wedge Sealed Dual 15" Enclosure Fi Car Audio Alpha 15 · Dual 15" Sealed enclosure

The Alpha 15, yes, in all three coil versions: they share the frame, the 13.875 in cutout and the 7.375 in mounting depth, and the motor clears the cabinet with 4.65 in to spare. Fi publishes one parameter set for the Alpha 15 that covers the S1, S2 and S4 single coil versions, so one alignment serves all three. Run two of the same coil so the chambers match.

Any other 15, no. The airspace is worked out from the Alpha 15's own Qts and Vas, and a different woofer that happened to drop into the hole would not see the alignment printed on this page.

1.80 cubic feet is Fi's own minimum recommended sealed volume for the Alpha 15. On Fi's figures (Fs 34.7 Hz, Qts 0.52, Vas 61.00 liters) that is Qtc 0.771 with half power at 47.5 Hz. Above Qtc 0.707 the response carries a small lift near 51 Hz before the rolloff, and F3 sits below fc.

Fi's own recommended sealed range for the Alpha 15 is 1.8 to 3 cubic feet, which reads Qtc 0.77 to 0.68. At 0.707 the dual would be 23.4 inches deep at the base, so it is built to Fi's figure instead.

Fi rates the Alpha 15 at 1000 watts RMS per driver, so target 700 to 1000 watts RMS per driver at the final load.

A pair lands on 0.5 ohm parallel or 2 ohms series with S1 coils, 1 ohm parallel or 4 ohms series with S2, and 2 ohms parallel or 8 ohms series with S4. Anything under 1 ohm, and many amplifiers at 1 ohm, is outside what the amplifier is built for; series wiring is the safe default. Confirm your amplifier's minimum stable impedance first.

The seat-side face carries a 20 degree rake that matches the lean of a factory rear seat back, so the cabinet sits flush against it and the cargo space in front stays usable. The baffle is the vertical trunk-lid-side face, so the drivers fire rearward into the trunk cavity.

The trunk acts as an acoustic extension for the sealed box, and the cabin adds real output below 50 Hz, so in-vehicle response runs deeper than the anechoic 47.5 Hz on this page.

Two drivers sharing one sealed volume load each other: each driver's compression stroke becomes the pressure the other works against, which blurs the alignment either would have alone. A bonded full-height 3/4 inch divider gives each driver its own 1.80 cubic feet and its own polyfill.

It also means a failure in one driver does not unload the other, and the alignment on this page is the alignment in each chamber, not an average across a shared box.

Anechoic F3 is 47.5 Hz per chamber, with system resonance at 51.4 Hz. Above Qtc 0.707 the response carries a small lift near 51 Hz before the rolloff, and F3 sits below fc.

Those figures exclude cabin gain. In a car the cabin's transfer function adds real output below 50 Hz, so in-vehicle response runs deeper than the numbers above.

No. Polyfill is already installed in both chambers as a light wall lining to control internal reflections, and the quantity is chosen so it does not change the acoustic volume.

Do not add more. Packing a sealed box with fill makes it behave as though it were larger, which walks the alignment away from the 1.80 cubic feet per chamber this cabinet was built to.

No driver. This is an enclosure only. You receive the cabinet finished in plush black carpet with the gloss black acrylic baffle insert, one side-mounted pre-wired terminal cup to both positions, the polyfill lining already in place, and a threaded insert kit with stainless machine screws.

The inserts ship loose on purpose: Fi prints no bolt circle for the Alpha 15, so you set the driver in the recess, mark its holes, drill and press the inserts. The pair of Fi Car Audio Alpha 15 is purchased separately, and the cabinet is cut direct fit to it, not a general purpose 15 inch box.