Proline X

Performance Optimized Wedge Sealed 15" Enclosure | Fi Car Audio Alpha 15

Q: Does this fit the Fi Car Audio Alpha 15 in S1, S2 and S4, and will any other 15 inch fit?

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 2.68 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.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.

Q: Why 2.54 cubic feet, and what alignment does it target?

2.54 cubic feet is the Butterworth volume, inside Fi's recommended 1.8 to 3.0 cubic feet. On Fi's figures (Fs 34.7 Hz, Qts 0.52, Vas 61.00 liters) that is Qtc 0.707 with half power at 47.2 Hz. At Qtc 0.71 the response is maximally flat into the rolloff, so F3 and fc coincide.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.

Q: What amplifier power should I run, and how should I wire it?

Fi rates the Alpha 15 at 1000 watts RMS, so target 700 to 1000 watts RMS at the final load.The single coil comes as S1, S2 or S4: 1, 2 or 4 ohms, wired straight to the amplifier. Confirm your amplifier's minimum stable impedance first.

Q: Why the wedge angle, and why rear-firing into the trunk?

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 driver fires 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 47.2 Hz on this page.

Q: Why is the front baffle double-layered?

The baffle is two bonded 3/4 inch sheets, 1.5 inches through. A single 3/4 inch panel flexes under the pressure a 15 inch woofer puts on it at 1000 watts, which smears output and works the driver loose over time; the second layer stiffens the face and gives the mounting hardware twice the material to bite.The outer layer is machined as a flush recess to the 15.5 inch frame, so the driver bolts to the inner layer and sits flush with the gloss black acrylic insert.

Q: What is the expected low-frequency extension?

Anechoic F3 is 47.2 Hz, with system resonance at 47.2 Hz. At Qtc 0.71 the response is maximally flat into the rolloff, so F3 and fc coincide.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.

Q: Do I need to add polyfill or damping?

No. Polyfill is already installed 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 2.54 cubic feet this cabinet was built to.

Q: Is the driver included, and are the threaded inserts installed?

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, 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 Fi Car Audio Alpha 15 is purchased separately, and the cabinet is cut direct fit to it, not a general purpose 15 inch box.

SKU: PO 15-S W FI ALPHA BB

QUICK SPECS

Best For: Fi Car Audio Alpha 15 Trunk-Fit Builds
Net Volume: 2.54 cu ft (2.69 cu ft Gross)
Driver Match: Fi Car Audio Alpha 15 (S1, S2 or S4)
Construction: Rear-Firing Wedge | V-Groove/Dado | Double 1.5" Baffle | Threaded Insert Kit

Shipping Included No freight charges at checkout. What you see is what you pay.

Built in Tennessee CNC cut from 3/4" Langboard Elite MDF. V-groove and dado joinery.

Advanced Terminal Cup Built in-house. ABS/Carbon Fiber. Stainless steel hardware. Copper ring terminals.

$ 349.99

Acrylic Insert Color

In Stock and Ready to Ship

Financing available. Choose Shop Pay Installments at checkout.

Performance Optimized Wedge Sealed 15" Enclosure | Fi Car Audio Alpha 15

Enclosure Type Sealed

External Dimensions 29.50" W x 18" H x 16.72" D at base tapering to 10.17" D at top

Gross Volume 2.69 cu ft

Net Volume 2.54 cu ft

Shipping Weight 59 lbs

Front Panel Thickness 1.5"

INSTALLER'S TAKE

2.54 cubic feet, Qtc 0.707. That is the Butterworth volume, inside Fi's recommended 1.8 to 3.0 cubic feet.

Flat to 47 Hz anechoic. The 7.375 inch motor clears the raked back with 2.68 to spare; the volume sets the rest of the cabinet.

Cut is 13.875, Fi's own figure. Inserts ship in the kit, not pressed: Fi prints no bolt circle. Mark from the driver, drill, press. It is sealed, so leave the polyfill as it is and run it at 1000 watts or under.

Product Details

Performance Optimized Wedge | Fi Car Audio Alpha 15

15" Sealed Wedge Enclosure | Rear-Firing Trunk-Load Design

2.54 ft3 Net2.69 ft3 GrossQtc 0.71Single Chamber Wedge29.50" x 18" x 10.17/16.72"Built in Tennessee

The Fi Car Audio Alpha 15 in a Sealed Wedge

The Alpha 15 is the 15 inch from Fi Car Audio's Alpha v1 line: 1000 watts RMS, 17 mm of one-way excursion on 810 square centimeters of cone, and 7.375 inches of mounting depth. Every figure comes from the Thiele-Small and technical tables Fi prints on its Alpha Series product page, including the 0.14 cubic foot driver displacement, and the parameters check against each other: the printed Qts reproduces from Qes and Qms, and Fi's moving mass agrees with its Fs, Vas and cone area.

2.54 cubic feet net is the Butterworth volume, inside Fi's recommended 1.8 to 3.0 cubic feet. On Fi's figures, Fs 34.7 Hz, Qts 0.52 and Vas 61.00 liters, that is Qtc 0.707 with half power at 47.2 Hz. 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. 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.

Trunk-Load Wedge Design Explained

The driver mounts on the vertical trunk-lid-side face and fires rearward into the trunk cavity.

Seat-Side Face Matches Seat Lean

Angled face matches the roughly 20 degree factory seat lean.

Trunk-Lid-Side Face Fires the Woofer

Vertical baffle, cone firing rearward into the trunk cavity.

Trunk Cavity Loading

The trunk acts as an acoustic extension for a smoother presentation.

Preserves Trunk Cargo Space

The wedge tucks against the seat back and the space in front stays usable.

Engineered for the Fi Car Audio Alpha 15

Half power lands at 47.2 Hz anechoic, before any cabin gain. At Qtc 0.71 the response is maximally flat into the rolloff, so F3 and fc coincide. Fi publishes the driver displacement, 0.14 cubic feet, and it is taken out of the net figure.

Alignment Numbers

Driver	System Qtc	fc	F3
Alpha 15 S1, S2 or S4	Qtc 0.707	47.2 Hz	47.2 Hz

Mounting Geometry

The baffle is cut to 13.875 in, Fi's published cutout, under the 15.5 in frame. The rear panel rakes 20 degrees, so the cavity deepens from 10.17 in at the top to 16.72 in at the base, and the 7.375 in motor clears it with 2.68 in to spare. Fi prints no bolt circle for the Alpha 15, so the baffle ships undrilled with a threaded insert kit and stainless machine screws: set the driver in the recess, mark its holes, drill, press the inserts and bolt it down.

What the Numbers Rest On

Langboard Elite MDF

Premium 3/4 in MDF throughout.

V-Groove and Dado Joinery

CNC cut V-grooves at every panel intersection, sides and brace seated in dados.

Double-Layer 1.5" Front Baffle

Two bonded 3/4 in MDF sheets at the driver face, the outer layer machined as a flush recess.

Fi's Published Parameters

Fs 34.7 Hz, Qts 0.52, Qes 0.56, Qms 8.10, Vas 61.00 L, Sd 810 cm2, Cms 0.0663 mm/N, Mms 326 g, Bl 10.0 / 14.1 / 20.2 N/A (S1 / S2 / S4), Re 0.8 / 1.6 / 3.3 ohms, sensitivity 88.5 dB 1W/1m.

Wiring the Fi Car Audio Alpha 15 to Your Amplifier

The single coil comes as S1, S2 or S4: 1, 2 or 4 ohms, wired straight to the amplifier. Confirm the final load against your amplifier's minimum stable impedance before powering the system.

Power handling is 1000 watts RMS per driver. Aim for roughly 700 to 1000 watts at the impedance you wire.

Built in Tennessee, Backed by Audio Intensity

Cut and assembled in Tullahoma, Tennessee on our own CNC equipment. Finished in plush black carpet with the gloss black acrylic baffle insert, one pre-wired terminal cup, 12 gauge OFC to the driver position. Polyfill wall lining installed. Threaded insert kit and stainless machine screws in the box. 1 year limited warranty. Enclosure only, the driver is purchased separately. Call or text 931-563-5151.

Fi Car Audio Alpha 15: Cut for the Driver

2.54 cubic feet net at Qtc 0.707, F3 47.2 Hz, cut direct fit and built to order in Tennessee.

Specifications

Specification	Value
Series	Proline X Performance Optimized Wedge
Compatible Driver	Fi Car Audio Alpha 15 S1, S2 or S4, quantity 1. Direct fit
Configuration	Single 15 inch, single chamber sealed wedge
Mounting Position	Trunk or cargo area, tall face against the rear seat back
Firing Direction	Rearward into the trunk. Not a down fire design, no feet included
External Width	29.50 in
External Height	18 in at the seat face
External Depth	16.72 in at the base, tapering to 10.17 in at the top
Internal Volume, Gross	2.69 cu ft
Internal Volume, Net Acoustic	2.54 cu ft after the brace and Fi's published 0.14 cu ft displacement
Driver Mounting Depth	7.375 in required; 2.68 in clearance above the motor against the raked rear panel
Alignment	Qtc 0.707, fc 47.2 Hz, F3 47.2 Hz anechoic
Fi Recommended Sealed	1.8 to 3 cu ft (Qtc 0.77 to 0.68)
Woofer Opening	13.875 in, Fi's published cutout
Frame Outer Diameter	15.5 in, Fi's published figure
Bolt Circle	Not printed by Fi; threaded insert kit included
Driver Displacement	0.14 cu ft, Fi's published figure
Fs / Qts / Qes / Qms / Vas	34.7 Hz / 0.52 / 0.56 / 8.10 / 61.00 L
Driver Sd	810 cm2, published by Fi
Xmax	17 mm one way, Fi spec
Driver Power Handling	1000 W RMS, Fi spec
Wiring Options	The single coil comes as S1, S2 or S4: 1, 2 or 4 ohms, wired straight to the amplifier.
Parameter Source	ficaraudio.com Alpha Series product page, Thiele-Small and Technical Specifications tables
Material	Langboard Elite MDF
Construction	CNC cut, V-groove and dado joinery
Panel Thickness	0.75 in
Front Baffle	1.5 in double baffle, outer layer machined as a flush recess
Internal Bracing	Windowed brace, dadoed to the shell
Driver Mounting	Threaded insert kit included, NOT pre-installed: Fi prints no bolt circle
Terminal Cups	One Proline X ABS and carbon fiber composite, side mounted, pre wired 12 gauge OFC
Acrylic Insert	Gloss black acrylic baffle insert standard; None variant minus $20
Damping	Light polyfill wall lining, does not alter acoustic volume
Carpet	Plush black
Shipping Weight	59 lb
Warranty	1 Year Limited
Built In	Tullahoma, Tennessee

Performance Graphs

Net airspace

2.54

cu ft

System Qtc

0.71

sealed

Half power

47.2

Hz · F3

Xmax at

957

W RMS · no filter

With 25 Hz subsonic

1273

W RMS · +33%

Amplifier

1000

W RMS recommended

This enclosure holds 2.54 cu ft net . In it the Fi Car Audio Alpha 15 runs a system Qtc of 0.71 with half power at 47.2 Hz, and the cone reaches its 17.0 mm travel limit at 957 W, below the driver’s 1000 W thermal rating, so travel is what stops you here, not heat. Run a subsonic filter at 25 Hz and that becomes 1273 W — +317 W, or 33 percent more power for the same 17.0 mm of travel.

01Is it a sealed driver?

Efficiency bandwidth product places the motor before anything else is decided.

EBP = Fs ÷ Qes = 62.0

single 1, 2 or 4 ohm · EBP 62.0

At 62.0 the Alpha 15 sits in the transition band, not the sealed band. That is not a mismatch: a transition motor works in either alignment, and sealed buys a smaller box and a gentler roll-off at the cost of some low-end output. Stage 02 shows what this airspace actually does with it.

02What the box does to it

2.54 cu ft against a 61.0 L Vas.

α = Vas ÷ Vb = 0.85 · Fc = Fs√(α+1) = 47 Hz · Qtc = 0.71

single 1, 2 or 4 ohm · Qtc 0.71, Fc 47 Hz, F3 47.2 Hz

The box lifts resonance from 34.7 Hz free-air to 47 Hz and half power lands at 47.2 Hz, anechoic, before any cabin gain. Figures are Fi Car Audio’s published Alpha 15 parameters from its Alpha Series product page, including the 0.14 cubic foot driver displacement. One parameter set covers the S1, S2 and S4 single coil versions.

03The watts that reach Xmax (in this enclosure)

Travel scales with the square root of power, so there is one wattage that puts the cone exactly on 17.0 mm — and a bigger one once a subsonic filter stops it wasting travel below the passband.

P_Xmax = 957 W unfiltered → 1273 W with a 25 Hz subsonic

single 1, 2 or 4 ohm · 957 W, no filtersingle 1, 2 or 4 ohm · 1273 W with subsonic at 25 Hz, 24 dB/octXmax 17.0 mm one-way

Every trace touches the limit and no further, because each is drawn at the exact power that puts that coil on 17.0 mm. Unfiltered that is 957 W, reached first at 20 Hz. It sits under the 1000 W the driver can take thermally, so the enclosure sets the ceiling, not the voice coil. The dashed traces are the same driver behind a subsonic filter at 25 Hz, 24 dB/oct: stopping the cone wasting travel below the passband lets it take 1273 W before hitting the same 17.0 mm. That is 33 percent more amplifier for the price of one filter setting.

Param	Value	Unit	Note
Fs	34.70	Hz	free-air resonance
Qts	0.520		single 1, 2 or 4 ohm
Vas	61.00	L	equivalent compliance
Sd	810.0	cm²	effective piston area
Xmax	17	mm	one-way linear travel
Pe	1000	W	continuous RMS
SPL	88.4	dB	1 W / 1 m, derived from T/S

Driver parameters as published by Fi Car Audio Alpha 15 product page. Curves computed against this enclosure’s actual net airspace.

Customer Installs

Materials

Material

Built From a Spec Sheet, Not a Sales Sheet

Every Proline X Performance Series enclosure is CNC-cut from Langboard Elite MDF, 3/4" (18mm) panels produced to published mill specifications. Most enclosure manufacturers source generic MDF without published data. We can tell you the internal bond rating, the modulus of elasticity, the modulus of rupture, and the face and edge screw holding strength for every panel in this enclosure, because we chose the material based on those numbers, not on price or availability.

Langboard Elite is produced in the United States. The specifications below are pulled directly from Langboard's published mill data and are verifiable. This is the same panel stock used in our full Proline X lineup.

Specifications

Langboard Elite MDF: Published Mill Data

Property	Value	Why It Matters
Panel Stock	Langboard Elite MDF	Named, verifiable panel source, not generic "high-density MDF"
Thickness	3/4" (18mm)	Industry standard for subwoofer enclosures; panels below this flex under pressure
Density	48.5 lbs/ft³	Higher density = less porosity, better screw retention, reduced resonance
Internal Bond	200 psi	Measures resistance to delamination under sustained internal pressure. The most enclosure-relevant spec on this sheet
Modulus of Rupture (MOR)	4,200 psi	Bending strength before the panel breaks, relevant for larger unsupported baffle spans
Modulus of Elasticity (MOE)	410,000 psi	Stiffness under load; higher MOE means less panel flex under bass pressure, less resonance
Face Screw Holding	325 lbs	How firmly the panel surface holds a screw; critical for baffle joint integrity over time
Edge Screw Holding	250 lbs	Panel edge retention, relevant for every joint that isn't face-to-face

Internal Bond

200 psi: The Number That Actually Matters for Enclosures

The internal bond rating measures how well the MDF panel resists delamination under sustained pressure applied perpendicular to the face. In a subwoofer enclosure, every bass transient generates a pressure pulse that pushes outward on every panel simultaneously. A panel that begins to delaminate internally loses stiffness gradually. The symptoms are slow: a box that starts to sound softer, less defined, less punchy over months of use. 200 psi internal bond is the spec that determines how long this enclosure holds its acoustic integrity under daily use at real power levels.

Stiffness

410,000 psi MOE: Less Flex, Less Resonance

The modulus of elasticity measures panel stiffness, its resistance to bending under load. A stiffer panel flexes less under the pressure generated by a high-excursion driver at volume. Panel flex is a source of acoustic coloration: the panel itself begins to radiate sound at its own resonant frequency, adding distortion to the output. 410,000 psi stiffness combined with V-groove and dado joinery means the panels stay rigid and the bracing controls resonance at the structure level, not the panel level. The bass you hear comes from the driver, not the box.

Screw Holding

325 lb Face / 250 lb Edge: Joints That Stay Locked

Screw holding strength determines how firmly the panel grips a fastener at the face and at the edge. This matters most at the baffle joint, where the front panel meets the side panels and where the driver is mounted. A panel with low screw holding strength allows fasteners to loosen over time under vibration, leading to air gaps at the joint and loss of acoustic seal. 325 lb face screw holding strength means the baffle hardware stays put. Combined with dado joinery and adhesive, the front panel on a Proline X Performance Series enclosure is not going anywhere.

Sourcing

Named Panel Stock: Not "Premium MDF"

The phrase "premium MDF" or "high-density MDF" appears on marketing copy for enclosures across every price point, from $40 to $400. It is not a specification. It is a description with no defined standard. Langboard Elite is a named, sourced, mill-documented product with published data. We name it in our copy because naming it holds us accountable to the spec. You can look up Langboard Elite independently, verify the numbers on this page, and confirm you are getting what we say you are getting. That is the point.

Why Proline X?

Most enclosures are built to a price point. Proline X is built to a spec. Every box starts with published-data MDF, gets CNC-cut to exact internal volume, and is assembled by hand in our Tennessee facility. The difference shows up in the measurements, and in how the box holds up two years into daily use.

CNC-Cut to Spec

Panels are machine-cut on ShopSabre CNC routers and joined with V-groove and dado joinery. Consistent geometry, no hand-cut variance, no gaps at the joints.