Fi Audio X15 D4
15″ subwoofer · dual 4 Ω · 750 W RMS
The Fi Audio X15 D4 is a 15″ subwoofer. With Qts 0.53 and EBP 53 it works both sealed and ported: sealed is tighter and smaller, ported is deeper and louder.
| Source | web.archive.org · manufacturer data |
|---|---|
| Checked against source | October 2026 |
| Thiele/Small parameters | complete |
| Mounting dimensions | 3 of 4 — missing: magnet diameter; the designer will use an estimate |
Recommended box
For a sedan, one sub, 750 W. It’s the same math as the designer’s step-by-step mode, where you can fit the box to your space, vehicle and amp and get plans, a cut sheet and DXF.
Sealed
smaller, tighter| Bass character (Qtc) | Tight and punchy · 0.84 |
|---|---|
| Fc | 47 Hz |
| F3 (−3 dB, no cabin gain) | 43 Hz |
| Example box (outside) | 31 1/2″ × 18 1/8″ × 11 7/8″ |
Ported
deeper, louder| Tuning (Fb) | 33 Hz |
|---|---|
| Port | slot 2 3/16 × 16 11/16″, 20 15/16″ long |
| Port velocity at 750 W | 63 ft/s |
| F3 (−3 dB, no cabin gain) | 32 Hz |
| Example box (outside) | 31 1/2″ × 18 1/8″ × 15 11/16″ |
Deeper: 4.87 ft³ (138 L) tuned to 33 Hz, 31 1/2″ × 18 1/8″ × 21″ outside — a bigger box with flatter deep bass. Open in the calculator
Projects with this subwoofer
Boxes that users designed and published: plans, cut sheet and response graph.
Thiele/Small parameters
| Size | 15″ |
|---|---|
| Voice coils | dual 4 Ω |
| RMS power | 750 W |
| Fs | 30 Hz |
| Qts | 0.53 |
| Qes | 0.57 |
| Qms | 7.48 |
| Vas | 4.28 ft³ |
| Re (one coil) | 2.8 Ω |
|---|---|
| Sd | 125.6 in² |
| Xmax | 0.79″ |
| Vd | 98.9 in³ calc. |
| Mms | 216 g calc. |
| BL | 14.1 T·m calc. |
| EBP | 53 — sealed or ported calc. |
| Sensitivity, 1 W / 1 m | 89.6 dB calc. |
Mounting dimensions
| Cutout diameter | 14 1/8″ |
|---|---|
| Mounting depth | 7 1/2″ |
| Overall diameter | 15 5/8″ |
“≈” means the value isn’t in the spec sheet and a typical value for the size is used; “calc.” means it’s calculated from the published parameters. Check the manufacturer’s spec sheet before you build.
Data sources
- web.archive.org/web/20151204181607/http://www.ficaraudio.com:80/product/x-series
- web.archive.org/web/20181115115452/http://www.ficaraudio.com:80/product/x-series
Voice coil versions
The same sub with different voice coils: the parameters are nearly the same — pick by your amp and wiring.
| Model | Voice coils | Re | Qes | Vas | Sealed | Ported |
|---|---|---|---|---|---|---|
| Fi Audio X15 S2 | 2 Ω | 1.9 Ω | 0.51 | 4.99 ft³ | 2.65 ft³ | 3.71 ft³ · 34 Hz |
| Fi Audio X15 S4 | 4 Ω | 2.9 Ω | 0.51 | 4.99 ft³ | 2.65 ft³ | 3.71 ft³ · 34 Hz |
| Fi Audio X15 D2 | dual 2 Ω | 1.4 Ω | 0.57 | 4.28 ft³ | 2.86 ft³ | 3.43 ft³ · 33 Hz |
| Fi Audio X15 D4 | dual 4 Ω | 2.8 Ω | 0.57 | 4.28 ft³ | 2.86 ft³ | 3.43 ft³ · 33 Hz |
Other Fi Audio 15″ models
- Fi Audio Alpha v1 15 S1 15″ · 1 Ω
- Fi Audio Alpha v1 15 S2 15″ · 2 Ω
- Fi Audio Alpha v1 15 S4 15″ · 4 Ω
- Fi Audio Alpha v2 15 S2 15″ · 2 Ω
- Fi Audio Alpha v2 15 S4 15″ · 4 Ω
- Fi Audio BL-NEO 15 D1 15″ · dual 1 Ω
- Fi Audio BL-NEO 15 D2 15″ · dual 2 Ω
- Fi Audio BL15 D1 15″ · dual 1 Ω
- Fi Audio BL15 D2 15″ · dual 2 Ω
- Fi Audio BL15v3 D1 15″ · dual 1 Ω
- Fi Audio BL15v3 D2 15″ · dual 2 Ω
- Fi Audio BL15v3.1 D1 15″ · dual 1 Ω
Similar 15″ subwoofers from other brands
Close in Fs, Qts and Vas — a box of similar volume will suit them.
- Hertz Hi-Energy HX 380 (SVC 4 ohm) 2.79 ft³ sealed · 3.50 ft³ ported
- Sundown Audio Ev2-15 D4 2.90 ft³ sealed · 3.53 ft³ ported
- Sundown Audio Ev2-15 D2 2.68 ft³ sealed · 3.43 ft³ ported
- Hifonics ZXS15D2 2.93 ft³ sealed · 3.39 ft³ ported
- Audiobahn AW150XS 2.83 ft³ sealed · 3.67 ft³ ported
- Audiobahn AW1500N 2.65 ft³ sealed · 3.04 ft³ ported
FAQ
What box does the Fi Audio X15 D4 need?
Recommended net volume: 2.86 ft³ sealed, 3.43 ft³ ported tuned to 33 Hz — for one sub in a sedan at 750 W. Fit the box to your space and amp in the free calculator, with plans and a cut sheet.
What are the Fi Audio X15 D4 Thiele/Small specs?
Fs 30 Hz, Qts 0.53, Vas 4.28 ft³, Xmax 0.79″, Sd 125.6 in².
What is the Fi Audio X15 D4 mounting depth?
Mounting depth is 7 1/2″, cutout diameter 14 1/8″. The inside depth of the box must exceed the mounting depth by at least 1″.
How much power does the Fi Audio X15 D4 handle and how do I wire it?
RMS power rating: 750 W. Dual 4 Ω voice coils: series gives 8 Ω, parallel gives 2 Ω at the amp. Wiring for several subs and amp power — in the wiring calculator.
Is the Fi Audio X15 D4 better sealed or ported?
EBP 53: both work — sealed 2.86 ft³ is tighter and smaller, ported 3.43 ft³ tuned to 33 Hz is deeper and louder.
What port does the Fi Audio X15 D4 need?
For the 3.43 ft³ ported box tuned to 33 Hz: slot 2 3/16 × 16 11/16″, 20 15/16″ long, area 36.1 in²; or a round port Ø6 13/16″, ≈ 20 15/16″ long. Port air speed at 750 W: 63 ft/s.
What box do two Fi Audio X15 D4 subs need?
Two subs need twice the volume: 5.72 ft³ sealed, or 6.85 ft³ ported at the same 33 Hz tuning with twice the port area. One shared chamber or one per sub — whatever fits your vehicle.