Fi Audio X15 S4
15″ subwoofer · 4 Ω · 400 W RMS
| 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, 400 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 · 0.80 |
|---|---|
| Fc | 57 Hz |
| F3 (−3 dB, no cabin gain) | 54 Hz |
| Example box (outside) | 31 1/2″ × 18 1/8″ × 11 1/8″ |
Ported
deeper, louder| Tuning (Fb) | 34 Hz |
|---|---|
| Port | slot 1 3/4 × 16 11/16″, 13 15/16″ long |
| Port velocity at 400 W | 59 ft/s |
| F3 (−3 dB, no cabin gain) | 36 Hz |
| Example box (outside) | 31 1/2″ × 18 1/8″ × 15 7/8″ |
Deeper: 4.87 ft³ (138 L) tuned to 37 Hz, 31 1/2″ × 18 1/8″ × 19 5/8″ 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 | 4 Ω |
| RMS power | 400 W |
| Fs | 33.4 Hz |
| Qts | 0.47 |
| Qes | 0.51 |
| Qms | 6.05 |
| Vas | 4.99 ft³ |
| Re | 2.9 Ω |
|---|---|
| Sd | 125.6 in² |
| Xmax | 0.71″ |
| Vd | 89 in³ calc. |
| Mms | 149 g calc. |
| BL | 13.3 T·m calc. |
| EBP | 65 — sealed or ported calc. |
| Sensitivity, 1 W / 1 m | 92.1 dB calc. |
Mounting dimensions
| Cutout diameter | 14 1/8″ |
|---|---|
| Mounting depth | 7 3/4″ |
| 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
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.
- Audiobahn AW150XS 2.83 ft³ sealed · 3.67 ft³ ported
- Sundown Audio Ev2-15 D2 2.68 ft³ sealed · 3.43 ft³ ported
- ESX QE1522 (2x2 Ohm) 1.66 ft³ sealed · 3.07 ft³ ported
- Audiobahn AW1571Q 2.75 ft³ sealed · 3.57 ft³ ported
- Audiobahn AW1571SE 2.75 ft³ sealed · 3.57 ft³ ported
- Audiobahn AW1571T 2.75 ft³ sealed · 3.57 ft³ ported
FAQ
What box does the Fi Audio X15 S4 need?
Recommended net volume: 2.65 ft³ sealed, 3.71 ft³ ported tuned to 34 Hz — for one sub in a sedan at 400 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 S4 Thiele/Small specs?
Fs 33.4 Hz, Qts 0.47, Vas 4.99 ft³, Xmax 0.71″, Sd 125.6 in².
What is the Fi Audio X15 S4 mounting depth?
Mounting depth is 7 3/4″, 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 S4 handle and how do I wire it?
RMS power rating: 400 W. Wiring for several subs and amp power — in the wiring calculator.