Fi Audio X15 S2

15″ subwoofer · 2 Ω · 400 W RMS

33.3 HzFs, resonance

0.47Qts

4.99 ft³Vas

0.71″Xmax

400 WRMS

The Fi Audio X15 S2 is a 15″ subwoofer. With Qts 0.47 and EBP 65 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, 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

2.65 ft³ (75 L)

Net volume

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″

Open sealed box in designer

Ported

deeper, louder

3.71 ft³ (105 L)

Net volume

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″

Open ported box in designer

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

Where to buy Amazon

Sealed Ported · SPL in dB at 1 W / 1 m, no cabin gain

Thiele/Small parameters

Size	15″
Voice coils	2 Ω
RMS power	400 W
Fs	33.3 Hz
Qts	0.47
Qes	0.51
Qms	6.02
Vas	4.99 ft³

Re	1.9 Ω
Sd	125.6 in²
Xmax	0.71″
Vd	89 in³ calc.
Mms	150 g calc.
BL	10.8 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

web.archive.org/web/20110712231226/http://www.ficaraudio.com:80/sess/utn;jsessio

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.

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 S2 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 S2 Thiele/Small specs?

Fs 33.3 Hz, Qts 0.47, Vas 4.99 ft³, Xmax 0.71″, Sd 125.6 in².

What is the Fi Audio X15 S2 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 S2 handle and how do I wire it?

RMS power rating: 400 W. Wiring for several subs and amp power — in the wiring calculator.

Is the Fi Audio X15 S2 better sealed or ported?

EBP 65: both work — sealed 2.65 ft³ is tighter and smaller, ported 3.71 ft³ tuned to 34 Hz is deeper and louder.

What port does the Fi Audio X15 S2 need?

For the 3.71 ft³ ported box tuned to 34 Hz: slot 1 3/4 × 16 11/16″, 13 15/16″ long, area 29.6 in²; or a round port Ø6 1/8″, ≈ 13 15/16″ long. Port air speed at 400 W: 59 ft/s.

What box do two Fi Audio X15 S2 subs need?

Two subs need twice the volume: 5.30 ft³ sealed, or 7.42 ft³ ported at the same 34 Hz tuning with twice the port area. One shared chamber or one per sub — whatever fits your vehicle.