Fi Audio IB315 S4

15″ subwoofer · 4 Ω · 550 W RMS

22.8 HzFs, resonance

0.60Qts

6.47 ft³Vas

1.18″Xmax

550 WRMS

The Fi Audio IB315 S4 is a 15″ subwoofer. With Qts 0.60 and EBP 33 it suits a sealed box: tight bass in a small enclosure; a ported box would need a lot of volume. Long excursion (Xmax 1.18″) — built for loud music and high power.

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, 550 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

2.86 ft³ (81 L)

Net volume

Bass character (Qtc)	Boomy, with a bump · 1.09
Fc	41 Hz
F3 (−3 dB, no cabin gain)	33 Hz
Example box (outside)	31 1/2″ × 18 1/8″ × 11 7/8″

Open sealed box in designer

Ported

deeper, louder

3.71 ft³ (105 L)

Net volume

Tuning (Fb)	30 Hz
Port	slot 1 9/16 × 16 11/16″, 16 13/16″ long
Port velocity at 550 W	64 ft/s
F3 (−3 dB, no cabin gain)	29 Hz
Example box (outside)	31 1/2″ × 18 1/8″ × 15 15/16″

Open ported box in designer

Deeper: 4.87 ft³ (138 L) tuned to 31 Hz, 31 1/2″ × 18 1/8″ × 20 1/4″ 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	4 Ω
RMS power	550 W
Fs	22.8 Hz
Qts	0.60
Qes	0.7
Qms	4.28
Vas	6.47 ft³

Re	2.8 Ω
Sd	125.6 in²
Xmax	1.18″
Vd	148.3 in³ calc.
Mms	247 g calc.
BL	11.9 T·m calc.
EBP	33 — suits sealed calc.
Sensitivity, 1 W / 1 m	86.9 dB calc.

Mounting dimensions

Cutout diameter	14 1/8″
Mounting depth	8″
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/20110712231407/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 IB315 S2	2 Ω	1.5 Ω	0.74	6.47 ft³	2.86 ft³	3.71 ft³ · 30 Hz
Fi Audio IB315 S4	4 Ω	2.8 Ω	0.7	6.47 ft³	2.86 ft³	3.71 ft³ · 30 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.

Kicker Solo-Baric L7R 15" 4-Ohm DVC (45L7R154) 3.71 ft³ sealed · 4.52 ft³ ported
Rockford Fosgate Punch P115S4 (Stage-1, 2004) 3.04 ft³ sealed · 3.96 ft³ ported
Rockford Fosgate Punch HE2 RFP4115 2.86 ft³ sealed · 3.71 ft³ ported
Diamond D315D4 3.07 ft³ sealed · 3.99 ft³ ported
Rockford Fosgate Punch P3D4-15 3.14 ft³ sealed · 2.40 ft³ ported
Rockford Fosgate Punch P1S4-15 3.11 ft³ sealed · 2.40 ft³ ported

FAQ

What box does the Fi Audio IB315 S4 need?

Recommended net volume: 2.86 ft³ sealed, 3.71 ft³ ported tuned to 30 Hz — for one sub in a sedan at 550 W. Fit the box to your space and amp in the free calculator, with plans and a cut sheet.

What are the Fi Audio IB315 S4 Thiele/Small specs?

Fs 22.8 Hz, Qts 0.60, Vas 6.47 ft³, Xmax 1.18″, Sd 125.6 in².

What is the Fi Audio IB315 S4 mounting depth?

Mounting depth is 8″, 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 IB315 S4 handle and how do I wire it?

RMS power rating: 550 W. Wiring for several subs and amp power — in the wiring calculator.

Is the Fi Audio IB315 S4 better sealed or ported?

EBP 33: a sealed box (2.86 ft³) suits it best — tight bass in a compact enclosure. Ported is possible (3.71 ft³, tuned to 30 Hz) but larger.

What port does the Fi Audio IB315 S4 need?

For the 3.71 ft³ ported box tuned to 30 Hz: slot 1 9/16 × 16 11/16″, 16 13/16″ long, area 26.3 in²; or a round port Ø5 13/16″, ≈ 16 13/16″ long. Port air speed at 550 W: 64 ft/s.

What box do two Fi Audio IB315 S4 subs need?

Two subs need twice the volume: 5.72 ft³ sealed, or 7.42 ft³ ported at the same 30 Hz tuning with twice the port area. One shared chamber or one per sub — whatever fits your vehicle.