Fi Audio IB318 S4

18″ subwoofer · 4 Ω · 550 W RMS

22.5 HzFs, resonance

0.63Qts

13.45 ft³Vas

1.18″Xmax

550 WRMS

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

4.27 ft³ (121 L)

Net volume

Bass character (Qtc)	Boomy, with a bump · 1.28
Fc	46 Hz
F3 (−3 dB, no cabin gain)	35 Hz
Example box (outside)	31 1/2″ × 20 7/8″ × 14 7/8″

Open sealed box in designer

Ported

deeper, louder

5.54 ft³ (157 L)

Net volume

Tuning (Fb)	30 Hz
Port	slot 1 9/16 × 19 7/16″, 11 13/16″ long
Port velocity at 550 W	56 ft/s
F3 (−3 dB, no cabin gain)	31 Hz
Example box (outside)	31 1/2″ × 20 7/8″ × 19 1/2″

Open ported box in designer

Deeper: 7.27 ft³ (206 L) tuned to 35 Hz, 31 1/2″ × 20 7/8″ × 24 3/8″ 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	18″
Voice coils	4 Ω
RMS power	550 W
Fs	22.5 Hz
Qts	0.63
Qes	0.77
Qms	3.41
Vas	13.45 ft³

Re	2.8 Ω
Sd	187.6 in²
Xmax	1.18″
Vd	221.5 in³ calc.
Mms	272 g calc.
BL	11.8 T·m calc.
EBP	29 — suits sealed calc.
Sensitivity, 1 W / 1 m	89.5 dB calc.

Mounting dimensions

Cutout diameter	16 3/4″
Mounting depth	9 1/2″
Overall diameter	18 1/2″

“≈” 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

Other Fi Audio 18″ models

Fi Audio BL-NEO 18 D1 18″ · dual 1 Ω
Fi Audio BL-NEO 18 D2 18″ · dual 2 Ω
Fi Audio BL18v3 D1 18″ · dual 1 Ω
Fi Audio BL18v3 D2 18″ · dual 2 Ω
Fi Audio BL18v3.1 D1 18″ · dual 1 Ω
Fi Audio BL18v3.1 D2 18″ · dual 2 Ω
Fi Audio BTL-NEO18 D1 18″ · dual 1 Ω
Fi Audio BTL-NEO18 D2 18″ · dual 2 Ω
Fi Audio BTL3 18 D1 18″ · dual 1 Ω
Fi Audio BTL3 18 D2 18″ · dual 2 Ω
Fi Audio BTL4 18 D1 18″ · dual 1 Ω
Fi Audio BTL4 18 D2 18″ · dual 2 Ω

Similar 18″ subwoofers from other brands

Close in Fs, Qts and Vas — a box of similar volume will suit them.

Sundown Audio Xv1-18 D4 4.10 ft³ sealed · 5.30 ft³ ported
Dayton Audio UMII18-22 4.17 ft³ sealed · 5.44 ft³ ported
Sky High Car Audio BMX-18 D1 4.45 ft³ sealed · 5.01 ft³ ported
PSI Platform 2 18 4.34 ft³ sealed · 5.65 ft³ ported
Sky High Car Audio FMX-18 D1 4.45 ft³ sealed · 5.16 ft³ ported
Earthquake Pro-X184 4.13 ft³ sealed · 5.33 ft³ ported

FAQ

What box does the Fi Audio IB318 S4 need?

Recommended net volume: 4.27 ft³ sealed, 5.54 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 IB318 S4 Thiele/Small specs?

Fs 22.5 Hz, Qts 0.63, Vas 13.45 ft³, Xmax 1.18″, Sd 187.6 in².

What is the Fi Audio IB318 S4 mounting depth?

Mounting depth is 9 1/2″, cutout diameter 16 3/4″. The inside depth of the box must exceed the mounting depth by at least 1″.

How much power does the Fi Audio IB318 S4 handle and how do I wire it?

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