Sky High Car Audio FXB-18 D4

18″ subwoofer · dual 4 Ω · 1,500 W RMS

27.3 HzFs, resonance

0.82Qts

4.69 ft³Vas

1.18″Xmax

1,500 WRMS

Source	skyhighcaraudio.com · 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, 1,500 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

4.31 ft³ (122 L)

Net volume

Bass character (Qtc)	Boomy, with a bump · 1.19
Fc	39 Hz
F3 (−3 dB, no cabin gain)	30 Hz
Example box (outside)	31 1/2″ × 20 1/2″ × 15 1/4″

Open sealed box in designer

Ported

deeper, louder

3.74 ft³ (106 L)

Net volume

Tuning (Fb)	30 Hz
Port	slot 2 3/16 × 19 1/16″, 27 7/16″ long
Port velocity at 1,500 W	64 ft/s
F3 (−3 dB, no cabin gain)	29 Hz
Example box (outside)	31 1/2″ × 20 1/2″ × 16 1/16″

Open ported box in designer

Deeper: 7.31 ft³ (207 L) tuned to 30 Hz, 31 1/2″ × 20 1/2″ × 27 7/16″ 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	dual 4 Ω
RMS power	1,500 W
Fs	27.3 Hz
Qts	0.82
Qes	0.94
Qms	6.59
Vas	4.69 ft³

Re (one coil)	3.2 Ω
Sd	188.8 in²
Xmax	1.18″
Vd	223 in³ calc.
Mms	538 g calc.
BL	17.7 T·m calc.
EBP	29 — suits sealed calc.
Sensitivity, 1 W / 1 m	86.6 dB calc.

Mounting dimensions

Cutout diameter	16 5/8″
Mounting depth	10 3/8″
Overall diameter	18 1/2″

Cone area (Sd) calculated from Fs, Vas and Mms: 188.8 in².

“≈” 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

skyhighcaraudio.com/sky-high-car-audio-fxb-series-18-subwoofer/

Other Sky High Car Audio 18″ models

Sky High Car Audio BMX-18 D1 18″ · dual 1 Ω
Sky High Car Audio BMX-18 D2 18″ · dual 2 Ω
Sky High Car Audio FEX-18 D2 18″ · dual 2 Ω
Sky High Car Audio FEX-18 D4 18″ · dual 4 Ω
Sky High Car Audio FMX-18 D1 18″ · dual 1 Ω
Sky High Car Audio FMX-18 D2 18″ · dual 2 Ω
Sky High Car Audio FXB-18 D2 18″ · dual 2 Ω
Sky High Car Audio FXXL-18 D1 18″ · dual 1 Ω
Sky High Car Audio FXXL-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.

DD Audio 3518i ESP D2 4.13 ft³ sealed · 4.13 ft³ ported
DD Audio 818f D1 4.13 ft³ sealed · 4.38 ft³ ported
DD Audio 818f D2 4.13 ft³ sealed · 4.38 ft³ ported
B2 Audio ISx18 D1 4.13 ft³ sealed · 4.10 ft³ ported
Dayton Audio UM18-22 4.31 ft³ sealed · 4.10 ft³ ported
URAL Bulava 18 (2+2) 4.20 ft³ sealed · 4.66 ft³ ported

FAQ

What box does the Sky High Car Audio FXB-18 D4 need?

Recommended net volume: 4.31 ft³ sealed, 3.74 ft³ ported tuned to 30 Hz — for one sub in a sedan at 1,500 W. Fit the box to your space and amp in the free calculator, with plans and a cut sheet.

What are the Sky High Car Audio FXB-18 D4 Thiele/Small specs?

Fs 27.3 Hz, Qts 0.82, Vas 4.69 ft³, Xmax 1.18″, Sd 188.8 in².

What is the Sky High Car Audio FXB-18 D4 mounting depth?

Mounting depth is 10 3/8″, cutout diameter 16 5/8″. The inside depth of the box must exceed the mounting depth by at least 1″.

How much power does the Sky High Car Audio FXB-18 D4 handle and how do I wire it?

RMS power rating: 1,500 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.