Sky High Car Audio FMX-18 D1

18″ subwoofer · dual 1 Ω · 3,500 W RMS

22.3 HzFs, resonance

0.57Qts

6.43 ft³Vas

1.46″Xmax

3,500 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, 3,500 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

4.45 ft³ (126 L)

Net volume

Bass character (Qtc)	Tight and punchy · 0.90
Fc	35 Hz
F3 (−3 dB, no cabin gain)	30 Hz
Example box (outside)	31 1/2″ × 20 1/2″ × 15 11/16″

Open sealed box in designer

Ported

deeper, louder

5.16 ft³ (146 L)

Net volume

Tuning (Fb)	30 Hz
Port	slot 4 3/4 × 19 1/16″, 44″ long
Port velocity at 3,500 W	64 ft/s
F3 (−3 dB, no cabin gain)	27 Hz
Example box (outside)	31 1/2″ × 20 1/2″ × 25 1/2″

Open ported box in designer

Deeper: 7.56 ft³ (214 L) tuned to 26 Hz, 31 1/2″ × 20 1/2″ × 34 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	dual 1 Ω
RMS power	3,500 W
Fs	22.3 Hz
Qts	0.57
Qes	0.663
Qms	4.26
Vas	6.43 ft³

Re (one coil)	0.65 Ω
Sd	194.9 in²
Xmax	1.46″
Vd	283.9 in³ calc.
Mms	628 g calc.
BL	9.3 T·m calc.
EBP	34 — suits sealed calc.
Sensitivity, 1 W / 1 m	86.8 dB calc.

Mounting dimensions

Cutout diameter	16 5/8″
Mounting depth	12 3/16″
Overall diameter	18 1/2″

Judging by its value, the published Re is for both coils in series — converted to per coil.

“≈” 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-fmx-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 D2 18″ · dual 2 Ω
Sky High Car Audio FXB-18 D2 18″ · dual 2 Ω
Sky High Car Audio FXB-18 D4 18″ · dual 4 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.

Sundown Audio Xv1-18 D4 4.10 ft³ sealed · 5.30 ft³ ported
Fi Audio SP4 18 D1 4.27 ft³ sealed · 4.34 ft³ ported
Fi Audio SP418 v2.1 D1 4.27 ft³ sealed · 4.34 ft³ ported
Fi Audio HT-1 18 S2 4.27 ft³ sealed · 5.54 ft³ ported
Fi Audio SP4 18 D2 4.27 ft³ sealed · 4.34 ft³ ported
Fi Audio SP418 v2.1 D2 4.27 ft³ sealed · 4.34 ft³ ported

FAQ

What box does the Sky High Car Audio FMX-18 D1 need?

Recommended net volume: 4.45 ft³ sealed, 5.16 ft³ ported tuned to 30 Hz — for one sub in a sedan at 3,500 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 FMX-18 D1 Thiele/Small specs?

Fs 22.3 Hz, Qts 0.57, Vas 6.43 ft³, Xmax 1.46″, Sd 194.9 in².

What is the Sky High Car Audio FMX-18 D1 mounting depth?

Mounting depth is 12 3/16″, 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 FMX-18 D1 handle and how do I wire it?

RMS power rating: 3,500 W. Dual 1 Ω voice coils: series gives 2 Ω, parallel gives 0.5 Ω at the amp. Wiring for several subs and amp power — in the wiring calculator.