Sky High Car Audio FEX-18 D4
18″ subwoofer · dual 4 Ω · 1,250 W RMS
The Sky High Car Audio FEX-18 D4 is a 18″ subwoofer. With Qts 0.94 and EBP 27 it suits a sealed box: tight bass in a small enclosure; a ported box would need a lot of volume.
| 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, 1,250 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| Bass character (Qtc) | Boomy, with a bump · 1.60 |
|---|---|
| Fc | 50 Hz |
| F3 (−3 dB, no cabin gain) | 36 Hz |
| Example box (outside) | 31 1/2″ × 20 1/16″ × 15 11/16″ |
Ported
deeper, louder| Tuning (Fb) | 32 Hz |
|---|---|
| Port | slot 2 3/8 × 18 11/16″, 15″ long |
| Port velocity at 1,250 W | 61 ft/s |
| F3 (−3 dB, no cabin gain) | 32 Hz |
| Example box (outside) | 31 1/2″ × 20 1/16″ × 21 1/16″ |
Deeper: 7.35 ft³ (208 L) tuned to 40 Hz, 31 1/2″ × 20 1/16″ × 26 1/8″ outside — a bigger box with flatter deep bass. Open in the calculator
Projects with this subwoofer
Boxes that users designed and published: plans, cut sheet and response graph.
Thiele/Small parameters
| Size | 18″ |
|---|---|
| Voice coils | dual 4 Ω |
| RMS power | 1,250 W |
| Fs | 29.1 Hz |
| Qts | 0.94 |
| Qes | 1.067 |
| Qms | 7.96 |
| Vas | 8.28 ft³ |
| Re (one coil) | 3.3 Ω |
|---|---|
| Sd | 189.9 in² |
| Xmax | 0.75″ |
| Vd | 142.1 in³ calc. |
| Mms | 271 g calc. |
| BL | 12.4 T·m calc. |
| EBP | 27 — suits sealed calc. |
| Sensitivity, 1 W / 1 m | 89.4 dB calc. |
Mounting dimensions
| Cutout diameter | 16 1/4″ |
|---|---|
| Mounting depth | 9 3/8″ |
| 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
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 |
|---|---|---|---|---|---|---|
| Sky High Car Audio FEX-18 D2 | dual 2 Ω | 2.15 Ω | 0.79 | 7.87 ft³ | 4.34 ft³ | 5.62 ft³ · 30 Hz |
| Sky High Car Audio FEX-18 D4 | dual 4 Ω | 3.3 Ω | 1.067 | 8.28 ft³ | 4.34 ft³ | 5.62 ft³ · 32 Hz |
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 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 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.
- Fi Audio IB318 v2 S2 4.27 ft³ sealed · 5.54 ft³ ported
- Fi Audio IB318 v2 S4 4.27 ft³ sealed · 5.54 ft³ ported
- 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
- CT Sounds TROPO-18 D4 3.99 ft³ sealed · 5.26 ft³ ported
FAQ
What box does the Sky High Car Audio FEX-18 D4 need?
Recommended net volume: 4.34 ft³ sealed, 5.62 ft³ ported tuned to 32 Hz — for one sub in a sedan at 1,250 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 FEX-18 D4 Thiele/Small specs?
Fs 29.1 Hz, Qts 0.94, Vas 8.28 ft³, Xmax 0.75″, Sd 189.9 in².
What is the Sky High Car Audio FEX-18 D4 mounting depth?
Mounting depth is 9 3/8″, cutout diameter 16 1/4″. The inside depth of the box must exceed the mounting depth by at least 1″.
How much power does the Sky High Car Audio FEX-18 D4 handle and how do I wire it?
RMS power rating: 1,250 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.
Is the Sky High Car Audio FEX-18 D4 better sealed or ported?
EBP 27: a sealed box (4.34 ft³) suits it best — tight bass in a compact enclosure. Ported is possible (5.62 ft³, tuned to 32 Hz) but larger.
What port does the Sky High Car Audio FEX-18 D4 need?
For the 5.62 ft³ ported box tuned to 32 Hz: slot 2 3/8 × 18 11/16″, 15″ long, area 44.1 in²; or a round port Ø7 1/2″, ≈ 15″ long. Port air speed at 1,250 W: 61 ft/s.
What box do two Sky High Car Audio FEX-18 D4 subs need?
Two subs need twice the volume: 8.69 ft³ sealed, or 11.23 ft³ ported at the same 32 Hz tuning with twice the port area. One shared chamber or one per sub — whatever fits your vehicle.