Fi Audio SP3 Plus 18 D1
18″ subwoofer · dual 1 Ω · 2,500 W RMS
| 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, 2,500 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| Bass character (Qtc) | Dry and tight · 0.49 |
|---|---|
| Fc | 54 Hz |
| F3 (−3 dB, no cabin gain) | 90 Hz |
| Example box (outside) | 20 1/16″ × 20 1/16″ × 14 5/8″ |
Ported
deeper, louder| Tuning (Fb) | 34 Hz |
|---|---|
| Port | slot 5 1/2 × 19 7/16″, 36 3/16″ long |
| Port velocity at 2,500 W | 65 ft/s |
| F3 (−3 dB, no cabin gain) | 32 Hz |
| Example box (outside) | 31 1/2″ × 20 7/8″ × 25 13/16″ |
Deeper: 3.60 ft³ (102 L) tuned to 35 Hz, 31 1/2″ × 20 7/8″ × 19 7/8″ 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 1 Ω |
| RMS power | 2,500 W |
| Fs | 32.2 Hz |
| Qts | 0.29 |
| Qes | 0.3 |
| Qms | 7.81 |
| Vas | 4.38 ft³ |
| Re (one coil) | 0.9 Ω |
|---|---|
| Sd | 187.6 in² |
| Xmax | 1.34″ |
| Vd | 251.1 in³ calc. |
| Mms | 408 g calc. |
| BL | 15.7 T·m calc. |
| EBP | 107 — suits ported calc. |
| Sensitivity, 1 W / 1 m | 93.4 dB calc. |
Mounting dimensions
| Cutout diameter | 16 3/4″ |
|---|---|
| Mounting depth | 12 1/4″ |
| 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
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.
- Ascendant Audio SMD Signature 18 D2 2.95 ft³ sealed · 5.54 ft³ ported
- Ascendant Audio SMD Signature 18 D1 2.95 ft³ sealed · 5.54 ft³ ported
- Sundown Audio Uv1-18 D2 1.80 ft³ sealed · 5.90 ft³ ported
- Sundown Audio Uv1-18 D4 1.80 ft³ sealed · 5.90 ft³ ported
- Sundown Audio NSv4-18 D2 2.86 ft³ sealed · 5.23 ft³ ported
- Edge EDX18D1-E7 1.62 ft³ sealed · 5.33 ft³ ported
FAQ
What box does the Fi Audio SP3 Plus 18 D1 need?
Recommended net volume: 2.38 ft³ sealed, 5.54 ft³ ported tuned to 34 Hz — for one sub in a sedan at 2,500 W. Fit the box to your space and amp in the free calculator, with plans and a cut sheet.
What are the Fi Audio SP3 Plus 18 D1 Thiele/Small specs?
Fs 32.2 Hz, Qts 0.29, Vas 4.38 ft³, Xmax 1.34″, Sd 187.6 in².
What is the Fi Audio SP3 Plus 18 D1 mounting depth?
Mounting depth is 12 1/4″, 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 SP3 Plus 18 D1 handle and how do I wire it?
RMS power rating: 2,500 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.