Fi Audio SP3 Plus 18 D2

18″ subwoofer · dual 2 Ω · 2,500 W RMS

32.2 HzFs, resonance
0.29Qts
4.38 ft³Vas
1.34″Xmax
2,500 WRMS

The Fi Audio SP3 Plus 18 D2 is a 18″ subwoofer. With Qts 0.29 and EBP 107 it suits a ported box: deeper and louder; sealed, the bass sounds a bit dry. Long excursion (Xmax 1.34″) — built for loud music and high power.

Sourceweb.archive.org · manufacturer data
Checked against sourceOctober 2026
Thiele/Small parameterscomplete
Mounting dimensions3 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
2.38 ft³ (67.5 L)
Net volume
Bass character (Qtc)Dry and tight · 0.49
Fc54 Hz
F3 (−3 dB, no cabin gain)90 Hz
Example box (outside)20 1/16″ × 20 1/16″ × 14 5/8″
Open sealed box in designer

Ported

deeper, louder
5.54 ft³ (157 L)
Net volume
Tuning (Fb)34 Hz
Portslot 5 1/2 × 19 7/16″, 36 3/16″ long
Port velocity at 2,500 W65 ft/s
F3 (−3 dB, no cabin gain)32 Hz
Example box (outside)31 1/2″ × 20 7/8″ × 25 13/16″
Open ported box in designer

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

Where to buy Amazon
203040506080100150200667278849096HzdB
Sealed Ported · SPL in dB at 1 W / 1 m, no cabin gain

Thiele/Small parameters

Size18″
Voice coilsdual 2 Ω
RMS power2,500 W
Fs32.2 Hz
Qts0.29
Qes0.3
Qms7.81
Vas4.38 ft³
Re (one coil)1.8 Ω
Sd187.6 in²
Xmax1.34″
Vd251.1 in³ calc.
Mms408 g calc.
BL22.3 T·m calc.
EBP107 — suits ported calc.
Sensitivity, 1 W / 1 m93.4 dB calc.

Mounting dimensions

Ø18 1/2″cutout Ø16 3/4″mounting depth 12 1/4″magnet Ø≈ 10 7/8″
Cutout diameter16 3/4″
Mounting depth12 1/4″
Overall diameter18 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.

ModelVoice coilsReQesVasSealedPorted
Fi Audio SP3 Plus 18 D1dual 1 Ω0.9 Ω0.34.38 ft³2.38 ft³5.54 ft³ · 34 Hz
Fi Audio SP3 Plus 18 D2dual 2 Ω1.8 Ω0.34.38 ft³2.38 ft³5.54 ft³ · 34 Hz

Other Fi Audio 18″ models

Similar 18″ subwoofers from other brands

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

FAQ

What box does the Fi Audio SP3 Plus 18 D2 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 D2 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 D2 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 D2 handle and how do I wire it?

RMS power rating: 2,500 W. Dual 2 Ω voice coils: series gives 4 Ω, parallel gives 1 Ω at the amp. Wiring for several subs and amp power — in the wiring calculator.

Is the Fi Audio SP3 Plus 18 D2 better sealed or ported?

EBP 107: a ported box (5.54 ft³, tuned to 34 Hz) suits it best — deeper and louder. Sealed (2.38 ft³) the bass is drier.

What port does the Fi Audio SP3 Plus 18 D2 need?

For the 5.54 ft³ ported box tuned to 34 Hz: slot 5 1/2 × 19 7/16″, 36 3/16″ long, area 107.2 in²; or a round port Ø11 11/16″, ≈ 36 3/16″ long. Port air speed at 2,500 W: 65 ft/s.

What box do two Fi Audio SP3 Plus 18 D2 subs need?

Two subs need twice the volume: 4.77 ft³ sealed, or 11.09 ft³ ported at the same 34 Hz tuning with twice the port area. One shared chamber or one per sub — whatever fits your vehicle.