Polk Audio db840

8″ subwoofer · 4 Ω · 180 W RMS

42 HzFs, resonance

0.56Qts

0.54 ft³Vas

0.24″Xmax

180 WRMS

Source	web.archive.org · manufacturer data
Checked against source	October 2026
Thiele/Small parameters	complete
Mounting dimensions	2 of 4 — missing: frame outer diameter, magnet diameter; the designer will use an estimate

Recommended box

For a sedan, one sub, 180 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

0.49 ft³ (14 L)

Net volume

Bass character (Qtc)	Tight and punchy · 0.81
Fc	61 Hz
F3 (−3 dB, no cabin gain)	57 Hz
Example box (outside)	10 1/4″ × 10 1/4″ × 13 3/16″

Open sealed box in designer

Ported

deeper, louder

0.57 ft³ (16 L)

Net volume

Tuning (Fb)	40 Hz
Port	round port Ø2 1/2″, 12 7/8″ long
Port velocity at 180 W	105 ft/s
F3 (−3 dB, no cabin gain)	40 Hz
Example box (outside)	17 1/4″ × 15 3/4″ × 6 1/2″

At 180 W this example’s port will be noisy: air speed 105 ft/s, while the limit is 82 ft/s. It stays quiet up to about 110 W. Full power needs a port of at least 6.4 in², which means a bigger box than this example — size it in the designer.

Open ported box in designer

Deeper: 1.38 ft³ (39 L) tuned to 34 Hz, 31 1/2″ × 15 3/4″ × 9 9/16″ 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	8″
Voice coils	4 Ω
RMS power	180 W
Fs	42 Hz
Qts	0.56
Qes	0.58
Qms	13.4
Vas	0.54 ft³

Re	3.5 Ω
Sd	35.7 in²
Xmax	0.24″
Vd	8.6 in³ calc.
Mms	70 g calc.
BL	10.5 T·m calc.
EBP	72 — sealed or ported calc.
Sensitivity, 1 W / 1 m	84.9 dB calc.

Mounting dimensions

Cutout diameter	7 1/16″
Mounting depth	3 15/16″

“≈” 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

web.archive.org/web/20121103004816/http://www.polkaudio.com/downloads/manual/dbS

Other Polk Audio 8″ models

Polk Audio db804 8″ · 4 Ω
Polk Audio db804DVC 8″ · dual 4 Ω
Polk Audio db840DVC 8″ · dual 4 Ω
Polk Audio dX8-4 8″ · 4 Ω
Polk Audio dX8-8 8″ · 8 Ω
Polk Audio DXi804 DVC 8″ · dual 4 Ω
Polk Audio DXi804 SVC 8″ · 4 Ω
Polk Audio EX8 (EX Series II) 8″ · 4 Ω
Polk Audio MM840 8″ · 4 Ω
Polk Audio MM840DVC 8″ · dual 4 Ω
Polk Audio MM2084 8″ · 4 Ω
Polk Audio MM2084 DVC 8″ · dual 4 Ω

Similar 8″ subwoofers from other brands

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

Kicker Comp 8" 4-Ohm SVC (43C84) 0.57 ft³ sealed · 0.53 ft³ ported
Morel Ultimo Titanium 804 (SVC 4 ohm) 0.46 ft³ sealed · 0.64 ft³ ported
Ground Zero GZIW 200X-II (4 Ohm) 0.42 ft³ sealed · 0.49 ft³ ported
Rockford Fosgate Punch P18S4 (Stage-1, 2004) 0.35 ft³ sealed · 0.49 ft³ ported
Hertz Cento CS 200 S4 (SVC 4 ohm) 0.35 ft³ sealed · 0.53 ft³ ported
Rockville W8K5S4 0.39 ft³ sealed · 0.92 ft³ ported

FAQ

What box does the Polk Audio db840 need?

Recommended net volume: 0.49 ft³ sealed, 0.57 ft³ ported tuned to 40 Hz — for one sub in a sedan at 180 W. Fit the box to your space and amp in the free calculator, with plans and a cut sheet.

What are the Polk Audio db840 Thiele/Small specs?

Fs 42 Hz, Qts 0.56, Vas 0.54 ft³, Xmax 0.24″, Sd 35.7 in².

What is the Polk Audio db840 mounting depth?

Mounting depth is 3 15/16″, cutout diameter 7 1/16″. The inside depth of the box must exceed the mounting depth by at least 1″.

How much power does the Polk Audio db840 handle and how do I wire it?

RMS power rating: 180 W. Wiring for several subs and amp power — in the wiring calculator.