Proline X PO 12D-S DF XLT sealed down-fire enclosure, three-quarter view of both flush-mount baffle openings and the side terminal cup. Optimized for PowerBass XLT.A black woofer enclosure insert shown on a dark blue gradient background with measurement labels indicating 15 inches tall by 7.28 inches wide, plus text reading 'Performance Optimized Series' and 'Part # PO 12D-S DF XLT' with Proline branding.A product information graphic for a Proline PO 12D-S DF XLT subwoofer. The black subwoofer is shown on a dark blue gradient background with white and blue text detailing its dimensions (30 inches by 7.

Performance Optimized · Datasheet

Performance Optimized Down Fire Sealed Dual 12" Enclosure | PowerBass XL-1240T Dual 12" Sealed enclosure

Net volume
0.80 cu ft total, 0.40 per driver after 0.1260 displacement each
Max mounting depth
3.8 in required, shallow mount
External dimensions
30.00 in W × 7.28 in in the down fire position H × 15 in D
Front baffle
1.5 in double baffle, outer layer machined as a flush recess
Buy at Audio Intensity
  • Alignment: 0.40 cu ft net per driver. Qtc 1.145, fc 71.9 Hz, F3 53.6 Hz
  • Driver: PowerBass XL-1240T, 400 W RMS, 7.5 mm Xmax, 3.8 in mounting depth, single 4 ohm
  • Rear Venting: Vented rear motor, pole vent may be fully obstructed
  • Standoffs: Included, 1.75 in, uninstalled. Installed height near 9.03 in
  • Cabinet: 30.00"W x 15"D x 7.28"H in down fire position, excludes standoffs

Technical specifications

SpecificationValue
SeriesProline X Performance Optimized Down Fire
Compatible DriverPowerBass XL-1240T, quantity 2. Direct fit
ConfigurationDual 12 inch shallow mount, sealed down fire, dual chamber
Mounting PositionTrunk or cargo floor, baffle facing down on standoffs
Firing DirectionDownward. Feet ship uninstalled, so front or up firing is also possible
External Width30.00 in
External Depth15 in
External Height7.28 in in the down fire position, excludes standoffs
Installed Height9.03 in including the 1.75 in standoff
Internal Volume, Gross1.0520 cu ft total
Internal Volume, Net Acoustic0.80 cu ft total, 0.40 per driver after 0.1260 displacement each
PowerBass Published Target0.40 cu ft optimum, 0.30 to 0.85 acceptable. Their figures are GROSS and include driver displacement
AlignmentQtc 1.145, fc 71.9 Hz, F3 53.6 Hz anechoic
Woofer Opening11.0000 in (281 mm) direct fit, both positions
Driver Mounting Depth3.8 in required, shallow mount
Driver MotorRear vented, pole vent may be fully obstructed
Driver Power Handling400 W RMS, 800 W peak each
Driver Voice CoilSingle 4 ohm, 2 in
Wiring2 ohms parallel or 8 ohms series
MaterialLangboard Elite MDF
ConstructionCNC cut, V-groove and dado joinery, square corners
Panel Thickness0.75 in
Front Baffle1.5 in double baffle, outer layer machined as a flush recess
Internal BracingFull chamber divider
Driver MountingThreaded inserts, stainless machine screws included
Terminal CupsOne Proline X ABS and carbon fiber composite, side mounted, pre wired 12 gauge OFC
StandoffsIncluded, 1.75 in, shipped uninstalled
DampingLight polyfill wall lining, does not alter acoustic volume
CarpetPlush black
Weight35 lb
Warranty1 Year Limited
Built InTullahoma, Tennessee

Performance in this enclosure

Net airspace
0.40
cu ft per chamber
System Qtc
1.15
sealed
Half power
53.6
Hz · F3
Xmax at
425
W RMS · no filter
With 25 Hz subsonic
425
W RMS · +0%
Amplifier
400
W RMS recommended
This enclosure holds 0.40 cu ft net per chamber. In it the PowerBass XL-1240T runs a system Qtc of 1.15 with half power at 53.6 Hz, and the cone reaches its 7.5 mm travel limit at 425 W, above the driver’s 400 W thermal rating, so heat stops you before travel does. A 25 Hz subsonic filter adds little here: 425 W against 425 W, 0 percent. The box already resonates at 72 Hz, so its own rolloff has removed most of what the filter would have stopped.

01Is it a sealed driver?

Efficiency bandwidth product places the motor before anything else is decided.

EBP = Fs ÷ Qes = 55.3
SEALED 0–50 TRANSITION 50–100 PORTED 100–150 0 50 100 150 XL-1240T 55.3
XL-1240T · EBP 55.3
At 55.3 the XL-1240T sits in the transition band, not the sealed band. That is not a mismatch: a transition motor works in either alignment, and sealed buys a smaller box and a gentler roll-off at the cost of some low-end output. Stage 02 shows what this airspace actually does with it.

02What the box does to it

0.40 cu ft against a 42.4 L Vas.

α = Vas ÷ Vb = 3.74  ·  Fc = Fs√(α+1) = 72 Hz  ·  Qtc = 1.15
20 30 50 70 100 150 200 +6 +0 -6 -12 -18 -24 FREQUENCY · Hz LEVEL · dB −3 dB
XL-1240T · Qtc 1.15, Fc 72 Hz, F3 53.6 Hz
The box lifts resonance from 33.0 Hz free-air to 72 Hz and half power lands at 53.6 Hz, anechoic, before any cabin gain.

03The watts that reach Xmax (in this enclosure)

Travel scales with the square root of power, so there is one wattage that puts the cone exactly on 7.5 mm, and in this alignment a subsonic filter barely moves it.

PXmax = 425 W unfiltered  →  425 W with a 25 Hz subsonic
20 30 50 70 100 150 200 0 4 8 FREQUENCY · Hz CONE TRAVEL · mm PAST LINEAR TRAVEL · Xmax 7.5 mm 425 W reaches Xmax here
XL-1240T · 425 W, no filterXL-1240T · 425 W with subsonic at 25 Hz, 24 dB/octXmax 7.5 mm one-way
Every trace touches the limit and no further, because each is drawn at the exact power that puts that coil on 7.5 mm. Unfiltered that is 425 W, reached first at 56 Hz. It sits above the 400 W the driver can take thermally, so the voice coil gives up before the suspension does. The dashed traces are the same driver behind a subsonic filter at 25 Hz, 24 dB/oct, and they sit almost on top of the solid ones: 425 W against 425 W. At 72 Hz the filter sits well below resonance, where the sealed alignment has already rolled the cone off at 12 dB/oct, so there is little wasted travel left for it to recover. Set it anyway to keep road rumble out of the cone, but do not size the amplifier around it.
ParamValueUnitNote
Fs33.00Hzfree-air resonance
Qts0.526XL-1240T
Vas42.37Lequivalent compliance
Sd510.7cm²effective piston area
Xmax7.5mmone-way linear travel
Pe400Wcontinuous RMS
SPL85.9dB1 W / 1 m, derived from T/S

Driver parameters as published by powerbassusa.com. Curves computed against this enclosure’s actual net airspace.

Installer's take

Watch the volume convention on this brand. PowerBass quotes 0.40 cu ft and that is GROSS, displacement included. This cabinet is 0.40 NET, which is 0.5260 gross. If a customer says the box looks big against the sheet, that is why, and it is inside their 0.30 to 0.85 acceptable range.

Measure the installed height, not the cabinet height. 7.28 tall plus 1.75 of standoff puts you at 9.03 inches of stack. Footprint is 30.00 by 15.

Rear vented motor, so the pole vent can be blocked. That is why this cabinet has no rear clearance allowance and why it comes out as short as it does.

Single 4 ohm coil. A pair gives 2 ohms parallel or 8 ohms series. Nothing to get wrong.

Qtc 1.145 at 0.40 net. This is a shallow driver in a small box and it reads that way: tight rather than deep. Set the expectation before the sale.

Questions

Will this enclosure fit any 12 inch subwoofer?

No. The baffle is cut direct fit at 281 mm for the PowerBass XL-1240T and the airspace is built to that driver. The XL-T is a shallow mount driver with a 3.8 inch mounting depth, and a conventional 12 needs both a deeper cabinet and more air.

Even where another driver dropped into the hole, it would be loading into an airspace engineered around a Qts of 0.526, and the alignment printed on this page would no longer describe what you own.

PowerBass says 0.40 cubic feet. Is this cabinet the same size?

Almost, and the difference is worth understanding. PowerBass quotes 0.40 cubic feet as internal airspace that INCLUDES the driver's displacement, so their figure is gross. We publish net everywhere, meaning the air the cone actually loads against after the driver's 0.1260 cubic feet is taken out.

This cabinet is 0.40 net, which is 0.5260 gross. That puts it slightly on the larger side of their optimum and comfortably inside the 0.30 to 0.85 cubic foot range they list as acceptable.

What amplifier power and impedance?

The XL-1240T is rated 400 watts RMS and 800 peak. It is a single 4 ohm voice coil. A matched pair gives 2 ohms in parallel or 8 ohms in series.

Single voice coil means the only wiring choices are those two so there is no low impedance trap to fall into on this driver. Target 800 watts RMS across the pair and set gains properly rather than to a number.

How tall is this once the standoffs are on?

The cabinet is 7.28 inches tall measured in the down fire position, excluding standoffs. They hold 1.75 inches under the baffle, so plan around a finished stack of roughly 9.03 inches.

The footprint is 30.00 inches wide by 15 deep.

Does the pole vent need clearance under the cabinet?

No. The XL-T uses a vented rear motor, so the pole vent can be obstructed without harming the driver. That is what allows this cabinet to be built without the rear clearance allowance most drivers require, and it is a large part of why a shallow driver suits a down fire cabinet at all.

The standoffs are there to give the cone room to breathe in front of the baffle, which faces the floor, not to relieve the pole vent.

What is the expected low-frequency extension?

Anechoic F3 lands at 53.6 Hz with system Qtc 1.145 and fc 71.9 Hz, calculated from PowerBass's published parameters at the 0.40 cubic feet net this cabinet holds.

Those figures are anechoic and exclude cabin gain. In a car the transfer function of the cabin adds real output below 50 Hz, so in-vehicle response runs deeper. A shallow mount driver in a small sealed box is a packaging decision, and the alignment reflects that honestly rather than promising otherwise.

Do I need to add polyfill or damping?

No. Polyfill is already installed as a light wall lining to control internal reflections, and the quantity is chosen so it does not change the acoustic volume.

Do not add more. Packing a sealed box with fill makes it behave as though it were larger, which on a cabinet this small would move the alignment noticeably. PowerBass also warns against letting fill reach the vented pole piece.

Are the PowerBass XL-1240T included?

No. This is an enclosure only. You receive the cabinet finished in plush black carpet, with threaded inserts at both driver positions, stainless machine screws, one side-mounted pre-wired terminal cup, the standoffs and the polyfill lining already in place.

The two drivers are purchased separately. We do not stock PowerBass drivers.

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