Car Subwoofers and Infinite Baffle (IB) Explained
A rear subwoofer should add the last few octaves of bass that the front speakers cannot reach, so smoothly that you cannot tell where it is. Done well, it adds body and extension; done badly, the bass turns bloated and unfocused. The core idea of Peter Steinbacher of Pssound is to use infinite baffle (IB, where the shut cabin or the outside air acts as the sub's 'box') whenever the car allows it, and a generously sized sealed box when it does not.
The short version
A rear sub should add only the lowest octaves the front cannot play, at a level where you cannot locate it.
Infinite baffle lets the cone move freely, so the sub plays deeper and cleaner than a box, on very little power.
True IB vents to the outside, usually through the floor, and trunk IB uses the boot as air space; both need a driver made for IB.
Mount everything rigidly: a flexing floor, wall or bumper causes rattles and costs accuracy.
If you cannot cut the car, use a sealed box bigger than a typical ready-made (prefab) one, not a ported (vented) box tuned high.
The car limits a rear sub: expect little useful output from the back above about 50 Hz.
What a rear sub is for
Peter's view is that you always need a sub, unless the car is so quiet that the front midbass (the front speakers just above the sub's range) alone plays low with full detail. The front sub (see our front subwoofer guide) plays the upper bass, and the rear sub plays only the last few octaves. Once the system is tuned, you cannot locate the rear one (watch).
Typical split: the BMW 6GT's rear sub plays only below 40 Hz and is timed to match the front, so it disappears and the bass seems to come from the top of the dash.
Level target: with two 15s in the boot, 135 to 138 dB gives plenty of headroom over the front end. If your music goes beyond 140 dB, build the front end to match (watch).
How infinite baffle works, and where it falls short
To make bass, a speaker must keep the sound from the front of the cone apart from the sound from the back, or the two cancel. IB uses the car as the box, with you sitting inside it. In true IB the back of the cone breathes to the open air outside, which is effectively infinite space (watch). With nothing squeezing the cone, Peter says, IB bass is cleaner and more effortless.
It is not slow bass. He calls IB snappy, punchy and controlled, with enough output to overpower a decent front end.
Its sweet spot is deep: IB subs come alive under 25 Hz and excel down to single digits, which he says no box can do.
Its weak spot is 25 to 50 Hz, where it usually lacks the feel of a ported box (watch). On loudness contests (SPL), his line is: "If you do SPL you wouldn't use IB." For sound quality, he says he will always use it when he can (watch).
True IB, venting outside, is his pick for the very lowest notes; trunk IB rolls off a bit sooner in his modelling, but not steeply.
IB also saves weight, but installation takes time that many customers will not pay for.
Building true IB through the floor
True IB puts the sub over a hole in the floor, usually in the spare-wheel well, so its back wave goes out under the car. Once the tailgate is shut, the interior is the box. A manifold is a mounting frame that lets a sub fire through to the outside rather than into a closed box.
Hole size: for a 15in, a 320 mm round hole equals 100% of the cone area, so nothing restricts it, and he says it could have been smaller still (watch).
Rigidity comes first: his S-Class sub sits on a birch-ply ring bolted through the steel of the spare-wheel well.
Wire two subs in a manifold exactly the same, not out of phase. Both cones must push air into the cabin together and pull it out together (watch).
If you vent behind the bumper, support it. The 6GT has a thick plastic panel underneath its bumper to take the pressure; without one, the bumper could possibly fly away (watch).
Cover the hole with rain guard foam, the sound-transparent foam used on PA speaker grilles. By 2023 he said a single layer is enough.
Water is less of a risk than it sounds. Air flows under a moving car and spray is thrown outward, so nothing goes up. Only a flood deeper than the floor, about 10 to 12in, reaches the driver (watch).
True IB picks up outside bass like a microphone, and cold weather stiffens its soft parts. At an ice-rink event one car's front true-IB drivers measured about 3 dB down the next morning (watch). Re-check the tune every morning at an outdoor competition.
Trunk IB and getting bass out of the boot
When you cannot cut the floor, trunk IB mounts the sub on a rigid baffle (a board or wall) between boot and cabin, so the boot is its air space. Peter's first rule is a very rigid mounting, fixed to the car itself rather than the folding rear seat, which can rattle.
Leaks matter less than you think. What counts is the ratio of cone area to leak, so a gap of a few square inches barely affects two 18s or two 15s. Seal the shelf and side edges and you should be fine, unless you chase absolute SPL (watch).
Let the bass through. In a test, folding down one side of the rear seat (about a third) gained a good 7½ dB around 25 to 26 Hz, and the other side only 1 to 1.5 dB more. He advises a ski hatch (an opening between boot and cabin) or a folded seat (watch).
A sealed saloon (sedan) boot is the hard case. One Mazda owner was told he needed more power, but his sub simply could not play into the cabin.
Power, cone area and choosing a driver
IB subs need surprisingly little power. Nothing pushes back on the cone, so the sub's impedance (how hard it is for the amplifier to drive) climbs very high at low frequencies, and the lower you play, the less power it draws. Peter says not to chase a dearer sub amp: good bass needs a good location, a good application and plenty of power, especially voltage. As he puts it, "Stable voltage is king" (see our power and wiring guide).
In a clamp-meter test, two Acoustic Elegance (AE) IB15AU subs on one amp channel drew about 350 W at 35 Hz for 135.5 dB. At 20 Hz they made 138.7 dB from just 67 W (watch).
The limit is how far the cone can move, not heat. To find it, play your lowest wanted frequency (15 to 20 Hz) and raise the level slowly. Past the limit you can hear the spider, the suspension under the cone, stretch (watch). He uses no subsonic filter (which cuts the deepest signals) on true IB subs.
Cone area matters most: a big cone moving a little beats a small one moving a lot. For plain output, one sub is enough unless you listen very loud. For a proper IB experience, he said in 2025 to plan for at least a single 18 or two 15s.
Choose a driver made for IB, not for a box (AE is his standing answer for IB sound quality). Look for a low Fs (the cone's natural resonance) around 20 Hz or below, a high Qts (a damping figure) above 0.5, high sensitivity (how loud it plays per watt) and low inductance (which helps it play higher notes). Soft suspension is good; the idea that IB subs need stiff suspension is a myth (watch).
If you cannot cut the car: sealed boxes, and why not ported
When the car cannot be cut, Peter's fallback is a generously sized sealed box. Sealed boxes have low, even group delay (how much the bass lags behind the music), so they blend easily with a front end. Most ported boxes, which use a tuned vent, have very high group delay.
Give the sub more air than a prefab box allows. He aims for a Qtc (a figure for how tightly the box controls the driver; more air lowers it) of about 0.7, where the sub is best for power handling and low-end reach. Prefabs are so small that Qtc is about 1 or above: punchy above 40 Hz, but they roll off sooner.
Example: a VW Tiguan's prefab of about 0.5 to 0.6 cu ft (roughly 16 to 18 L) became a custom box of about 1.2 to 1.3 cu ft (about 35 L). It plays clean to 29 to 30 Hz and also freed boot space (watch).
From a sealed 10in, expect 30 Hz solid and 25 Hz just about; anything lower is a bonus. For near-IB depth he uses a JL 13W7 in a very oversized sealed box, only for the lowest octaves.
Ported suits hip hop and bass you feel. But he dislikes a boot-filling ported 12 or 15 that only plays below 35 Hz, because its group delay can never be integrated with the front end (watch). Never boost EQ far below a ported box's tuning: the output falls off a cliff and you lose control.
What the car does to rear-sub bass
A point Peter repeats often: the limit is the car, not the sub.
In his Honda no rear sub played above about 50 Hz at the seat, even very fast sealed boxes (watch). So the claim that true IB subs do not play high is mostly a location myth.
In most longer cars the sub's wave travels forward, bounces off the windscreen and cancels where you sit, somewhere around 55 to 80 Hz (watch). A different sub, or sealed versus ported, does not change it. The fix is a sub or large midbass up front that plays low enough to meet the rear sub (see our crossovers, EQ and time alignment guide).
If a sub seems not to play the lows, it usually is, but the level is too low next to the big bass peak that short cabins create. Flatten the peak with EQ (tone adjustment) and win the output back with gain (turning the sub up) (watch).
To make the low end audible, the measured level has to keep rising as notes go lower, by at least about 3 dB per octave below the sub crossover. Only true IB can do that in a car, he says.
Common mistakes to avoid
Expecting punch above about 50 Hz from a rear sub, then adding power.
Mounting on anything that flexes: a thin floor, the folding rear seat, or a bumper with no support panel.
Using a box sub as an IB driver, or choosing a driver on one number such as Qts.
Wiring one sub of a manifold pair out of phase.
Trusting a prefab box's recommended size, or never pushing on the drivers to check a box for leaks.
Filling the boot with a ported box that only plays below 35 Hz.
Keeping a weak, noisy sub: a bad sub is worse than none.
Forgetting to re-check a true-IB tune when the temperature changes.
Watch Peter explain it
This is an independent, plain-English summary of Peter Steinbacher's public YouTube videos, written by VA3 Car Studio. It is not affiliated with or endorsed by Pssound. The links jump to the moment in each video where he makes the point.
Part of our series: Car audio lessons from Pssound, start here
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