Midbass in Car Audio: Why Location Beats the Driver
Midbass is the speaker job in the punchy range between the subwoofer and the midrange, roughly 50 to 200 Hz in a three-way system (separate midbass, midrange and tweeter). It carries the snap of drums and the low end of voices and instruments, so it decides whether a car sounds tight or muddy. Peter Steinbacher of Pssound calls it the hardest part of any car system, and his core idea is that you win it by where the speaker sits and how solidly it is mounted, not by what the driver costs.
The short version
Midbass is the hardest part of a car system, and Peter compares it to the drummer in a band.
Good midbass is tight, accurate and free of buzzes, not deep, loose bass that shakes your trousers.
Where the speaker sits and how rigidly it is mounted matter far more than its price.
Doors flex, rattle and create dips in the bass that tuning cannot fully fix, so Peter avoids them.
His order of preference: kick panels, then under the seat or on the floor in front of it, with a sealed box on the door as the fallback.
If you are stuck with the door, seal it, don't overspend and don't try to boost a dip with EQ.
Midbass works best in a three-way system, with a midrange that plays low above it and a sub below it.
What good midbass sounds like
Peter's picture is a band. The bassist, sax player and singer can all be superb, but if the drummer plays badly everything falls apart, and midbass is the drummer (watch). For tight drums in rock or fast electronic music, the midbass rather than the sub does the work.
The first job is no buzz and no rattle. After that Peter wants accuracy and enough output, like heavy floorstanding speakers at home, rather than sheer impact.
Bass that reaches down to 30 Hz and shakes your trousers does not impress him, because it is loose and uncontrolled. The range that matters is about 60–70 Hz up to 200 Hz.
A 6.5-inch or 8-inch driver is not a kick drum. Real big drums are 24–32 inches across, so a larger cone always sounds more dynamic down low (watch).
Installation matters more than the driver
Peter keeps returning to one point: good midbass comes from the location and the application, meaning what the driver is mounted in and how rigid that is. Price comes a long way behind, and as he puts it, "you can't fix problems with tuning" (watch).
Audi TT test: Audible Physics AP 6.25s (under £200), installed properly, beat the owner's Dynaudio Esotar 650s (£600–800 then, about £1000 now). From the same small kick box the AP matched the Dynaudio's response and was 5–6 dB louder, needing about a quarter of the power for the same volume (watch).
Honda test: for a while Peter's Honda ran an £89 SB Acoustics 8–9 inch driver on one side and a custom Accuton costing over £1.2k on the other. After tuning, listeners could not tell which was which (watch).
Until a midbass is solidly mounted, in a sealed box or in the kicks with its back open to the outside, he reckons you leave probably around 70% of its performance unused.
Why the door is a poor home for midbass
Peter absolutely hates midbass in the door. He sees it as the wrong place for a speaker covering roughly 50–400 Hz, with acoustic problems you cannot fix with a DSP, the digital processor that tunes the system (watch). The door moves, and its position creates dips.
Peter's view is that a door always vibrates. Even with the outer skin, inner skin and door card treated, something still rattles or flexes (see our guide to deadening and rattles).
Even without audible rattles the panel flexes, so sound comes from the whole door. The bass wraps around you instead of coming from one point, which kills the pinpoint image and the chest kick.
On his scale a treated door reaches maybe 5–6 out of 10 for accuracy, realism and zero buzz, against a good 8 out of 10 from an alternative mounting (watch).
Sound reaching your ears directly and the same sound reflected around the cabin arrive out of step and cancel at some pitches, a dip called a null. Door speakers sit low, one close to you and one far, so left and right never match.
The classic driver-side null sits somewhere between 60 and 100 Hz (in a Golf Mk8, deepest at 67 Hz) (watch). Play that note and you hardly hear it. Peter says a hole around 70–90 Hz appears in about 80% of cars, and it comes from the location, not the speaker (watch).
Peter says not to let a door midbass play above about 400 Hz: above that the response gets messy, and a passenger's leg changes it heavily.
Making a door work, and when to build a box on it
About 90–95% of cars can only use the factory door position. For a daily car, Peter's advice is to soundproof it well, fit the driver and hope for the best.
Do not spend big on a door midbass; he fitted an inexpensive Steg QK6 in a Volkswagen Transporter T6.
Seal every gap, and avoid long, deep spacer rings, which suffocate the driver around its basket.
Leaks can fool you. An Audi RS6 owner blamed his amplifier, but a 5–8 mm gap between driver and mounting plate acted like an extra port, giving a big peak near 170 Hz and almost no bass below about 120–130 Hz. A 10 mm adapter plate, a gasket and a shallower driver fixed it (watch).
The big step up is a sealed enclosure (an airtight box) built onto the door. Peter's first one was mind-blowing, because the sound now comes from the speaker rather than the door.
Volume guide: in 4 L most 6.5-inch drivers struggle, as they need at least 6–8 L. With 10–12 L suitable drivers are easier to find, and with 15–20 L it is easy, even for an 8-inch (watch).
A box cannot remove a null, because the null comes from the seat position and the cabin. In the Audi TT the driver-side door midbass played 100 Hz, but with the door shut it could not be heard from the seat (watch).
Better homes: kicks, firewall and under the seat
Peter's order of preference is the kicks first (the lower front corners of the footwell, beside your feet), then under the seat or on the floor in front of it. The firewall (the steel wall between engine bay and cabin) and the kicks are the most rigid parts of the car, so nothing nearby rattles.
In the kicks, a dedicated midbass gets a much better response across about 60–200 Hz, without the door's cancellations. That holds for a three-way; used as a midrange in a two-way, it has the same trouble above 200–300 Hz as a door (watch).
Aim for a sealed box of at least 0.2–0.3 cubic feet, not a tiny 2–3 L shoebox (watch). Only the air volume matters, not the shape, and it must be airtight. Left and right boxes need not match; only a gap of around double would show.
The costs are legroom, and in right-hand-drive cars the pedals leave little space. The reward is a midbass that seems to vanish, especially alongside a front sub (see our front-woofer guide).
Many Mercedes models from about 2013 carry an 8-inch midbass in the firewall behind the footwell carpet. Despite the carpet, Peter says it plays fine in the 50–200 Hz it needs (watch). It cannot play much above 140–150 Hz, so the car needs a midrange of at least 4 inches, or low-playing pillar widebands (watch); see our midrange guide.
BMW-style under-seat drivers only play to about 150–160 Hz, because the seat and your body filter the sound, so they too need a midrange that plays low. Tuned well, you hear the bass on top of the dash, not under you.
More power into a plug-and-play under-seat driver means more pressure and more rattle. Peter's fix is a hole under each driver so it breathes out under the car, making it infinite baffle (watch).
True infinite baffle
Infinite baffle (IB) here means the back of the driver is not boxed in: the front plays into the cabin and the rear wave goes outside the car, through a wheel arch or the floor. Given the chance and suitable drivers, Peter would always choose kick-mounted true IB for the finest sound. The driver sits on a very solid part of the car, so nothing flexes, and in a three-way he finds no cancellations or dips between 60 and 200 Hz. In his words, "if you can do true IB in a car it's magical" (watch).
It needs the right driver. Check the Thiele-Small parameters (the spec-sheet numbers that describe how a driver behaves): a higher Qts and a low Fs suit IB, and the wrong numbers will not sound controlled or tight.
Peter's rule is to keep at least 50% of the cone area open to the outside, so the driver can breathe.
Water is less of a worry than it sounds: doors take in just as much muck, and two layers of rain guard foam restrict water but let air through.
The real downsides are that IB midbass is audible outside the car and that you must cut the car, which is where many people back away. The kicks in a VW Amarok pickup took about 80 hours.
Driver choice, crossover and how to test
Peter wants the driver to suit its location, and the tuning to respect it.
Check Qts, a spec-sheet number that hints at how much air a driver wants. For doors, around 0.2–0.3 means it needs an enclosure, while 0.6–0.7 and above suits a door. A high-Qts driver in a small sealed box will not reach down well (watch).
Sealed boxes are easier and sound more accurate than ported ones; tuning a tiny 3 L box to 40 Hz just chokes the driver.
Do not push a door 6.5-inch below about 70 Hz; in a well-designed system he would not go below 90 Hz. But with a boot sub, the front midbass must still reach at least 60–70 Hz to blend.
Never EQ a null up to the other side's level. Even +6 to +10 dB does not come back, and it drives the cone to its limit and shakes the door apart. Relocate the driver rather than buying a new one.
To judge midbass, turn the sub off and play loud: you hear at once whether the front is solid. Another test is Blue Monday by New Order, whose triple kick should hit like a shotgun with the speaker hardly moving (watch).
At home, try a bookshelf speaker on the floor against the door midbass over the same range, such as 70–250 Hz.
Common mistakes to avoid
Spending big on a midbass driver and then mounting it in a door.
Blaming the amplifier when the real problem is an air leak or a flimsy mount.
Expecting a 6.5-inch to play the big kick drum; for more kick, Peter suggests a pair of them, an 8-inch or a front sub, not a bigger amp.
Pushing a dip up with EQ to match the other side.
Building a tiny shoebox for the kicks, or adding a port to a tiny box.
Dropping any driver into infinite baffle and expecting magic.
Running a kick, firewall or under-seat midbass without a midrange that can play low.
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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