Car Audio Fabrication and Mounting: Build It Rock-Solid
Fabrication is the building work behind a car audio system: speaker rings and spacers, enclosures, A-pillar pods, amp racks and the trim over the top. It matters for sound because a speaker can only pass its full energy to you if whatever holds it does not move, rattle or choke it. Peter Steinbacher of Pssound builds around one idea: everything must be rock-solid, safe and serviceable.
The short version
Bolt everything to the car's structure, at factory points or with rivnuts (threaded metal inserts squeezed into drilled body steel), so a box can shake the whole car without moving itself.
Build so parts can still come out for service, and so the car can go back to standard.
Use birch plywood, not MDF, anywhere damp can reach, especially in doors.
Mount drivers on machine bolts in threaded inserts, never wood screws, with thin, chamfered (bevelled) rings so they can breathe.
Milkshake, Peter's name for resin thickened with powder, seals boxes and stiffens pods and pillars.
Fill every sealed box with water to prove it is airtight.
Allow far more time than you expect, because a proper pair of A-pillars is at least two long days.
Mount it solid, safe and serviceable
Peter says solid mounting means no lost energy: all of the speaker's output reaches you, which is what gives attack and punch (watch). His summary is "Mounting is the most important." In his Megane build every board and the sub enclosure are bolted to the chassis, so moving the box moves the whole car and nothing rattles. He says that matters more than even the best subwoofer (watch).
His proof: with the carpet lifted in Ramsey's car and loud music playing, the floor midbass (a driver for upper bass) moved only about 1 mm and the front sub about 3–4 mm.
The shake test: rock a properly mounted box, rack or kick build. The whole car should move while the part does not give even a fraction of a millimetre (watch).
Use factory fixing points or rivnuts, not self-tapping screws. The S-Class boot rack uses four factory points plus two rivnuts, and Lee of Platinum In-Car says the Porsche Taycan he built with Peter needed no cutting or drilling, so it can return to standard.
Never rely on gravity: in one Chrysler 300C the old amp rack simply sat in the boot and slid around. Heavy boxes need real fixings. An Audi RS6 box over 30 kg (about 70 lb) bolts to a panel fixed into a factory rail point, with half-inch (12 mm) ply underneath carrying metric bolts into the floor (watch).
Stop hard parts touching, even when bolted: carpet an MDF box all over, bottom included, so it cannot buzz on its board. The one thing he softens is the amplifiers, which sit on small shock mounts in his Honda because its infinite baffle (IB) subs, which use the boot or the outside air as their box, shake them (see our subwoofer and IB guide).
Birch ply, not MDF
MDF (a board pressed from wood dust and glue) soaks up water, swells and falls apart, while plywood resists water and dries quickly. By 2021 Peter said he rarely built anything from MDF except trim panels, though he used it in earlier builds (watch).
Not in doors, even painted, unless heavily resin-coated. He expects a painted MDF ring to last no more than about a year in UK weather, and birch ply is not much dearer (watch). In one Tiguan, MDF door spacers had grown mould after about three years and swollen about 10 mm thicker.
His standard door spacer is birch ply, through-bolted with stainless bolts and locking nuts, with closed-cell foam (a foam that does not soak up water) on the back to seal it and stop rattles.
PU (polyurethane) wood glue is his favourite. It expands into small gaps and sets in about half an hour.
Machine bolts, threaded inserts and rivnuts
Peter fits drivers with machine bolts (ordinary metric bolts) into threaded inserts (small threaded metal sleeves set into the wood), never wood screws. A bolt lets a driver come out and go back as often as you like, while screws damage the wood after a few removals. For neat work and top-level competition he calls inserts essential, and they take only about three minutes (watch).
Fit inserts from the top, not underneath, because tightening the bolt pulls an insert in further. A dab of superglue stops it moving.
Use M4 inserts and bolts even where the basket takes M5, which leaves no wiggle room. With four to six bolts the driver stays put.
Rivnuts turn body steel into bolt points wherever there is a cavity behind it. Before drilling, check what is below: in most cars the fuel tank sits under the rear seats (watch).
Rings, spacers and letting the driver breathe
A driver needs open air around and behind its basket (the metal frame). A thick ring or baffle hugging the basket makes it sound boxed in, and Peter says no EQ (tone adjustment) can fix that. His cure is to chamfer the back of every ring or baffle at about 45 degrees, which he says makes the midrange cleaner and more natural (watch).
Spacers colour the sound. The further a driver sits back from the door's inner metal panel, the more of a collar forms around it (watch). Stacking three or four 18 mm rings into a deep spacer creates a tunnel.
Small drivers go on thin steel rings of about 3–4 mm, roughly as thin as the basket's lip.
Gasket the basket. A Skoda's kick midbass buzzed for about a year because the drivers did not seal to the baffle. Enough gasket, tightened until it squashed, cured it (watch). Never squash deadening (sound-damping material) under a driver, because it squeezes out and spoils the speaker.
For doors, mount drivers to the door itself, never the door card (the interior trim panel). Bolt the ring to the door first, then fit the driver with short bolts into inserts in the ring, so it comes out without stripping the door. If the inner panel is riveted, swap the rivets for rivnuts so it bolts back on. See our midbass guide for the drivers themselves.
Milkshake, fibreglass and the wooden puzzle method
Milkshake is Peter's name for resin thickened with powder, and it is no secret recipe. Any resin works (polyester is cheaper than epoxy for this). The powder is usually fibreglass powder, though sand, sawdust or even MDF dust will do (watch). Poured inside a box and moved around, it fills gaps, seals the box and bonds everything from the inside.
Add the catalyst (hardener) last. More catalyst sets it faster, but too much makes it shrink and bond less well. It is fully dry after about 12 hours, so never pull a part out early.
For extra weight he mixes in steel powder or fine steel shot. Inside a sub box, finish with a plain coat so no loose steel can stick to the magnet.
A sub box needs thick walls. Many fibreglass boxes have walls only 5–6 mm thick, and he says a good-sounding box needs at least half an inch however it is built (watch). His minimum has moved over the years (12 mm in 2018, 8–10 mm in 2021, half an inch in 2022).
Instead of a full fibreglass mould he often builds the shape from 18 mm (3/4-inch) ply pieces glued with PU glue, his wooden puzzle method, with leftover gaps filled by fibre filler (a glass-reinforced filler that bridges gaps). He says it is no slower than fibreglassing, and wood is far easier overhead.
A-pillars, dashes and kicks: build them on the car
On A-pillars (the posts either side of the windscreen), most people pull stretchy fabric over the plastic, add resin and trim it. For performance Peter builds the structure up to about half an inch thick around the driver mount, from wood, fibre filler and milkshake. The key step is drilling hundreds of holes through the pillar, so filler on the front and milkshake on the back bond through the plastic and clamp it from both sides (watch).
Load the back of the pillar with steel milkshake for more rigidity, more weight and a deader pillar, and deaden it as well (see our deadening guide). Kick builds get the same mass: each Audi TT kick, the panel beside your feet, holds about 2 kg of steel shot plus 1–1.5 L of milkshake.
Pillars cannot be built properly off the car. A pod may not fit because something sits behind the pillar, and you cannot work out the aiming or see where the airbag is (watch). Recess the drivers into the pillar rather than building pods out from it.
Peter says the fear that an airbag will throw a pillar speaker at your face is wrong, because in most cars the bag reaches only about halfway down the pillar (watch). The real check is what sits behind: if an airbag bracket is there, the speaker cannot go there.
On dashes, you cannot know the workload until the dash is out. Build enclosures modular (the Honda's go from finished to bare dash in about half an hour), and test a rough, airtight version in the car before finishing it.
A pair of A-pillars is never a one-day job: he says at least two hard days, and prefers three (watch). His hours have fallen, though. In 2023 he put a fully trimmed pair at 16–20 hours between two people, against 30–40 hours about five years earlier.
Sealing and testing boxes, and building for service
A sealed box must be airtight, and Peter's test is to fill the finished box with water, which also gives its true volume. As he puts it, "If the water doesn't come out, air doesn't come out." (watch). Do this only with a plywood or fibreglass box, never MDF.
Leave the water in. No seepage after about five minutes means it is sealed, and another 20 minutes shows other leaks. Seepage shows where to fill from outside.
No water to hand? Push the cone in and let go. It should return very slowly. If it pops back quickly, the box leaks.
Most sealed boxes lose air at the speaker terminal, so he uses a cable gland (a fitting whose rubber grommet tightens around the cable) (watch). Plywood can leak between its layers, so seal the edges too.
In a boot, treat the base plate as the foundation: the 300C's sub box bolts to it with four bolts, and the trim panels bolt to the box. One RS6 box is bolted at 8 points.
Design for removal and daily use. Leave at least 40 mm above the box top for trim panels, build amp racks to slide out of the boot, and leave about 2 mm between trim panels, which vanishes once they are covered.
If you must cut the car, do it properly. The cut can be reversed by keeping the cut-out piece and welding it back (watch). Grind and deburr the edges, then clean, seal and paint them against rust.
Common mistakes to avoid
Leaving anything held by gravity, amps loose on the floor or screwed to the rear seat, or an amp board glued down.
Using MDF rings or spacers in doors, even painted ones.
Using wood screws or self-tappers that bite into nothing.
Stacking thick rings into a tunnel, or making a baffle that surrounds the whole basket.
Skipping the gasket under the basket, or squashing deadening under it.
Building pillars or door panels without the car, or onto the pillar surface with no holes drilled through.
Making a fibreglass sub box with 5–6 mm walls, or never leak-testing it.
Drilling or cutting without checking what is behind, such as a fuel tank, or leaving cut steel edges bare.
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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