TL;DR

  • Sound deadening uses three different materials for three different jobs: damping mats (stop panels ringing), a mass barrier (blocks airborne noise) and closed-cell foam (decouples and absorbs).
  • Damping mats don’t need full coverage: a partial patch in the middle of each large flat panel stops it resonating. A mass barrier only works with near-full coverage, with no gaps.
  • Do it with the shell stripped, before the wiring and interior go back in, and keep drain holes, harness routes and service openings clear.

Why bother on an E36

A 25-plus-year-old E36 has tired factory insulation, rattling trim and drier seals. On the Projekt 36 car the swap from the four-cylinder M43 to the six-cylinder M50 will also change how the engine and exhaust sound in the cabin. Sound deadening isn’t about silence: it’s about removing the drone, rattles and boom that make a long drive tiring.

The three materials

1. Damping mats (constrained-layer damping, CLD)

A butyl layer with an aluminium foil top, rolled onto the metal. It adds mass and damping to a panel so it stops ringing. Tap an untreated door skin and you hear it ring; with a patch of damping mat it just thuds.

  • Where: large flat panels: door outer skins, floor, boot floor, roof, rear quarter panels, spare wheel well.
  • How much: partial coverage of the centre of each panel is the usual practice. Covering every square centimetre adds weight for little extra benefit.
  • Choose: butyl-based mats from established car-audio brands. Avoid asphalt-based mats, which can smell and soften in summer heat.

2. Mass barrier (mass-loaded vinyl, MLV)

A heavy, flexible sheet that blocks airborne noise (road, exhaust, engine). It works only as a continuous layer: gaps let the noise through. It goes on top of a foam decoupling layer, not straight onto the metal.

  • Where: floor and transmission tunnel, bulkhead (cabin side), boot floor.
  • Trade-off: it’s the heaviest part of the system. For a road car the comfort is usually worth it; for a track car, skip it.

3. Closed-cell foam

Decouples the mass barrier from the metal, stops trim rattling against panels and absorbs some sound.

  • Use closed-cell only. Open-cell foam soaks up water and holds it against the floor: a direct route to rust.
  • Where: under the mass barrier; behind door cards and trim panels; between plastic trim and metal.
Sound deadening layers on the floor Cross-section from the bottom up: the steel floor panel; damping mat patches covering part of the panel; a continuous layer of closed-cell foam; a continuous mass barrier; then the carpet. Carpet Mass barrier (MLV) continuous, no gaps Closed-cell foam decouples, full layer Damping mat (CLD) patches, centre of panels Steel floor Layers drawn apart for clarity; in the car each layer sits directly on the one below. Not to scale.
The three materials do different jobs, so they're applied differently: damping mats as patches, foam and mass barrier as continuous layers.

Where to put what in an E36 sedan

AreaDamping matFoamMass barrier
Front floor and tunnelCentre of the flat sectionsFull layerFull layer
Bulkhead (cabin side)Where accessibleFull layerFull layer, as far as you can reach
DoorsOuter skin, centre; patch the large openings in the inner skinBehind the door cardOptional
Rear floor, under the seatCentre of the flat sectionsFull layerFull layer
Boot floor and spare wheel wellCentre of the flat sectionsOver the floorOptional
RoofCentre sections between the bowsThin layerNot usually
Rear quarter panels and parcel shelfCentre sectionsBehind trimOptional

Keep these clear

  • Drain holes in the floor and door bottoms. Water must still be able to get out.
  • The door’s water path: the outer door skin gets wet on the inside, which is normal. Keep mats away from the bottom edge and drain holes, and renew the vapour barrier on the inner door skin (see the door panel guide).
  • Wiring routes and grommets, so the harness still fits.
  • Service openings: fuel pump access under the rear seat, sender access, and any bolts you’ll need again.
  • Hot areas: near the exhaust tunnel, use materials rated for the temperature.

Application tips

  1. Clean and degrease every panel; the butyl only sticks to clean metal.
  2. Work warm. In a cold garage, warm the mats and panels so the butyl bonds.
  3. Roll every mat down firmly with a hard roller, especially the edges. Air bubbles under the foil reduce the effect.
  4. Treat rust first. Never cover rust or bare metal: weld, prime and seal it before deadening goes on. See the rust map.

When it fits in the Projekt 36 build

The plan is to fit sound deadening after the metalwork and rust repair and before the new wiring harness, the air ride installation and the interior go back in:

  1. Teardown
  2. Metalwork and rust repair
  3. Sound deadening (bare shell, everything accessible)
  4. Wiring for the M50 swap
  5. Air ride: the tank and compressor go in the boot, so the boot layout is planned before any deadening there
  6. Interior reassembly

Before-and-after noise measurements will be documented in the build journal when the work is done.

See also: air ride kit comparison