iHow to read this

Ten steps in build order. Each: objective · spec · method · one caution. All dimensions & STC/RT60 values come from the study engine (sheets A-301/A-302, report §4–6) — not site guesses. Step 7 (HVAC) is a placeholder until the fan recording + unit specs arrive. Do the steps in order: isolation is built from the slab out.

0Site preparation & demolition

Return the raw basement shell to a clean, flat, sealed base — nothing acoustic performs over an uneven or unsealed substrate.

Method

• Level the floor (self-levelling screed / grind high spots) — the floating floor needs a flat base.
• Fill concrete honeycombing & wall cavities; build the new West closing wall to the neighbour's meeting room.
• Demolish the fireplace recess and fill flush — it is a weak point in the party wall.
• Map services; keep pipes ≥20 mm clear of the future inner shell, routed outside the envelope.
• Photograph & log the shell condition (provenance).

⚠ Caution: No acoustic layer recovers a bad base. Do Step 4 (sealing) thinking starts here.

1Floating floor — the isolation base

Decouple the whole inner box from the slab so structure-borne noise (including the stairwell fan) cannot transmit.

Spec: Poured RC ~75–100 mm or ply platform on EPDM/rubber bearings · continuous 25 mm perimeter gap · zero rigid bridges.

Method

• Lay resilient bearings/mat on the levelled slab. Grade bearings by static load — more small pads under point loads, never bigger pads (S-2).
• Cast the RC slab (or lay the ply platform) floating on top — not touching any wall.
• Keep a continuous 25 mm perimeter gap; fill with a resilient strip, never rigid mortar.
• Never anchor the float to the slab — no screw, conduit, or skirting may bridge the gap.

⚠ Caution: A single rigid bridge shorts the entire floor. Inspect the whole perimeter before building on it.

2Decoupled inner-leaf walls (non-parallel)

Build the inner-box walls to each boundary's STC target, on the floating floor, with splayed (non-parallel) faces for LF & flutter control.

Method

• Frame double / staggered studs on the float floor (never the slab); fill cavities with mineral wool 45–60 kg/m³.
• 2× gypsum each side with damping compound between layers (random pattern); stagger all joints.
• Build the Control side walls splayed: 2.40 m at the glass → 2.90 m at the rear (3.5°/side) per sheet A-301.
• Seal every stud-to-floor line and corner airtight.

⚠ Caution: A wall rigidly tied to the outer shell, or an oversized observation glass, wastes the whole wall's STC.

3Decoupled ceiling — sloped

Hang an independent ceiling that isolates impact from the occupied floor above and breaks the floor↔ceiling mode.

Spec: 230 mm decoupled (resilient hangers/clips + 2× gypsum + wool) · field STC ~59–63.

Method

• Hang on resilient hangers/clips from the slab — zero rigid bridge (S-2).
• Build it sloped (sheet A-302): in the VO room it follows the concrete step — hug the low ~2.85 m bay at the window, ramp up under the ~3.97 m zone; no flat parallel bay.
• 2× gypsum + wool in the plenum; seal the perimeter airtight to the wall leaves.
• Treat the vertical step face (the 'knee') as a broadband soffit trap — not bare concrete.

⚠ Caution: The low street-side bay has the least headroom — confirm the box-in-box assembly fits with full decoupling before framing.

4Airtight sealing

Make the inner box airtight — isolation is only as good as its weakest seal.

Method

• Acoustic sealant at every perimeter, penetration and service pass-through.
• Backer rod + sealant on all gaps; no daylight, no air path.
• Pipes ≥20 mm clear of the inner shell, rubber-lined clamps, lagged, routed outside the envelope.

⚠ Caution: A ~1% open area can cost 10+ STC points. Smoke- or light-test before closing any wall or ceiling.

5Observation glazing + sealed clerestory

See between Live and Control without breaking isolation; seal the street windows.

Spec: View glass ≤ 1.5 m² → composite ≥ STC 59 · 2× heavy laminated lites, large unequal air gap, one lite tilted 5–7°, absorptive reveal.

Method

• Keep the opening small (≤1.5 m²) — the glass is the weakest element in the 225 mm wall.
• Two laminated lites of different thickness, unequal gap; tilt one 5–7° to kill flutter between them.
• Absorptive reveal in the cavity; fully gasketed frame.
• Seal the street clerestory windows (mass + airtight) — the weakest point on the north side.

⚠ Caution: Oversizing the glass throws away the whole wall's STC (1.0 m²→61, 1.5→59, 2.0→58).

6Acoustic doors — 2-door sound lock

Enter and exit without an isolation leak at the door.

Method

• A 2-door sound lock at the entrance off the staircase (replaces the wooden door).
• Each door: heavy gasketed acoustic leaf, drop seal at the threshold, sealed frame — spec the full set.
• Wall element order: door → window → bass traps → panels → QRD (last) (S-1); verify zero collisions.

⚠ Caution: Doors + seals are the most common field-STC failure; the leaf alone is not the product — the set is.

7HVAC & silencers — pending vendor PENDING VENDOR

Quiet ventilation to NC-15–20 without breaking the envelope. Design intent below; final sizing needs the fan recording + the chosen unit's specs.

Method

• Awaiting: stairwell-fan recording (on/off) + HVAC unit Lw, airflow, static pressure.
• Intent: lined baffle boxes (Z/U), duct silencers, vibration-isolate the unit, pausable in takes.
• Air velocity ≤500 fpm at registers (≤300 quietest).

⚠ Caution: Do NOT cut the sealed envelope for ducts until the silencer design is fixed — coordinate every penetration back into Step 4.

8Acoustic treatment install

Hit the RT60 targets (Control 0.10–0.21 s; Live 0.17↔0.49 s variable) without over-damping.

Spec: Order (S-1): bass traps at corners only → panels → diffusion, then curtains.

Method

• Fix bass traps in the vertical corners first (corners only — S-1).
• Mount panels at first-reflection points — positions finalize once monitor placement is set.
• Hang the deployable curtains on track in the Live room (deployed = dead 0.17 s, retracted = live 0.49 s).
• Lay carpet + ply diffusion in Control.

⚠ Caution: Set traps and curtains now; hold panel positions until the monitor / listening layout is confirmed.

9Commissioning & QC

Prove the build meets the study before sign-off.

Method

• Float floor: no rigid bridges (visual + tap test).
• Seals: airtight (smoke / light test) before closing.
• STC: field-test each boundary vs target (61–65 / 60–64 / …).
• RT60: measure per band vs target; tune curtains / panels.
• NC: measure at design airflow once HVAC is in (Step 7).

⚠ Caution: Log everything to the ledger (R-10). NC is not signed off until HVAC is installed and measured.