Overview
The sound lock is a double-door acoustic airlock that provides the primary acoustic isolation path between VO Live (B304, NC-12 target) and the Control Room (B311, NC-18 target). It uses the buffer spine corridor (Wall W5, STC 55) as an intermediate acoustic buffer zone. When both doors are closed simultaneously, the system achieves approximately 90 dB total isolation — critical for maintaining VO Live’s NC-12 noise floor during recording sessions.
The airlock concept is simple: two high-STC doors separated by an acoustically treated buffer zone. Sound must pass through Door D2, cross the buffer volume, and then pass through Door D3. Each boundary subtracts energy, and the combined path loss far exceeds what any single door could achieve.
Airlock Diagram
CONTROL ROOM (B311) BUFFER SPINE VO LIVE (B304)
NC-18 target Corridor NC-12 target
┌─────────────────┐ ┌──────────────────┐ ┌─────────────────────┐
│ │ │ │ │ │
│ Equipment │ │ 15 cm wide │ │ Talent mic │
│ noise sources │◄──►│ acoustic buffer │◄──►│ position │
│ (monitors, │ D2 │ │ D3 │ │
│ computer, │ │ W5 walls both │ │ NC-12 requires │
│ Apollo x4) │ │ sides (STC 55) │ │ maximum isolation │
│ │ │ │ │ │
└─────────────────┘ └──────────────────┘ └─────────────────────┘
◄── D2: STC 45 ──► ◄── W5: STC 55 ──► ◄── D3: STC 45 ──►
TOTAL COMPOSITE ISOLATION PATH: ~90 dB
(D2 STC 45) + (buffer air gap) + (D3 STC 45) = effective ~90 dB
Components
Airlock Elements
Door & Wall Schedule
| Element | Type | STC Rating | Dimensions | Notes |
|---|---|---|---|---|
| D2 | Acoustic door | 45 | 90 × 220 cm | Buffer west face → swings west into Control [AC] |
| D3 | Acoustic door | 45 | 80 × 220 cm | VO/Control partition → swings north into VO Live [AC] |
| W5 | Buffer spine wall | 55 | 15 cm width | Both sides of buffer corridor |
Isolation Chain
Composite STC Path
How the Airlock Achieves ~90 dB Isolation
Sound traveling from the Control Room to VO Live must pass through three acoustic barriers in series:
| Barrier | STC | Role |
|---|---|---|
| D2 — acoustic door (west face of buffer) | 45 | First barrier: blocks Control Room equipment noise |
| Buffer air gap (15 cm corridor) | — | Decoupling volume: breaks direct vibration path |
| D3 — acoustic door (VO/Control partition) | 45 | Second barrier: final attenuation before VO Live |
The composite isolation is not a simple arithmetic sum (45 + 45 = 90) — real-world performance depends on flanking paths, seal quality, and the buffer volume. However, with both doors properly sealed and no flanking leaks, the combined path loss approaches 90 dB across the mid-frequency range.
Key principle: The buffer air gap decouples the two doors mechanically. They share no common frame, stud, or structural member. This prevents vibration short-circuiting the isolation chain.
Clearances & Geometry
Buffer Spine Dimensions
Critical Measurements
| Parameter | Value | Source |
|---|---|---|
| Buffer spine width | 15 cm | x = 213.6 to x = 228.6 (DXF) |
| Buffer spine X start | 213.6 cm | DXF verified |
| Buffer spine X end | 228.6 cm | DXF verified |
| D2 location | West face of buffer spine | x ≈ 213.6 cm |
| D3 location | VO/Control partition (Wall W6) | North of buffer |
| D2 clear width | 90 cm | Door schedule |
| D3 clear width | 80 cm | Door schedule |
| Door height (both) | 220 cm | Door schedule |
| Ceiling height | 275 cm AFF | SSOT |
Tight clearance: The 15 cm buffer width means D2 and D3 are very close together. Verify door swings do not interfere — D2 swings west (away from buffer), D3 swings north (away from buffer). Neither door swings INTO the buffer corridor.
Operational Sequencing
Recording Protocol
The Cardinal Rule: Never Open Both Doors Simultaneously [AC]
The airlock only provides its full isolation when at least one door remains closed. Opening both doors simultaneously creates a direct acoustic path from Control Room to VO Live, completely defeating the isolation and exposing the talent microphone to equipment noise.
Entry Procedure (Control → VO Live)
- Confirm recording is paused or talent is aware of entry
- Open D2 (step into buffer corridor) — D3 remains closed
- Close D2 fully behind you — verify seal engagement
- Open D3 (step into VO Live) — D2 is now closed
- Close D3 behind you — airlock restored
Exit Procedure (VO Live → Control)
- Open D3 (step into buffer corridor) — D2 remains closed
- Close D3 fully behind you — verify seal engagement
- Open D2 (step into Control Room) — D3 is now closed
- Close D2 behind you — airlock restored
Do NOT: Hold both doors open simultaneously during recording sessions. Do NOT prop either door open with wedges or stops. Do NOT remove door closers — they ensure automatic seal engagement.
Recommendation: Install a visual indicator (e.g., “RECORDING” light) outside D2 in the Control Room to remind operators to follow the airlock protocol. [HP]
Seal Requirements
Door Sealing System
Both D2 and D3 Require
| Seal Type | Location | Purpose |
|---|---|---|
| Perimeter compression seals | All four edges of door frame | Block airborne sound at jamb, head, and threshold edges |
| Automatic drop seal (mortise) | Bottom of door leaf | Seals the gap between door bottom and threshold when door closes [AC] |
| Acoustic threshold | Floor at door position | Provides a flat, smooth surface for drop seal to press against |
| Magnetic or cam-lift gaskets | Frame perimeter (optional upgrade) | Increases seal compression force for higher STC performance |
Critical: A 1 mm gap around an acoustic door can reduce its effective STC by 10–15 dB. Seal integrity is the single most important factor in door performance.
Seal Maintenance
- Inspect compression seals monthly for wear, cracking, or compression set
- Test drop seals by sliding paper under closed door — paper should not pass
- Replace seals immediately if any degradation is found
- Lubricate drop seal mechanism annually
Installation Notes
Construction Sequence
Airlock Installation Order
- Complete buffer spine wall framing (W5 both sides) and insulation [AC]
- Install door frames for D2 and D3 — frames must be mechanically isolated from each other (no shared studs)
- Seal frame-to-wall junction with acoustic sealant (continuous bead, no gaps)
- Hang door leaves — verify swing clearances
- Install perimeter compression seals on all frame edges
- Install automatic drop seals on both door leaves
- Install acoustic thresholds — must be level and smooth
- Adjust door closers for reliable, complete closure
- Test: close each door and inspect all edges for light leaks (light = sound leak)
Hold Point: After installation, perform a light-leak test on both doors. Shine a torch from one side while the room is dark on the other side. Any visible light indicates a seal failure that must be corrected before proceeding. [HP]
Do NOT: Share frame members between D2 and D3 — this creates a structural flanking path. Do NOT use standard (non-acoustic) door hardware. Do NOT omit the drop seal — the floor gap is the most common failure point.
QC Checklist
Verification
Sound Lock Acceptance Criteria
- D2 frame isolated from D3 frame — no shared structural members [AC]
- Both doors close fully and latch without assistance
- Perimeter seals compress visibly when doors close
- Drop seals deploy and retract smoothly
- Light-leak test: no visible light around any closed door edge [HP]
- Paper test: paper cannot slide under either closed door
- Door swings verified: D2 west (into Control), D3 north (into VO Live)
- No door swing interference between D2 and D3
- Buffer spine walls (W5) continuous and sealed top to slab
- Frame-to-wall acoustic sealant continuous on both doors
Acceptance: Sound lock is accepted when all items pass and the light-leak test shows zero leaks on both doors.