Concert Halls Acoustic Design Checklist

1) Project overview: what, where, who, and why

In early 2024, the City of Redbrook commissioned a 1,200-seat concert hall inside a renovated 1970s civic theater footprint in the downtown arts district. The project was managed by Alder & Pike Construction (GC) with architectural design by Studio Lattice. Our team at SonusGearFlow was brought in as the acoustic and audio systems documentarian during design development through commissioning, with a brief that was both simple and unforgiving: achieve a hall that works for unamplified symphonic repertoire, chamber ensembles, and spoken-word events without relying on “fix it in DSP” after the fact.

The venue, branded the Redbrook Philharmonic Hall, needed to host an 80-piece orchestra, touring jazz acts, small amplified ensembles, and municipal events. The owner’s stated business case was clear: reduce changeover time between event types, minimize external rentals of acoustic shells and portable treatment, and avoid post-opening complaints about intelligibility or uneven coverage that would force disruptive retrofits.

Stakeholders included the owner’s rep, the orchestra’s artistic director, the house audio lead, and a code consultant. From day one, we treated the project as a checklist-driven acoustic design effort: define performance targets, validate them with modeling and measurement plans, and lock construction details that would reliably hit the numbers.

2) Challenges and requirements at the outset

The inherited building shell set the tone. The existing structure had a low roofline over the audience chamber, a shallow stage house, and a heavy concrete perimeter that limited reconfiguration. Early site surveys revealed three specific issues:

Mechanical noise risk: Two air-handling units sat directly above the rear of the hall, with duct paths that historically transmitted low-frequency rumble into the room.
Geometry constraints: The room could not be widened without major structural changes, so lateral energy and diffusion had to be designed into a relatively narrow plan.
Variable-use requirement: The orchestra wanted a warm, supportive field for strings, while the city required high speech intelligibility for public meetings and lectures.

The owner approved performance targets as contract requirements, not “aspirational goals.” The initial checklist looked like this:

Reverberation time (RT60): 1.8–2.0 s at 500 Hz for symphonic mode, 1.3–1.5 s for speech mode (occupied).
Background noise: NC-15 in the audience chamber, NC-20 on stage during HVAC operation.
Speech intelligibility: STI ≥ 0.60 for seats beyond 25 m from the stage lip in speech mode (with reinforcement), avoiding excessive gain-before-feedback limitations.
Uniformity: ±3 dB broadband level consistency across the main floor for reinforced events, with no more than 6 dB seat-to-seat variance in the upper balcony.
Isolation: Minimum 65 dB airborne sound insulation to adjacent rehearsal rooms and lobby, with special attention to bass transmission.

The schedule was tight: design development to construction documents in 14 weeks, construction in 11 months, commissioning in 3 weeks, and opening night in week 56 from kickoff.

3) Approach and methodology chosen

We structured the work in four parallel tracks, each with clear deliverables and decision gates:

Room acoustics modeling: Early geometry options tested in CATT-Acoustic and cross-checked with Odeon for RT, clarity (C80), and early decay time targets.
Noise and vibration control: Mechanical noise modeling, duct breakout calculations, and vibration isolation coordination with MEP.
Reinforcement system integration: EASE modeling for loudspeaker coverage and interaction with room modes, making sure the system complemented the acoustic intent.
Commissioning plan: A measurement protocol drafted during design, including target metrics, mic positions, and acceptance tolerances to avoid “we’ll see later” ambiguity.

The core method was iterative: set numeric targets, simulate, choose constructible details, and lock them early enough that procurement wouldn’t dilute performance. Every major decision had a corresponding checklist item, an owner-approved metric, and a test plan.

4) Step-by-step execution narrative

Week 1–3: Baseline survey and constraints

We started with an as-built survey and ambient noise logging. Overnight measurements captured a persistent 31.5 Hz component from the rooftop units and intermittent structure-borne noise from a nearby service alley. We used a Class 1 sound level meter (NTi XL2 with M2230 mic) for long-term logging and a tri-ax accelerometer spot-check on the existing steel to identify likely vibration paths.

Simultaneously, we mapped audience seating geometry and sightlines. The narrow plan meant that lateral reflections—critical for spaciousness—would be harder to achieve without careful wall shaping and diffusion.

Week 4–7: Concept modeling and material strategy

Three room concepts were modeled: a classic shoebox variant, a modified fan, and a “compressed vineyard” with shallow side terraces. The shoebox performed best for early lateral energy and uniformity, but the existing footprint limited wall-to-wall width. The compromise solution retained shoebox proportions in the main audience chamber while introducing angled, faceted side-wall panels to increase scattering without stealing seats.

We targeted RT60 through a combination of volume, surface absorption, and adjustable elements. The final strategy included:

Fixed diffusion: Hardwood diffusive panels on side walls (QRD-inspired, but built as multi-depth wells for manufacturability) from 2.5 m to 9 m above finished floor.
Adjustable absorption: Motorized banners (heavy velour with backing) concealed in ceiling pockets over rear stalls and under balcony.
Stage enclosure: A modular acoustic shell with 50 mm MDF core and hardwood face, tuned for strength and reflectivity, plus overhead clouds.

Material submittals were treated as performance-critical. We required octave-band absorption and scattering data for all major finishes. Where manufacturer data was incomplete, we used conservative assumptions and added contingency absorption in banner sizing.

Week 8–14: Construction documents and coordination

The highest-risk coordination topic was HVAC. To achieve NC-15, the mechanical team agreed to lower duct velocities and add attenuation where it mattered, not everywhere. Key decisions:

Main supply duct velocity capped at 4.0 m/s near the AHU, stepping down to 2.5 m/s at branch runs feeding the hall.
Large-radius elbows and lined plenums at transitions to reduce turbulence noise.
Spring isolators on AHU bases (selected for 95% isolation at operating frequency) and flexible connectors at duct interfaces.
Return air paths routed to avoid direct line-of-sight from audience chamber to mechanical rooms, using two-bend paths with lined sections.

We also specified wall and ceiling assemblies for isolation: double-stud partitions with 2 layers of 16 mm gypsum each side, 100 mm mineral wool, and sealed penetrations. For the most sensitive boundary (hall to rehearsal room), we used a room-within-room approach for the rehearsal side and resilient hangers for the shared ceiling zone.

Month 4–10: Construction oversight and field verification

During framing, we caught an early deviation: the contractor substituted a lighter gauge hat channel for the ceiling isolation grid in a section under the balcony. The change would have reduced low-frequency isolation and increased rattling risk. We halted the area, documented the issue, and required the specified resilient hanger system with verified load ratings. The correction cost two days but avoided a likely post-opening defect.

We also ran a “rattle hunt” protocol once the shell was enclosed but before finishes: excite the room with a swept sine through a temporary subwoofer (d&b B22) and walk the structure to find buzzes. Two lighting conduit brackets and one HVAC damper linkage were re-secured and damped at this stage, when fixes were cheap.

Month 11–12: System installation, tuning, and commissioning

The reinforcement system was designed to be unobtrusive for classical use and capable for speech and light amplification. The installed package included:

Main L/R: L-Acoustics A10 Focus with A10 Wide fills, flown high and tight to the proscenium line to preserve sightlines.
Balcony underfill: L-Acoustics X8 distributed fills on time alignment zones.
Subwoofers: Two KS21 for amplified events, normally muted for orchestral programming.
Control and DSP: LA Network Manager and system processing, with console I/O via Dante to a Yamaha QL5 at FOH.
Measurement: Room EQ Wizard for quick checks, and Smaart v9 for transfer function, alignment, and verification.

Commissioning measurements followed a pre-agreed grid: 18 audience positions (stalls, side seats, balcony front, balcony rear) plus 6 on-stage positions. RT60, EDT, C80, and STI were captured with a dodecahedron source and calibrated mic chain. For speech mode, we measured STI with the system active and banners deployed.

5) Technical decisions and trade-offs made

Several decisions required explicit trade-offs, and documenting them helped the project team avoid conflicting expectations later.

RT vs. clarity: The orchestra wanted longer decay for romantic repertoire, but speech and contemporary programming demanded clarity. We set the baseline hall RT to land around 1.85 s at 500 Hz occupied, then used banners to pull it down to 1.4 s for speech mode. This avoided building a “dry” hall and trying to add reverberation electronically.
Diffusion vs. cost: True mathematically optimized diffusers across the full side walls were cost-prohibitive. We used a repeatable, multi-depth panel system with three well depths (40/80/120 mm) arranged in pseudo-random sequences, delivering audible scattering benefits without custom CNC for every panel.
Isolation vs. footprint: Increasing wall thickness reduced usable area. Where space was tight, we prioritized isolation at known noise boundaries (mechanical rooms, lobby) and accepted slightly thinner assemblies at low-risk partitions, but required airtight sealing everywhere.
Loudspeaker coverage vs. visual impact: Larger arrays would have improved throw, but the hall’s aesthetic favored minimal rigging. We chose compact mains and invested in distributed under-balcony fills, accepting more alignment work in exchange for a clean proscenium.

6) Results and outcomes with specific details

The final measurements were documented in an acceptance report delivered two weeks before opening. Key outcomes:

Background noise: With HVAC in normal operation, the audience chamber measured NC-16 average, with a slight rise to NC-18 at the rear balcony when the rooftop units cycled to high stage-load mode. After adjusting VFD ramp profiles and adding additional liner at a return transition, the rear balcony improved to NC-16–17.
RT60 (occupied simulation and post-opening verification): Symphonic mode (banners retracted) measured 1.82 s at 500 Hz, 2.05 s at 125 Hz, and 1.62 s at 2 kHz. Speech mode (banners deployed) measured 1.41 s at 500 Hz and 1.28 s at 2 kHz.
Clarity: C80 averaged -1.2 dB in symphonic mode at mid-hall seats, which the orchestra preferred, and improved to +2.8 dB in speech mode with banners deployed.
Speech intelligibility: STI in speech mode with reinforcement was 0.62–0.71 across the seating grid, with the lowest values at upper balcony rear before we adjusted underfill aiming and increased that zone by 2 dB.
Coverage consistency: After time alignment (mains to underfills ranging 14–32 ms delays by zone) and level shading, we achieved ±2.5 dB variation across the main floor and ±3.5 dB in the balcony.

Operationally, the hall staff reported that switching between modes became predictable: the presets included banner deployment positions, stage shell configuration notes, and console snapshots. Changeover from orchestral to spoken-word was reduced from an estimated 4 hours (with rented portable drape and ad hoc PA) to about 75 minutes, primarily driven by staging and mic placement rather than acoustic scrambling.

7) Lessons learned and what could be done differently

Three lessons stood out, each tied to a real friction point during the build:

Lock HVAC noise targets before duct layout hardens: NC-15 is not a “finish tweak.” The biggest gains came from duct velocity control and path geometry. If we had pushed for an earlier acoustic review of 30% MEP drawings, we likely would have avoided one late-stage liner retrofit.
Field verification beats assumptions: The rattle hunt caught issues that would not show up in a model. A two-hour sweep test saved days of troubleshooting later when the room was finished and occupied.
Document the intent behind every compromise: The narrow footprint forced design choices that looked arbitrary to non-acousticians. Writing down why certain surfaces were reflective, why banners were sized as they were, and what each element was expected to accomplish kept value engineering from removing “mysterious wood panels” that were actually doing the heavy lifting.

If we were to do it again, we would add a mid-construction “pre-finish acoustic check” with temporary seating absorption approximations. We relied on modeling for occupied conditions and validated after opening, but a pre-finish checkpoint could have identified one slightly over-live balcony zone earlier, when banner pocket access was easier.

8) Takeaways applicable to other projects

This project reinforced a practical acoustic design checklist that applies to any concert hall—new build or renovation—especially when multiple program types must coexist:

Convert “good sound” into numbers: RT60 bands, NC targets, STI goals, and coverage tolerances should be written into the project requirements with a test plan.
Prioritize noise control early: If you miss the background noise target, every other acoustic choice becomes a workaround. Control velocities, isolate rotating equipment, and eliminate line-of-sight air paths.
Design for variability without gimmicks: Adjustable absorption (banners, curtains) and a properly designed shell are more reliable than attempting to synthesize room characteristics electronically.
Coordinate acoustics with architecture: Diffusion and lateral energy need geometry. If the footprint is constrained, build scattering into surfaces and avoid large, uninterrupted parallel planes.
Integrate reinforcement to support the room, not fight it: Choose loudspeakers for pattern control and sightline constraints, then invest time in zoning, alignment, and level shading. Distributed fills can outperform “bigger mains” in challenging balconies.
Commission like it’s a deliverable, not an afterthought: Define measurement positions, metrics, and acceptance thresholds early. Use the same protocol for baseline and final verification to keep discussions objective.
Protect details during construction: Small substitutions—hanger types, sealant omissions, lighter bracing—can undo isolation and create rattles. A short list of “do not substitute” assemblies should be part of every site walk.

The Redbrook Philharmonic Hall opened on schedule in week 56 with a sold-out program: a chamber overture, a full symphonic work, and a mayoral address. The transition between acoustic and reinforced use was uneventful—exactly the point. The most useful compliment came from the house engineer: “I’m not fighting the room.” For project managers and audio teams, that’s the outcome the checklist is meant to deliver—repeatable performance backed by measured proof.