How to Design Classrooms for Optimal Acoustics

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

In spring 2025, SonusGearFlow was brought in as the acoustics and AV integration partner for a classroom refresh at East Ridge Community College in Madison, Wisconsin. The scope covered eight general-purpose classrooms in the Humanities building: six “standard” rooms (approximately 8.5 m x 10.5 m with 3.0 m ceilings, ~270 m³) and two larger active-learning rooms (approximately 12 m x 14 m with 3.4 m ceilings, ~570 m³).

The client team included Facilities (owner’s rep), the campus AV manager, and an architectural firm already under contract for new finishes and lighting. Our role was to design and document an acoustic treatment package that would improve speech intelligibility for instructors and hybrid learning, while staying inside a fixed construction window (10 weeks, summer break) and a capped budget of $180,000 for all eight rooms (materials, labor, commissioning).

The “why” was measurable, not aspirational. Student accessibility requests had increased, instructors reported vocal fatigue, and the hybrid capture system installed during 2021 was underperforming: remote participants complained of “echoey” audio and “muffled questions,” even when microphone gain was increased. The campus AV manager also wanted a repeatable recipe that could be used later in other buildings.

2) Challenges and requirements at the outset

The initial walk-through showed the classic classroom acoustics failure mode: hard parallel boundaries, low absorption, and multiple noise sources. Each standard classroom had painted CMU sidewalls, a gypsum board front wall, a glass sidelighted corridor wall, and an ACT ceiling with older, low-NRC tiles. Floors were sealed concrete or VCT. Furniture had changed from rows to pods, which increased student-to-student cross talk and raised the noise floor.

The baseline measurements confirmed the subjective complaints. In two representative rooms we measured:

Reverberation time (T60): 1.05–1.20 s at 500 Hz and 0.95–1.10 s at 1 kHz (occupied estimate still above 0.8 s).
Background noise: NC-40 to NC-45 with HVAC in occupied mode; dominant bands at 250 Hz and 500 Hz from diffuser velocity and fan noise.
Speech intelligibility: STI in the 0.48–0.55 range at the rear seats using a talker position at the instructor station.

Requirements were established with the owner and AV team:

Target reverberation: 0.50–0.65 s (500 Hz–2 kHz) in standard rooms; 0.55–0.70 s in active-learning rooms.
Target background noise: NC-30 to NC-35 in teaching mode (or as close as feasible within HVAC constraints).
Intelligibility goal: STI ≥ 0.65 at the rear seating area; uniformity variation less than 0.05 across positions.
Hybrid capture: Improve far-end clarity without increasing microphone gain or aggressive noise suppression.
Constraints: Keep wall-mounted treatments durable and cleanable; do not reduce projection surface; maintain code-required wall protection.

The biggest constraint was schedule. The building was occupied until May 20, and classes resumed August 1. That left roughly 10 weeks for design finalization, procurement, installation, and commissioning—with predictable procurement risk for acoustic materials.

3) Approach and methodology chosen

We treated this as an acoustic + electroacoustic system problem rather than “add panels until it sounds better.” The methodology combined field measurements, acoustic modeling, and coordination with mechanical and architectural trades.

Workflow in brief:

Capture baseline metrics (T60 by band, STI mapping, background noise spectra) and document noise sources.
Model each room type in EASE to estimate required absorption area for target T60 and to validate loudspeaker coverage.
Prioritize interventions that reduce both reverberation and noise: ceiling NRC upgrades, targeted wall absorption, and HVAC diffuser changes.
Integrate the acoustic plan with the existing AV: ceiling speakers, DSP AEC, and microphone placement.
Commission: verify T60, STI, and noise criteria; tune DSP with real-room acoustic data after treatment installation.

The team used a calibrated measurement chain: Earthworks M30 measurement mic, Focusrite Scarlett interface, and Room EQ Wizard for T60 and RT by octave band; NTi XL2 for spot-check SPL and NC curves; and a controlled speech source for STI mapping. The AV platform used Biamp TesiraFORTÉ for AEC and routing in each room; we planned to keep that and improve the room conditions so the DSP would work less aggressively.

4) Step-by-step execution narrative

Week 1–2: Baseline testing and stakeholder alignment

We measured two standard rooms and one active-learning room in detail, then used the results to set design targets for all eight. We also recorded 15-minute HVAC noise samples during occupied-mode airflow. The mechanical contractor confirmed the VAV boxes were operating at higher-than-design minimums due to a previous balancing workaround.

A key early decision was to define “teaching mode” HVAC settings. Facilities agreed to a new schedule profile: reduce minimum airflow during instruction while maintaining ventilation rates via extended pre-occupancy purge. That shifted the problem from “fix everything in sheet metal” to “optimize controls and address obvious noise generators.”

Week 3–4: Acoustic design and procurement

Modeling showed we needed roughly 45–55 m² of additional equivalent absorption area (A) per standard room to reach 0.60 s T60, assuming an average occupancy of 24 students. The existing ACT ceiling tile tested closer to NRC 0.55.

We specified a two-part treatment strategy:

Ceiling upgrade: Replace tiles with NRC 0.90 mineral fiber tiles (2' x 2') while keeping the grid. This provided a broad, durable absorption increase without consuming wall space.
Wall panels: Add 50 mm fiberglass core panels (nominal NRC 1.00) at first reflection zones and rear wall, with abuse-resistant fabric and edge protection.

For the active-learning rooms, the model indicated that ceiling treatment alone would not control lateral flutter between large glass areas and CMU. We added upper-wall absorption and limited diffusion to preserve “liveliness” without sacrificing intelligibility.

Procurement was timed aggressively. We issued submittals in week 3, approved in week 4, and pre-ordered ceiling tiles and panel hardware. Lead times were 4–6 weeks for the panels and 2–3 weeks for ceiling tiles. To reduce risk, we used a single panel line across all rooms and standardized sizes (600 mm x 1200 mm and 600 mm x 600 mm).

Week 5–7: Installation coordination with trades

Installation sequencing mattered. Ceiling work had to precede lighting trim and projector alignment. We coordinated a mock-up in one standard room: ceiling tile replacement, then panel placement, then AV tuning. The mock-up validated that the projector brightness and screen reflections were unaffected and that wall panels didn’t conflict with whiteboards.

The HVAC contractor swapped high-velocity diffusers for perforated-face diffusers in four rooms where measurements showed turbulent noise. Two VAV boxes received internal liner replacement and damper adjustment. Not every mechanical fix was possible in the schedule, so we focused on interventions with predictable results.

Week 8–10: Commissioning, DSP tuning, and documentation

After the acoustic materials were installed, we re-measured T60 and ran STI maps at three listening positions per room (front third, middle, rear). We then revisited the Biamp DSP configuration. In the pre-upgrade state, AEC was fighting late reflections and noise, so the integrator had used higher noise reduction and more aggressive gating. Post-upgrade, we reduced noise reduction depth by 3–6 dB, relaxed gate thresholds, and lowered overall gain structure by ~2 dB to improve naturalness while maintaining far-end clarity.

Documentation included as-built panel layout drawings, product data sheets, measurement reports (before/after), and a repeatable checklist for future classroom refreshes.

5) Technical decisions and trade-offs made

Several decisions were trade-offs between ideal acoustics and real-world constraints:

Ceiling NRC vs. durability and cost: NRC 0.90 tiles cost more than basic 0.70 products, but the ceiling offered the largest continuous surface area and the best “absorption per dollar” once labor was considered. We avoided specialty microperforated systems due to lead times.
Wall panel placement vs. room function: Instructors needed uninterrupted writing space and pin-up areas. We placed panels above chair rail height (generally 1.2–2.4 m AFF) and concentrated them on rear and side walls where they would not be damaged by backpacks and carts.
Absorption vs. over-deadening: The active-learning rooms initially modeled to a very low RT if we treated every available wall. Instead, we limited absorption to achieve ~0.65 s and added modest scattering using a pair of shallow 2D diffusive elements on the rear wall. The goal was to keep group discussion from feeling acoustically “flat” while still improving clarity.
Mechanical fixes vs. controls optimization: Full duct rerouting was not feasible. We focused on diffuser selection, VAV minimum airflow, and balancing. This improved NC by 5–8 points in most rooms without major construction.
Microphones vs. room acoustics: The temptation was to “solve it with better mics.” We kept the existing ceiling mic arrays in the two active-learning rooms and the instructor lavs in standard rooms, but prioritized acoustic treatment so the microphones operated with less gain and fewer artifacts.

6) Results and outcomes with specific details

The post-install measurements showed consistent improvement across all eight classrooms.

T60 (standard rooms): Reduced from 1.05–1.20 s to 0.55–0.68 s at 500 Hz; from 0.95–1.10 s to 0.52–0.62 s at 1 kHz.
T60 (active-learning rooms): Reduced from 1.25–1.40 s to 0.62–0.74 s at 500 Hz; to 0.58–0.70 s at 1 kHz.
Background noise: Improved from NC-40/45 to NC-32/37 depending on room, with the largest gains in rooms where diffuser and VAV adjustments were made. The low-mid peaks around 250–500 Hz were reduced by approximately 4–6 dB.
STI: Improved from 0.48–0.55 to 0.66–0.73 in the rear seating positions of standard rooms. Active-learning rooms achieved 0.64–0.70 depending on seating configuration.

On the hybrid learning side, remote participants reported fewer “roomy” artifacts and better pickup of student questions. Objectively, we saw a reduction in AEC tail length requirements and fewer instances of double-talk suppression triggering during group work. The AV manager also noted that instructors stopped increasing mic gain mid-class—an informal but meaningful indicator that the room was no longer fighting speech.

Budget and schedule performance was also trackable. Total spend landed at $172,400: $86,000 in ceiling tile materials and labor, $58,500 in wall panels and labor, $15,400 in HVAC adjustments and diffuser replacements, and $12,500 in measurement, commissioning, and documentation. The work completed in 9.5 weeks, leaving a few days of buffer before faculty returned.

7) Lessons learned and what could be done differently

Three lessons stood out.

First, the mock-up room prevented costly rework. It revealed that a proposed panel location conflicted with a door swing and that one lighting row created glare on a new whiteboard. Fixing those on paper saved a day of labor in every room.

Second, HVAC noise should be treated as a core acoustic variable, not a separate discipline. Even after improving reverberation, NC-38 in one classroom still masked consonants for soft-spoken instructors. If we had more time, we would have specified quieter terminal units in that branch and added duct attenuation rather than relying on control changes alone.

Third, panel durability details matter in schools. In one room, the first installed panel fabric was too light in color and showed scuffs from chair contact during summer workshops. We switched to a darker, heathered fabric with a higher abrasion rating for the remaining rooms. If we were starting over, we would select the final fabric from an on-site abuse test (chairs, bags, cleaning chemicals) rather than a spec sheet.

8) Takeaways applicable to other projects

Start with measurable targets: A classroom acoustic plan should begin with T60, STI, and NC goals tied to room volume and teaching style—not a generic “add absorption.”
Use the ceiling as your primary absorber: If the room has an accessible ACT ceiling, high-NRC tile replacement often provides the fastest and most uniform improvement per dollar.
Treat reflection paths, not just surfaces: First reflections on side walls and the rear wall typically contribute more to perceived “echo” and STI degradation than random placement.
Coordinate mechanical and acoustic work: Even small changes—diffuser type, VAV minimums, balancing—can yield meaningful NC improvements and make every other fix more effective.
Let room acoustics reduce DSP complexity: Better room conditions allow less aggressive AEC/noise reduction settings, resulting in more natural voice and fewer artifacts in hybrid capture.
Standardize for repeatability: A consistent panel family, standard sizes, and a documented placement logic make multi-room deployments predictable in cost and performance.
Commission like an audio project, not a construction closeout: Post-install measurements and DSP tuning should be scheduled and budgeted; without them, improvements can be inconsistent and hard to defend.

Designing classrooms for optimal acoustics is not a single product choice—it’s a sequence of decisions across architecture, mechanical noise control, and electroacoustic behavior. The East Ridge project was successful because the team treated intelligibility as the deliverable, measured it, and verified it under realistic operating conditions. For audio engineers and project managers, the repeatable pattern is straightforward: define targets, model and measure, spend money where it affects speech the most, and commission the room as a complete system.