Transmission Loss Report Template and Analysis

By James Hartley · April 29, 2026

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

In February of this year, Sonus Gear Flow was asked to document and validate a transmission loss (TL) upgrade for a three-room post-production suite in Austin, Texas. The facility occupied a renovated brick-and-joist building with a busy coffee shop below and a rooftop HVAC plant above. The client’s core issue wasn’t a lack of acoustic treatment inside the rooms; it was sound getting in and out—particularly low-frequency rumble from rooftop units and speech leakage between edit bays.

The project team consisted of a project manager (client-side), a general contractor (GC) familiar with tenant improvements, an MEP subcontractor responsible for ducting modifications, and our audio engineering lead acting as acoustics consultant. The “why” was straightforward: the facility had landed a long-term broadcast deliverables contract requiring consistent monitoring conditions and repeatable QC. The contract also required that “adjacent work areas must not audibly distract editorial operations,” which effectively meant measurable isolation performance, not just subjective “it feels quieter.”

The deliverable was twofold: (1) a practical transmission loss report template the client could reuse for future build-outs and (2) a full analysis of the upgraded partitions, doors, and critical flanking paths, including field measurements, recommendations, and a sign-off package for the client’s compliance documentation.

2. Challenges and requirements at the outset

The building presented typical—but sharp—constraints for isolation work:

Structure-borne noise: Rooftop HVAC compressors transmitted vibration into steel members spanning the suite ceiling.
Existing construction: Interior partitions were single-stud 2x4 with one layer of 5/8” gypsum each side. Doors were hollow-core. Ceiling was a shared plenum with lightweight grid and 2x4 mineral fiber tiles. This configuration created straightforward airborne leakage and massive plenum flanking.
Schedule: The client had a 6-week window to complete construction and commissioning before the next series delivery cycle.
Operational constraints: Work had to proceed while another tenant (downstairs coffee shop) operated 6 a.m.–6 p.m., limiting our ability to perform daytime baseline measurements with stable background noise.
Targets: The client wanted “as close as possible to STC 55” between rooms, but we reframed that into measurable goals: achieve at least DnT,w 50 between edit bays and DnT,w 55 between the mix room and corridor, plus NC 20–25 in the mix room with HVAC running.

A critical early requirement was reporting consistency. The client had previously received an “acoustic memo” full of general statements and wanted a repeatable template: what was measured, how, what the numbers mean, what failed, what was done, and what to do next time.

3. Approach and methodology chosen

We built the reporting method around field metrics that reflect real performance in the building rather than lab ratings:

Airborne isolation: Field measurements using ISO 16283-1 methodology (sound pressure level difference with reverberation time correction), reported as DnT and single-number DnT,w. This avoids the common trap of quoting lab STC values that ignore flanking and installation quality.
Background noise: Room criterion measurements with the HVAC in occupied mode, reported as NC curves and 1/3-octave spectra.
Flanking and weak-point identification: Near-field scanning around doors, glazing, electrical penetrations, ceiling plenum paths, and duct breaks, using narrowband checks where needed.

Instrumentation was chosen for speed, repeatability, and defensibility:

Class 1 measurement microphone and analyzer: NTi XL2 with Class 1 mic capsule (calibrated on-site with a 94 dB calibrator).
Omnidirectional source: dodecahedron loudspeaker with power amp, capable of stable output down to 80 Hz for practical field work.
Supplemental: handheld thermal anemometer for identifying duct leakage locations; laser distance meter for room volume and RT calculations.

The report template was structured around a “test chain” narrative: baseline condition → modification scope → test setup → results → deficiency list → corrective actions → re-test. The client’s PM requested a one-page executive summary plus appendices with raw curves.

4. Step-by-step execution narrative

Week 1: Walkthrough, baseline documentation, and risk mapping. We conducted an initial walkthrough with the GC and MEP lead. We photographed every partition-to-structure junction, cataloged door types, identified shared duct runs, and noted ceiling grid continuity across rooms. The most important discovery was that the edit bays shared a continuous ceiling plenum with the corridor and reception, effectively bypassing any wall STC value.

Baseline isolation measurements were scheduled at 10 p.m. to reduce coffee shop and street noise. We tested between Edit Bay A and Edit Bay B, and between Mix Room and corridor. Baseline results were consistent with the construction: DnT,w 34 between edit bays and DnT,w 38 mix-to-corridor. Spectra showed a major dip in the 125–250 Hz bands—typical of lightweight construction and door leakage.

Week 2: Define upgrade scope with the GC. The upgrade plan focused on three items: (1) improve wall isolation, (2) eliminate plenum flanking, and (3) fix doors and penetrations. The chosen assemblies were practical given the six-week schedule and existing footprint:

Edit bay demising wall: convert to double-stud (two independent 2x4 frames with a 1” air gap), R-13 mineral wool in each stud cavity, and two layers of 5/8” Type X each side with Green Glue damping compound between layers.
Mix room perimeter: where rebuilding wasn’t feasible, add resilient isolation clips and hat channel on the mix room side, then two layers of 5/8” Type X with damping compound.
Ceiling: build hard-lid gypsum ceilings above the edit bays and mix room, sealed and isolated where possible, rather than relying on tile. This was the single largest driver of performance improvement.
Doors: replace hollow-core with STC 45-rated solid-core acoustic doors (1-3/4”), continuous perimeter seals, and automatic door bottoms; add vestibule-style door pairs at the mix room entry where space allowed.

Week 3–4: Construction oversight and “acoustics punch list.” During framing and first-board drywall, we performed site visits specifically to catch isolation-killers: short-circuiting between stud frames, back-to-back electrical boxes, unsealed top plates, and unlined duct penetrations. We required acoustic sealant at all perimeters and confirmed that the double-stud wall frames were not tied together with blocking.

In the ceiling, we coordinated with MEP to route ducts so that supply and return penetrations used lined flex connectors and did not hard-couple the ductwork to the new hard lids. We specified 1” duct liner for the first 10 feet of critical runs and required that the diffuser boots be sealed to the gypsum with mastic and gasketed connections.

Week 5: First post-construction test and corrective actions. We ran the first post-construction isolation test before final paint to allow fixes. Results were improved but not yet on target:

Edit Bay A ↔ Edit Bay B: DnT,w 48
Mix Room ↔ Corridor: DnT,w 52

The mid/high bands looked strong, but 160 Hz and 200 Hz were still underperforming by 6–8 dB. A near-field scan localized two primary issues: (1) an unsealed conduit sleeve above the ceiling line and (2) a door bottom not fully engaging the threshold due to a slight floor dip.

The GC sealed the conduit sleeve with backer rod and acoustic sealant, and the door installer shimmed and adjusted the automatic door bottom. We also added a second layer of 5/8” gypsum on a short corridor return wall that was acting as a weak “leaf” adjacent to the mix room entry.

Week 6: Final verification and report delivery. We repeated the tests under the same nighttime conditions, with HVAC running in occupied mode for background noise measurements.

5. Technical decisions and trade-offs made

Hard-lid ceilings vs. “better tiles.” The client initially asked about upgrading ceiling tiles to high-CAC panels. We advised against relying on CAC in a shared plenum because it does not prevent flanking around the tile edges and through ductwork. Hard lids cost more and require coordination with sprinklers and lighting, but they convert a leaky plenum into a controlled boundary. This was the most consequential decision.

Double-stud vs. clips on both sides. For the edit bay demising wall, double-stud framing provided superior decoupling and more predictable low-frequency performance, but consumed an extra ~3 inches of floor width. For the mix room perimeter, the footprint couldn’t shrink, so we used isolation clips and channel—less ideal than full decoupling, but still effective when installed carefully.

Door strategy. One very high-performance door can still fail if the frame is poorly sealed or if there is no approach to controlling corridor noise. We chose STC 45 doors with robust seals and added a short vestibule at the mix room entry. The trade-off was minor loss of usable corridor space, accepted by the client because it stabilized isolation during busy hours.

Low-frequency expectations. The client’s initial “STC 55” target implied strong broadband isolation, but STC-weighted ratings can hide low-frequency weakness. We explicitly evaluated 1/3-octave results down to 80 Hz and documented that real-world performance below 125 Hz would be limited by building structure and rooftop equipment without major structural work.

6. Results and outcomes with specific details

The final verification produced the following:

Edit Bay A ↔ Edit Bay B: DnT,w 51 (up from 34). Improvements were strongest from 250 Hz to 2 kHz (often +18 to +25 dB). The 125 Hz band improved by ~10 dB after sealing and door adjustment.
Mix Room ↔ Corridor: DnT,w 56 (up from 38). The vestibule configuration reduced corridor speech intelligibility significantly; informal listening confirmed that normal conversation became indistinct in the mix position.
Background noise (HVAC on): Mix room measured NC 23 at the listening position, with a mild bump at 63 Hz attributed to rooftop equipment. Edit bays averaged NC 27, acceptable for editorial.

We also tracked practical outcomes the client cared about:

Re-recording mixer reported reduced need to monitor louder to “mask the building,” with typical monitoring level dropping from ~79 dB SPL (C-weighted slow) to ~74 dB SPL during dialogue editorial checks.
Internal QC notes citing “audible office activity” dropped to near zero in the first month after occupancy.
Client’s PM used the report package to standardize requirements for a second location, specifically reusing the door and ceiling details.

Timeline: baseline tests and scope finalization in the first two weeks, construction weeks 3–5, final test week 6, and report delivered three business days after final measurements. Total acoustic consulting effort: 28 hours across site visits, test sessions, and reporting.

7. Lessons learned and what could be done differently

Measure earlier, but also measure smarter. Our baseline tests were valuable, but we could have added a short vibration survey on the steel members during rooftop unit cycles. The persistent 63 Hz bump was not a surprise, but quantifying it early would have allowed a clearer cost option: spring isolators for specific rooftop curbs versus accepting the residual low-frequency noise.

Doors are a system, not a SKU. The only significant late-stage deficiency was door bottom engagement. Next time, we would specify a field verification checklist for door installers: gap measurements at multiple points, seal compression checks, and a flashlight test before hardware sign-off.

Flanking paths deserve their own line item. The conduit sleeve that compromised the first post-construction test was small and easy to miss. Future scopes should include explicit allowance for “acoustic sealing of all non-documented penetrations discovered after demo,” so it doesn’t become a change-order negotiation.

Don’t overpromise below 125 Hz. Even with good partition design, the building structure and rooftop equipment set a floor for achievable performance. Communicating that early helped, but we reinforced it in the final report with clear spectral plots and explanations in plain project-manager language.

8. Takeaways applicable to other projects

Use a field metric and report the spectrum. A reusable TL report template should prioritize DnT (or at minimum field STC with context) and include 1/3-octave curves. Single-number ratings alone are not enough for studios and post suites where low-frequency noise and speech privacy both matter.

Start your template with a “construction reality” section. Before results, document the as-built assemblies: stud type, layers, damping, insulation, ceiling type, door seals, and any shared plenums. Include photos. This prevents future teams from guessing why a room performed the way it did.

Make flanking a first-class citizen. In the template, include a dedicated checklist for flanking inspection: ceiling plenums, duct breaks, back-to-back boxes, floor slab continuity, window mullions, and conduit sleeves. Add a “weakest link” table that ranks likely leakage points and assigns corrective actions.

Test twice when schedule allows. A midstream test (before paint and final finishes) is often the difference between “good enough” and “repeatably compliant.” The cost of a second test session is usually lower than the cost of guessing and reopening walls later.

Be explicit about trade-offs. Project managers need decisions they can defend: why hard lids instead of CAC tiles, why a vestibule, why double-stud here but clips there. A strong TL report doesn’t just present numbers; it documents decisions and constraints so the next build can be faster and more predictable.

For this suite, the combination of hard-lid ceilings, disciplined sealing, and door system upgrades moved performance from “typical office build-out” to “purpose-built post-production.” The reusable outcome was the report template itself: a structured way to connect construction details to measured isolation, and measured isolation to operational impact—exactly what audio teams and project managers need when timelines are short and performance expectations are non-negotiable.