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The American Wood Council’s Technical Report No. 15 (TR15) offers an empirical way to estimate sound transmission ratings for certain wood-frame floor-ceiling assemblies. It is useful for comparing designs that fit the report’s scope, but it is not a universal formula for wood-framed construction or a guarantee of how quiet a finished building will be.
The model combines sound-transmission data for assembly layers with measured effects of the framing system and its components. It estimates laboratory-style STC and IIC ratings; field sound isolation also depends on sound paths through adjoining construction. The distinction matters: a calculated assembly rating and the noise a person hears in a completed room are related, but they are not the same result.
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What is the empirical model for wood-frame sound transmission?
TR15 is a calculation model for estimating sound transmission through selected wood-frame floor-ceiling assemblies. Published by the American Wood Council in January 2019, it uses empirical data—measurements from tested assemblies—to account for the way layers, framing, and other components work together. It is a bounded model, not a first-principles formula that can be applied to any wall or floor.
The AWC report says its initial compilation contained more than 300 STC and IIC data points from 17 sources. The model was then narrowed to specific floor-ceiling systems and components. That history is important: the size of the initial database does not mean every possible wood-frame design was represented or validated.
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For historical context, a 1985 USDA Forest Products Laboratory report summarized a DuPree compilation covering 194 interior partition designs and 55 floor-ceiling designs. That work illustrates the longer history of measuring and characterizing wood-frame sound transmission, but it is not a substitute for checking whether a particular design fits TR15’s more specific scope. Read the AWC Technical Report No. 15 and the USDA report for the source material.
How does TR15 estimate sound transmission?
For STC, TR15 estimates transmission loss (TL) across one-third-octave frequency bands, then fits the ASTM E413 reference contour to the estimated values from 125 Hz through 4,000 Hz. The resulting single-number classification is an STC estimate, not a direct prediction of every sound or frequency a listener might notice.
The report describes the central relationship as:
Estimated assembly TL = sum of layer TL values + baseline assembly system effect + applicable system-effect adjustments.
In other words, the model does not simply add the weight of the materials or treat each layer as though it performs independently. It starts with the transmission-loss contributions of the modeled layers and applies empirical terms for the assembly as a system and for relevant component variations. Those terms reflect the influence of framing and construction details in the measured data.
Mass is relevant, but it is not the whole explanation. TR15 gives the idealized mass-law relationship R = 20 log10(fm) − 47, where f is frequency and m is mass per unit area. The report cautions that measured assembly performance can differ significantly from this idealized behavior. A heavier layer alone therefore does not establish the STC of a complete framed floor-ceiling assembly.
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TR15 also estimates IIC, a rating for impact-generated, structure-borne sound. STC and IIC describe different sound pathways: STC relates to airborne sound, while IIC addresses impact sound. The report warns that IIC does not represent low-frequency footfall behavior below approximately 100 Hz. Neither single-number rating captures every sound a resident might hear.
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Use TR15 only when the assembly’s major components and configuration fit the systems documented in the report. In-scope floor-ceiling assemblies use gypsum-board ceilings attached to RC1 resilient channels and are framed with one of three framing types:
- Sawn lumber
- Prefabricated wood I-joists
- Metal-plate-connected wood trusses
The model confines resilient-channel and framing spacing to 16 or 24 inches on center. It includes selected fiberglass or mineral-wool batt insulation, some uninsulated assemblies, floor layers, and generic floor coverings. “Selected” matters: a component being broadly similar to one of those categories does not establish that its exact configuration is represented in the data.
The report’s documented scope is not a universal model for wood-framed walls. It also does not establish validated estimates for structural composite lumber or cross-laminated timber assemblies. In January 2019, the AWC described extending and validating models for those systems and for walls as future work. Unless a newer source establishes otherwise, treat them as outside TR15’s documented model scope.
Before relying on an estimate, list the assembly’s framing type and spacing, ceiling board layers, resilient-channel configuration and spacing, insulation, floor layers, and floor covering. Compare each item with the report’s allowed inputs and configurations. If a key component or arrangement falls outside that scope, do not present the resulting number as a validated TR15 prediction.
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How accurate is a calculated STC rating?
TR15 reports promising validation results for a limited set of in-scope bare-floor assemblies—not a universal accuracy guarantee. The AWC compared estimates with measured STC values for 35 bare-floor assemblies. Every estimate was between 3 STC points below and 2 points above the measured value, and approximately 83% were within plus or minus 1 point.
For the validated sawn-lumber and I-joist assemblies, the average difference was an underestimation of 0.8 points. The nine truss-framed assemblies showed an average overestimate of 0.4 points. The report notes a possible confounding factor: the validation trusses were 12 inches deep, while trusses in the model-development database were 14 to 18 inches deep.
These figures describe the report’s validation sample and its scope. They do not tell you that any new assembly will land within a particular margin, especially if it has components or construction details not represented in the model. The AWC concludes that the model appears reasonably accurate for estimating bare-floor STC within scope, while explicitly cautioning against assuming accuracy for assemblies outside that scope. See the report’s validation discussion for the underlying qualifications.
Also distinguish a model estimate from a standardized laboratory test. ASTM E90 is the laboratory method referenced for airborne transmission-loss measurements, and ASTM E413 is used to derive the STC classification. For impact sound, the report references ASTM E492 and E989. A calculated result can support design comparison; it is not itself a new measurement under those methods.
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Does a laboratory-style rating predict sound isolation in a finished building?
Not by itself. A laboratory-style rating describes the separating assembly under a defined measurement approach; sound heard between rooms in a building can also travel through adjoining elements and connections. Those indirect routes are called flanking paths, and they can affect apparent sound insulation in the completed building.
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The National Research Council Canada (NRC) explains the distinction in its Guide to Calculating Airborne Sound Transmission in Buildings. The guide notes that lightweight framed assemblies can have high internal loss factors, so laboratory transmission values can be used without adjustment to estimate direct transmission through the separating assembly in a finished building. But that statement is about direct transmission. The guide immediately qualifies it: standardized flanking calculations do not give reliable results for lightweight framed elements and require a different approach.
Therefore, a TR15 estimate is not the same as a room-to-room field result. It does not, on its own, quantify every path through adjoining floors, ceilings, walls, or other connections. If the project question is “What will occupants hear?” rather than “How does this separating assembly rate?”, a model number alone cannot answer it. The NRC’s discussion of apparent sound insulation in wood-framed buildings addresses this broader building-level issue.
The measurement setup also explains why a consumer sound meter is not a substitute for transmission testing. The NRC describes the E90 laboratory procedure as using full-scale assemblies between reverberation rooms, calibrated microphones, and multiple loudspeaker sources. Its report gives measurement bands from 50 Hz to 5,000 Hz, while STC is calculated over 125 Hz to 4,000 Hz. A handheld meter reading in a room does not recreate that controlled test or produce a valid assembly STC rating.
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First decide what you need to compare. If the question is which in-scope floor-ceiling assembly has the better estimated laboratory-style rating, use consistent inputs and compare the relevant STC or IIC estimates. If the goal is a target for a completed building, account separately for the limits of assembly ratings and the potential contribution of flanking transmission.
- Define the assembly and the question. Record the framing type, spacing, floor and ceiling layers, resilient-channel arrangement, insulation, and floor covering. Decide whether the concern is airborne sound (STC), impact sound (IIC), or field room-to-room performance.
- Check eligibility before calculating. Compare the design with TR15’s documented floor-ceiling systems and spacing limits. Flag walls, structural composite lumber, cross-laminated timber, or other unrepresented configurations as outside the documented scope unless supported by newer evidence.
- Use consistent model inputs. Compare designs on the same basis. If a change is made—for example, to a ceiling layer, insulation, floor covering, or framing configuration—identify that change rather than attributing a rating difference to “more mass” alone.
- Read the result as an estimate. Keep the rating tied to the modeled assembly and do not convert it into a claim about guaranteed occupant experience. TR15’s reported accuracy evidence applies to its 35-assembly bare-floor validation sample.
- Escalate decisions that depend on a specific outcome. If compliance, a contractual rating, or a sensitive acoustic target depends on the result, confirm the applicable test and rating requirements and consult an acoustics professional. Standardized testing may be appropriate when a calculated estimate is not enough.
When comparing options, the useful question is not simply which assembly has the highest number. Ask whether both are within scope, whether the same rating is being compared, what component changes drive the difference, how much weight to place on the validation evidence, and whether the project needs a laboratory-style assembly rating or building-level sound isolation.
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What code or compliance claims can you make from TR15?
Do not treat the report’s historical code discussion as a current, universal compliance determination. TR15 describes 2018 International Building Code requirements in their historical context. Code requirements depend on jurisdiction and the code edition in force, so verify local rules and the applicable project criteria before claiming that an assembly complies.
A model estimate may inform design, but compliance documentation can require a specific tested or listed assembly, a particular rating, or other evidence. The available data does not establish one set of current code requirements for every location. Where a rating is contractually or legally consequential, confirm what the authority having jurisdiction or project documents require and obtain appropriate professional guidance.
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What is the practical takeaway for designers and builders?
TR15 is most useful as a structured comparison tool for wood-frame floor-ceiling assemblies that match its documented components and configurations. Its advantage over a mass-only rule is that it incorporates empirical system effects; its limit is that its predictions are only as applicable as the underlying model scope and validation.
Keep three boundaries clear: the report models selected floor-ceiling assemblies, the reported validation covers 35 in-scope bare-floor assemblies, and an assembly rating does not fully describe apparent sound insulation in a finished building. For designs outside scope or decisions that depend on a defensible performance result, use a qualified acoustics professional and consider standardized testing rather than extrapolating the model.
Frequently Asked Questions
What do STC and IIC stand for?
STC is the Sound Transmission Class, a single-number classification for airborne sound transmission loss. IIC is the Impact Insulation Class, which addresses impact-generated sound; it is not a complete description of low-frequency footfalls.
Can I use a phone or handheld sound meter to calculate an assembly’s STC?
No. STC is derived from standardized transmission-loss measurements across frequency bands, not from a simple room sound-level reading. The NRC describes E90 laboratory testing as a controlled full-scale procedure using reverberation rooms, calibrated microphones, and multiple sound sources.
Why might footsteps still be audible when a floor has a reasonable IIC rating?
IIC does not represent footfall behavior below approximately 100 Hz, according to TR15. Low-frequency impact noise and paths through the rest of the building can therefore matter even when a single-number rating seems satisfactory.
Does TR15 cover wood-framed walls?
The model documented in the January 2019 report is for selected floor-ceiling assemblies, not a universal wall model. The report described wall-model development as future work, so do not apply its floor-ceiling estimates to walls without newer, applicable evidence.
