When contractors and building inspectors talk about wall and floor performance in Canada, two numbers come up consistently: the STC rating and the IIC rating. Both describe how well an assembly limits the passage of sound, but they measure entirely different phenomena. Understanding the distinction — and what the numbers actually represent — is the starting point for any practical soundproofing project.
What Is STC?
Sound Transmission Class (STC) is a single-number rating that summarizes how much airborne sound a wall, floor, ceiling, door, or window assembly blocks. Airborne sound includes speech, television audio, music, and most mechanical noise — anything that originates as pressure waves in air.
Testing follows ASTM E90 (laboratory) or ASTM E336 (field). Engineers measure transmission loss at 16 one-third octave frequency bands between 125 Hz and 4000 Hz, then fit that curve to a standard reference contour defined in ASTM E413. The STC number is the transmission loss in decibels at 500 Hz on that fitted contour.
A 10 dB increase in STC roughly corresponds to the perception of sound being reduced by half. Moving from STC 35 to STC 45 is not a small improvement — it's the difference between speech being intelligible through a wall and speech being audible only as a faint murmur.
Interpreting STC Values
The following ranges are widely cited as practical benchmarks:
- STC 25–30: Normal speech clearly audible and understandable.
- STC 35–40: Loud speech audible but not clearly intelligible.
- STC 42–45: Loud speech faint; adequate for most residential applications between own rooms.
- STC 50–54: Loud speech barely audible; typical target for party walls in multi-family housing.
- STC 55–65: Most sounds inaudible; used for recording studio partitions and home theatres.
These are perceptual guidelines, not engineering absolutes. The frequency balance of the noise source matters: a wall with an STC of 50 may perform well against speech but less well against low-frequency bass from a subwoofer, since STC does not penalise low-frequency deficiencies as heavily as the ear does.
What Is IIC?
Impact Insulation Class (IIC) measures how well a floor-ceiling assembly attenuates impact noise — footsteps, dropped objects, dragged furniture. Impact noise is structurally borne: it enters the building structure directly at the point of impact rather than travelling through the air.
Testing follows ASTM E492 (laboratory) or ASTM E1007 (field) using a standardised tapping machine with five steel-tipped hammers that strike the floor at a fixed rate. Microphones below the floor measure transmitted sound levels. The IIC number is derived by fitting the resulting curve to a reference contour.
A higher IIC number means less impact sound is transmitted. The minimum IIC for floor-ceiling assemblies in attached housing under the National Building Code of Canada (NBC) is 50 in laboratory conditions — though field-measured values, which account for flanking transmission, are typically 5 to 8 points lower than lab values for the same assembly.
Why IIC Is Harder to Achieve Than STC
Airborne sound can be attenuated substantially by adding mass — thicker drywall, additional layers, denser materials. Impact noise requires a different strategy: decoupling or resilient layers that absorb the energy before it enters the structure. A concrete slab floor may have an excellent STC of 58 or higher, yet an IIC of only 28 — meaning footsteps directly overhead are very audible despite the mass. Floor underlays, floating floors, and acoustic mats address IIC independently of mass.
National Building Code of Canada Requirements
Part 9 of the NBC (housing and small buildings) specifies minimum performance levels for suite separations in multi-unit residential construction:
- Walls between dwelling units: minimum ASTM E90 STC 50 (laboratory) or field STC 45.
- Floor-ceiling assemblies between units: minimum STC 50 and IIC 50 (laboratory).
- Walls separating dwelling units from common corridors: minimum STC 50.
These requirements apply to new construction. Renovations and retrofits in existing buildings are not subject to the same NBC requirements unless a building permit triggers a compliance review — though provincial codes and local bylaws may impose additional conditions.
Laboratory STC and field STC are not the same. Field measurements account for flanking paths — sound travelling around, over, or through gaps in the assembly. A partition that tests at STC 55 in a lab may measure only STC 47 in a real building if floor, ceiling, and electrical penetrations are not detailed properly.
OITC: An Alternative to STC for Low-Frequency Noise
The Outdoor-Indoor Transmission Class (OITC) is a newer rating that weights low-frequency performance more heavily than STC. It covers frequencies from 80 Hz to 4000 Hz and is considered a more accurate predictor of how well an assembly attenuates transportation noise — aircraft, trains, heavy trucks — and low-frequency music. If the dominant noise problem involves significant energy below 250 Hz, OITC provides a more useful comparison than STC alone.
Testing Methods: Lab vs. Field
Laboratory tests (ASTM E90, E492) are conducted in controlled test facilities where flanking is eliminated and only direct transmission through the specimen is measured. These produce the highest possible numbers for any given assembly design.
Field tests (ASTM E336, E1007) measure actual performance in a building as constructed. Flanking paths — through floor and ceiling connections, at wall-to-floor junctions, through electrical boxes, around door frames — consistently reduce measured values by 3–10 STC or IIC points compared to lab results. Published assembly data from manufacturers is almost always derived from laboratory testing.
When specifying an assembly for a renovation or new build, it is reasonable to subtract 5–7 points from the published lab STC to estimate field performance if flanking control measures are standard but not exceptional.
Practical Implications for Retrofit Projects
Most soundproofing complaints in Canadian homes fall into one of three categories:
- Airborne noise between rooms on the same floor — usually addressed with additional drywall layers, insulation, and careful sealing of penetrations.
- Impact noise from floors above — addressed primarily with floor underlays, floating floor systems, or resilient ceiling mounts below.
- Low-frequency noise (bass, HVAC, mechanical) — the most difficult to address; requires substantial mass and decoupling, and STC ratings alone understate the problem.
For each category, identifying the dominant transmission path before selecting materials prevents the common mistake of investing in the wrong component. Adding dense insulation to a wall cavity does little if the flanking path through the concrete slab is carrying most of the energy.
Sources: ASTM E413, National Research Council of Canada — Codes Centre, National Building Code of Canada 2020.
