Last updated: September 2026
What STC actually measures
Sound Transmission Class (STC) is a single-number rating derived from laboratory testing across a range of frequencies, primarily describing how well a wall, floor or door resists airborne sound—voices, television, music. The test happens in a controlled lab with a speaker on one side and a microphone on the other, measuring how much sound energy is lost passing through the assembly at each frequency band, then averaging that into one number.
Because it is a lab number, STC describes the assembly in isolation—no flanking paths, no doors, no electrical boxes, no real-world workmanship variation. An assembly tested at STC 60 in a lab will very rarely perform at STC 60 once it is built into an actual room with an entry door, ductwork and a shared corridor wall. That gap is normal and expected, not a sign that the product failed.
STC also underweights very low frequencies, which matters because a lot of what people describe as 'still hearing everything' after a soundproofing project is deep bass or bass-heavy speech that the STC curve does not penalize as strongly as it should for how annoying it actually is.
What IIC actually measures
Impact Insulation Class (IIC) uses a standardized tapping machine—a set of small hammers dropped onto the floor at a fixed rate—to measure how much impact energy transmits through a floor-ceiling assembly to the room below. It is the relevant rating for footsteps, dropped objects, dragged furniture and similar impact noise, as opposed to STC's focus on airborne sound like voices.
The tapping machine produces a specific, repeatable impact signature that does not perfectly match a 200-pound adult walking in hard-soled shoes, a child running, or a dog with untrimmed nails on hardwood. Real walking tends to include lower-frequency, heavier impacts than the standardized hammers reproduce well, which is one reason a floor with an excellent IIC rating on paper can still transmit an audible thud in practice.
In Toronto condos, IIC ratings usually appear in the context of flooring replacement rules: many corporations require a minimum tested IIC (sometimes FIIC, the field-tested equivalent) before hardwood, engineered flooring or tile can replace carpet, precisely because impact noise complaints between vertically stacked units are common.
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Why lab numbers and lived experience diverge
The biggest source of divergence is flanking transmission—sound bypassing the treated assembly through a continuous slab, a shared corridor door, a duct run, an electrical box, or the gap where a wall meets a window return. You can build a wall rated for excellent laboratory STC and still hear your neighbour clearly if the sound is flanking around it through an untreated path.
Workmanship matters just as much as material selection. A single missed bead of acoustic sealant at a perimeter joint, a screw that bridges an isolation clip, or an electrical box left untreated can measurably reduce field performance below the tested design value. This is why we describe project outcomes as design ranges rather than guaranteed scores—an estimated STC 55-65 design range communicates an expectation while acknowledging that as-built performance depends on execution.
Room size, furnishing, and background noise level in the receiving room also affect what a person actually perceives, independent of the wall's physical performance. A quiet, sparsely furnished bedroom will reveal more residual sound than a busy living room with soft furnishings and ambient noise from a television or white-noise machine.
How to use these ratings when comparing options
Use STC and IIC as directional indicators of an assembly's design intent, not as promises about your specific room. When a contractor gives you a target range, ask what assumptions the range depends on—base wall type, flanking paths addressed, whether outlets and doors are included in the scope—because the number without the assumptions is close to meaningless.
Be skeptical of any single product, such as a foam panel or a thin acoustic mat, advertised with an isolated STC or IIC number. Ratings apply to complete tested assemblies, not individual components; a damping compound or resilient channel tested as part of a specific double-gypsum wall does not transfer its rating to a different wall built with different materials.
- Ask what flanking paths were considered in a quoted target range
- Confirm whether doors, outlets and windows are included in the scope
- Treat a single-product rating as marketing, not an assembly guarantee
- Expect field results to sit below laboratory numbers, not match them
Where verification testing fits in
For most residential projects, a qualitative before-and-after listening comparison is sufficient to confirm a project met its goals. For owners who want documentation—often for condo board records, pre-sale disclosure, or dispute resolution—field sound testing measures apparent transmission loss or impact performance in the actual room, which is a more meaningful number than any laboratory rating because it captures flanking, workmanship and room conditions together.
Field testing does add cost and scheduling, and results are still influenced by ambient conditions on the day of the test, so we discuss whether it is worthwhile case by case rather than including it by default.
The bottom line for condo owners
STC tells you about airborne sound resistance—voices and television. IIC tells you about impact resistance—footsteps and dropped objects. Neither number describes your specific room once flanking paths, doors, furnishings and workmanship are accounted for, so treat both as planning tools rather than guarantees, and ask any contractor quoting a number to explain the assumptions behind it.