IIC and STC Ratings Explained: What They Mean for Noise

If you've shopped for flooring, you've seen these two acronyms sitting quietly in the spec sheet: IIC and STC, each followed by a number. Most buyers skip past them. They shouldn't - especially in a condo, an upstairs bedroom, or a rental replacing carpet with something hard.

Quick Answer

IIC (Impact Insulation Class) measures footstep and impact noise passing through a floor to the room below. STC (Sound Transmission Class) measures airborne noise - voices, TV, music. Both scales run higher-is-better. Code minimum sits around 50; many HOAs and condo boards want 60 or higher. McMillan's SPC vinyl (EVOLVED and SupremeCORE lines) tests at IIC 66 / STC 68. McMillan's laminate (EVOLVED Series) now carries an upgraded best-in-class attached underlayment tested at IIC 79 / STC 77, Intertek certified - among the highest published acoustic ratings in laminate flooring, ahead of most competitors regardless of category.

Two Ratings, Two Different Noises

IIC - the footsteps test

IIC is measured with a standardized tapping machine that mechanically strikes the floor, simulating footsteps, dropped objects, and kids running. A microphone in the room below records how much of that impact energy makes it through. Tested per ASTM E492 (lab) and E989 (rating calculation). This is the number that matters most to a downstairs neighbor.

STC - the voices and TV test

STC is measured by playing sound through a speaker at a range of frequencies and recording how much is blocked versus transmitted through the assembly. Tested per ASTM E90 (lab) and E413 (rating calculation). One honest limitation worth knowing: STC is calibrated around speech frequencies, so it under-represents low-frequency, bass-heavy sound - a subwoofer or a bass-heavy movie scene will transmit more than the STC number alone suggests.

What the Numbers Actually Mean

Both scales are logarithmic, not linear. A 2-point difference is close to imperceptible; a 10-point jump is dramatic. That's part of why McMillan's laminate result - a 13-point IIC gain and a 9-point STC gain over the SPC line - represents a meaningfully quieter floor, not just a bigger number.

The Fine Print That Actually Matters

• Ratings belong to the assembly, not the plank. An IIC/STC number describes the full tested system - subfloor, underlayment, and finish floor together - not the plank in isolation. That's why the same plank can test differently over concrete versus over wood joists.

• The underlayment is doing real work. For SPC, the attached 1.5mm antibacterial pad is part of the tested 66/68 result. For EVOLVED laminate, the upgraded best-in-class attached pad is what pushes the assembly to 79/77 - the acoustic performance is built into the plank, not something you add separately.

• Many brands don't publish acoustic ratings at all. Comparable competitor products (including Lifeproof) often list no IIC/STC figures, which makes an apples-to-apples comparison difficult - and makes a published, third-party-certified number a real differentiator on its own.

• Lab and field results differ. Expect roughly a 5-point gap between controlled lab conditions and real-world field testing, due to flanking noise paths (walls, doors, plumbing chases) that a lab assembly doesn't have.

When Does This Actually Become a Requirement?

For most single-family homeowners, IIC/STC is a nice-to-know. It becomes a real requirement in a few specific situations:

• Condos and HOAs: Many governing documents specify a minimum IIC and/or STC (60 is a common threshold) for any hard-surface flooring installed above another unit.

• Upper floors generally: Even without a formal requirement, second-story installations above occupied space benefit from a higher-rated assembly.

• Rentals replacing carpet: Swapping carpet for a hard surface is one of the most common triggers for noise complaints between units - and one of the most common reasons boards start asking for acoustic documentation.

Frequently Asked Questions

What's a good IIC rating for flooring?

50 is code minimum. 60+ is a common HOA/condo threshold and considered comfortable. Above 70 is genuinely excellent - McMillan's laminate at IIC 79 sits well into that range.

Is a higher IIC or STC number always better?

Yes - both scales run higher-is-better. Just remember they measure different things (impact versus airborne noise), so a strong IIC doesn't guarantee a strong STC and vice versa. McMillan's laminate happens to lead on both.

Does McMillan's laminate really outperform its own SPC vinyl on sound?

Yes. EVOLVED laminate's upgraded attached underlayment tests at IIC 79 / STC 77 (Intertek certified), ahead of the SPC line's IIC 66 / STC 68. Both are strong, published results - the laminate figure is simply the higher of the two and among the highest published in the laminate category generally.

Does underlayment change the IIC rating?

Significantly. A resilient underlayment between a hard plank and the subfloor is the largest single contributor to impact performance in a floating floor. Ratings are tested on the full assembly, so a floor with an attached pad - like both of McMillan's lines - carries its acoustic performance built in rather than dependent on a separate purchase.