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Specification guide

STC, IIC and NRC Explained

The three ratings that turn soundproofing from a guess into a specification — airborne blocking, impact blocking and absorption. Learn what the numbers mean so you can hold any quote to an honest target.

Acoustics stops being guesswork the moment you can put a number on it. Three ratings do most of the work — STC for airborne blocking, IIC for impact blocking, and NRC for absorption. Knowing what they mean lets you set a real target, compare options honestly, and hold any contractor (including us) to a measurable promise instead of a vague reassurance. None of them is a promise of silence; each is a scale, and this guide shows you how to read it.

STC — Sound Transmission Class

STC rates how well a wall, floor, door or window blocks airborne sound — voices, television, traffic, a generator. It’s a single number: higher means better blocking. The useful landmarks:

  • STC ~25–30 — normal speech is clearly audible through the wall. Most cheap partition walls sit here.
  • STC ~40 — loud speech is a murmur; you’re aware of it but can’t follow it.
  • STC ~50 — the recognised threshold for adequate privacy between dwellings; normal speech is inaudible and only loud sounds come through faintly.
  • STC ~55–60 — normal speech is inaudible; even loud sound is heavily muffled. This is the region studios, cinemas and confidential clinics aim for.
  • STC 60+ — specialist isolation for recording, live music and critical privacy.

Think of STC like an SPF rating for noise: a higher number protects you more, but it’s never a guarantee of zero exposure. A useful reality check — because decibels are logarithmic, moving from STC 45 to 55 is a large, clearly audible improvement, not a marginal one. STC drives the design of our wall soundproofing, soundproof doors and seals and acoustic windows and secondary glazing.

IIC — Impact Insulation Class

STC does not tell you about footsteps. For that you need IIC (Impact Insulation Class), which rates how well a floor/ceiling assembly blocks impact sound — footfall, dropped objects, dragged furniture — travelling down through the structure. Again, higher is better and lower means more thud below.

  • IIC ~50 — a common building-code minimum; footfall is audible but tolerable.
  • IIC ~55–60 — noticeably quieter; hard-heeled footsteps are softened.
  • IIC 60+ — the target above gyms, dance studios and busy flats.

The catch that surprises many owners: a wall or floor can have a high STC and still transmit footsteps badly, because impact energy enters the structure directly and simple mass does little against it. Fixing impact means resilient underlayment, floating floors and decoupled ceilings — the work behind our ceiling, floor and impact soundproofing. The airborne-versus-impact distinction is important enough that we give it its own guide: Airborne vs Impact Noise.

NRC — Noise Reduction Coefficient

The first two ratings are about blocking sound between rooms. NRC is about something entirely different: absorption inside a room. It’s a score from 0 to 1 describing how much sound a surface soaks up rather than reflects — how “thirsty” it is for sound. Soft carpet drinks sound; glossy tile spits it back.

  • NRC 0.0 — fully reflective (bare concrete, glass, tile).
  • NRC ~0.5 — moderately absorbent.
  • NRC 0.85–1.0 — highly absorbent (thick mineral-wool or PET panels).

The vital caveat: a high NRC does nothing to stop your neighbour hearing you. NRC measures echo control inside a room, not transmission between rooms. This is exactly the point at which people waste money — buying high-NRC foam expecting it to “soundproof,” when it only makes the room sound better inside. NRC governs our acoustic treatment and echo control, and it’s closely tied to reverberation time, covered in RT60 and Reverberation Explained.

Putting the Three Together

A well-specified room usually names all three. A home cinema, for example, might target STC 60 in the walls, IIC 55 in the floor, and an average room NRC around 0.7 for clear dialogue. A restaurant may need only high NRC to cut the din. Getting the mix right is the whole job — and the reason the three ratings exist separately rather than as one magic number. The wider framework is in The Four Pillars and How Soundproofing Actually Works.

The Honest Frame

Lab ratings assume perfect installation; the real world subtracts points for every unsealed gap and every flanking path (see Flanking Paths Explained). That’s why we don’t quote a rating and walk away — we design to a target, install carefully, and verify the actual result before and after so the number on paper becomes the number in your room. No honest firm promises silence; we promise a measured, checkable target.

Want a target STC, IIC or NRC set for your space? Book a free noise assessment: +233 20 531 3333.