Acoustics: STC, NRC, IIC, and the metrics that govern building sound

One-line orientation

Building acoustics balances two separate problems — absorbing sound within a room and blocking sound between rooms — and the exam tests which metric (STC, NRC, IIC) applies to which problem, plus the dB relationships that drive design decisions.

Key points

• Sound speed: approximately 1,125–1,128 ft/s in air at room temperature.
• Decibel (dB) rules:
  • Halving distance: +6 dB; doubling distance: −6 dB.
  • Doubling sound energy: +3 dB.
  • Perceived twice as loud: +10 dB; perceived four times as loud: +20 dB.
• Key frequencies for building acoustics: 63, 125, 250, 500, 1000, 2000, 4000 Hz (octave bands used for general building-acoustics discussion). STC itself is calculated from one-third-octave transmission-loss bands, not just this octave-band list.
• Three sound behaviors:
  • Transmission — sound passes through a material.
  • Absorption — energy absorbed (converted to heat); relates to air pockets in the material.
  • Reflection — sound bounces back from a surface.
• Absorption coefficient (alpha): alpha = 1 = no reflection (total absorption); alpha = 0 = total reflection.
• Reverberation time (RT60): time for sound to decay 60 dB after source stops. A long RT means sound lingers — wanted in a concert hall (≈ 1.8–2.2 s), wrong wherever speech must be understood: speech and conference spaces target ≈ 0.5–0.8 s, classrooms ≈ 0.4–0.6 s. RT60 is the one metric on this card a designer sets a target for.
• Early reflections (Haas effect): reflections arriving soon after the direct sound may be heard as part of the same event. Later reflections are heard as reverberation or echo.
• Key metrics:
  • NRC — average absorption coefficient at 250, 500, 1000, 2000 Hz; ranges 0–1; NRC > 0.70 = good absorber. Rates the material’s surface.
  • STC — single-number rating for how well an assembly (wall, floor) blocks airborne sound. Code minimum for dwelling-unit separations in IBC Group R is typically STC 50 lab-tested (45 if field-tested) — the same pair of numbers as IIC. Same STC wall can behave differently at low vs high frequencies: massive walls are better at blocking low-frequency noise.
  • IIC — rates floor-ceiling assembly resistance to impact (structure-borne) noise; measured with a tapping machine. Code minimum for dwelling-unit separations in IBC Group R is typically IIC 50 lab-tested (45 if field-tested); 60–65 is an enhanced comfort target.
  • TL (Transmission Loss) — frequency-dependent measure of how well an element blocks airborne sound; STC is derived from the TL curve.
  • dBA — A-weighted decibels, used for exterior noise levels.
  • NC (Noise Criteria) — rates steady background/HVAC noise by comparing octave-band levels to standard NC curves; the rating is the lowest curve the spectrum does not exceed. Lower NC = quieter. Select the target by occupancy.
• Material factors for sound: mass, smoothness, fiber orientation, porosity, airtightness, stiffness. Massive materials are better at blocking low-frequency sounds; softer materials address high-frequency impact noise.
• Structural discontinuity: resilient connections (decoupled framing, resilient channels) prevent vibration from transmitting through the structure.
• Flanking paths: sound bypasses the primary assembly through gaps, penetrations, or connected structure — the weakest point controls isolation.

Confusions / comparison

Metric	What it rates	Sound type	Scale / range	Higher = ?
STC	assembly (wall, floor, window)	airborne (speech, music)	single number (typical 25–65+); Group R code min usually 50 lab / 45 field	better blocking
NRC	surface material	absorption within a room	0 to 1.0	more absorption
IIC	floor-ceiling assembly	impact / structure-borne	single number; Group R code min usually 50 lab / 45 field	better impact isolation
TL	element at each frequency	airborne	dB loss by frequency band	more loss = better
NC	interior ambient noise level	background noise	typical targets NC 25–40 by room type	lower = quieter

Design goal	Tool / strategy
Reduce echo/reverberation within a room	Add absorptive materials (high NRC)
Block sound between rooms (airborne)	Increase mass/STC; seal flanking paths
Block footstep noise from floor above	Improve IIC (soft flooring, resilient underlayment)
Isolate mechanical vibration	Structural discontinuity / resilient mounts

NRC, STC, and IIC — three different acoustic problems

Ask where the sound is and what the rated material or assembly must do.

View diagram Hide diagram 3 panels

3 panels side by side — swipe to compare →

Key

• blue = sound path / material acting on it
• amber = unintended flanking path

Related

→ Systems: HVAC (mechanical noise, NC ratings for equipment rooms) · Construction: assemblies (wall and floor-ceiling STC/IIC details) · Code: IBC occupancy-specific acoustic requirements.