Acoustics Glossary: NRC, STC, RT, Modes & More

Acoustics jargon can make simple decisions harder. These are the terms most likely to appear in product data sheets and treatment discussions.

Absorption coefficient

A frequency-specific value describing how much incident sound energy a material absorbs under a particular test method. Read the test conditions; mounting changes performance.

NRC — Noise Reduction Coefficient

A single-number average derived from absorption measurements at selected mid frequencies. It is useful for quick comparison but can hide low-frequency differences.

Reverberation time (RT)

How long sound energy takes to decay by a specified amount after the source stops. Long decay can reduce clarity; the desirable value depends on room use and volume.

Flutter echo

A rapid, repeated reflection between hard, roughly parallel surfaces. It can sound like a metallic “zing” after a clap.

Room mode

A low-frequency resonance determined by room dimensions. Modes can create peaks and nulls that vary strongly with source and listener position.

Diffusion

Scattering reflected sound over multiple directions rather than simply absorbing it. Diffusion aims to reduce distinct reflections while preserving more acoustic energy.

STC — Sound Transmission Class

A single-number rating used to summarize airborne sound isolation of building assemblies over a defined frequency range. It is not a complete description of low-frequency isolation.

Mass law

A simplified principle that heavier, limp barriers tend to transmit less airborne sound as surface mass increases, though real walls are affected by resonance, coincidence, studs, gaps, and flanking.

Flanking path

An alternate route sound takes around the primary separating assembly—through ceilings, floors, ducts, side walls, structural connections, or gaps.

Decoupling

Reducing rigid mechanical connections between layers so vibration is less readily transferred. Examples include resilient channels and isolated framing when designed and installed correctly.

Broadband absorber

An absorber intended to work across a relatively wide frequency range. Greater depth and air space generally help extend porous absorption lower in frequency.

First reflection

An early reflected path from a loudspeaker or source to a listener. In critical listening rooms, controlling strong early reflections can improve imaging and clarity.