Sound Absorption vs. Sound Transmission Loss

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Frequently Asked Questions

Q. What is the difference between sound absorption (NRC), sound transmission loss (STC), and impact sound transmission (IIC)?

The following is a simple breakdown of NRC, STC, and IIC:

Term: Sound Absorption

Property: The ability of a material to reduce sound reflections, reverberation, and echo within an enclosed space

Example: “The room was too reverberant due to hard-surfaced floor, ceiling, and walls; therefore, sound-absorbing materials such as fabric-wrapped wall panels and acoustical tile ceiling were installed.”

Lab Test Method: ASTM C423 which provides Sound Absorption Coefficient, SAA (Sound Absorption Average), and NRC (Noise Reduction Coefficient) ratings.

Term: Sound Transmission Loss

Property: The ability of a material, panel, or wall to act as a barrier preventing airborne sound transmission from one space to another

Example: “We could hear people talking in the room next door, so we had to improve the sound transmission loss of the wall to gain sufficient privacy.”

Lab Test Method: ASTM E90, which provides sound transmission loss ratings at 1/3 octave bands and ASTM E413 which is the calculation method for the STC (Sound Transmission Class) rating.

Term: Impact Sound Transmission

Property: The ability of a floor/ceiling assembly to reduce footfall noise transmission to the space below

Example: “We could hear people walking on the floor above, so they installed a resilient underlayment beneath the floor covering.”

Lab Test Method: ASTM E492, which provides impact sound pressure level at 1/3 octave bands and ASTM E989 which is the calculation method for the IIC (Impact Insulation Class) rating.

Sound absorption is a property of materials whereby incident acoustic energy is converted into heat through various processes. This physical process contributes to both NRC and STC performance.

In acoustical engineering for buildings and architecture, the term sound absorption (NRC) refers to the property of treatments that reduce sound reflections and reverberation WITHIN the room. Sound Transmission loss (STC) is the ability of a material to provide sound insulation or noise control from one space to another. In practice, the materials and systems used for each are very different.

Sound absorption (NRC) materials are usually thick, porous (or fibrous) panels that effectively reduce sound reflections and reverberation when installed in the correct locations and quantity. Examples include fabric-wrapped fiberglass wall panels, acoustical ceiling tiles, and thick foam wedge panels. These materials will have very little if any benefit to the reduction of sound transferring to or from adjacent spaces.

Materials having a high sound transmission loss (STC) performance are typically heavy and impermeable. Properties that contribute to high sound transmission loss (STC) performance are mass (weight), limpness, impermeability, damping, and isolation of layers. Examples of high STC barrier systems include thick concrete wall structures, resiliently mounted gypsum board walls, and damped gypsum board panels. Sound-absorptive insulation may be employed in acoustical barrier systems to reduce resonance within the cavities of hollow structures, thus improving sound transmission loss for the assembly.

Note: Single number ratings (STC or NRC) should only be used for rough “shotgun” comparisons or for selecting materials for general applications.  Full 1/3 octave band sound absorption coefficients or sound transmission loss values over the full spectrum give a more complete indication of the system’s performance against the noise generated by the offending source. It is not possible to derive the full 1/3 octave band performance from the NRC or STC rating alone.  Unfortunately, it can be very difficult to obtain the full TL or sound absorption spectrum data.

References:
Long, Marshall Architectural Acoustics, Second Edition, Academic Press; 2014. Print. http://amzn.to/2y2nUAU

ASTM Standard C634, 2013, “Standard Terminology Relating to Building and Environmental Acoustics,” ASTM International, West Conshohocken, PA, 2013, https://www.astm.org/Standards/C634.htm

ASTM Standard C423, 2017, “Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method,” ASTM International, West Conshohocken, PA, 2017, https://www.astm.org/Standards/C423.htm

ASTM Standard E90, 2016, “Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method,” ASTM International, West Conshohocken, PA, 2016, https://www.astm.org/Standards/E90.htm

ASTM Standard E492, 2016, “Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method,” ASTM International, West Conshohocken, PA, 2016, https://www.astm.org/Standards/E492.htm