Sound Transmission Class (STC): Partition Attenuation

Sound transmission class (STC) is a single-number rating that describes the airborne sound isolation performance of building partitions such as walls, floors, doors, and glazing systems. Higher STC values indicate greater attenuation of speech and noise transmitted through the assembly. Designers use STC to meet code requirements, ensure occupant privacy, and enhance overall acoustic comfort.

This article details how STC is defined, traces the history of the rating system, explains laboratory and field testing procedures, and presents design strategies for achieving target values. Links to related noise metrics and calculators support integrated acoustic design.

Definition and Rating Procedure

Standardised contour method

STC ratings follow ASTM E413, which fits a reference contour to measured sound transmission loss (TL) data between 125 Hz and 4000 Hz. The contour is shifted in 1 dB increments until the total deficiency—the sum of negative differences between TL data and the contour—does not exceed 32 dB. The contour value at 500 Hz becomes the STC rating. Because STC emphasises mid-frequency performance, designers supplement it with other metrics for low-frequency noise such as mechanical hum or impact sound.

Interpreting ratings

An STC 30 wall allows normal speech to be heard clearly, while STC 50 or higher provides excellent speech privacy. Codes for multifamily housing often require STC ≥ 50 between dwelling units, and hospitals may target STC ≥ 55 for patient privacy. Pairing STC analysis with exposure metrics such as Leq ensures that interior noise meets both isolation and energy-based limits.

Historical Development

Origins in the mid-twentieth century

The STC system emerged in the 1950s as researchers sought a simplified metric for communicating transmission loss results. Early versions used different reference curves, but ASTM E413 standardised the contour in 1962, enabling consistent comparisons across laboratories. Subsequent revisions refined frequency weighting and addressed new construction materials.

Evolution alongside building codes

Building codes adopted STC requirements to protect occupants from intrusive noise, particularly in multifamily housing, hotels, and healthcare facilities. As construction practices evolved, STC testing informed innovations like resilient channels, staggered studs, and laminated glazing. Today, green building programmes and wellness certifications also reference STC to support occupant comfort.

Testing and Measurement

Laboratory testing (ASTM E90)

Laboratory STC measurements follow ASTM E90, which places a test specimen between reverberant source and receiving rooms. Broadband noise is emitted, and microphones capture sound levels to compute transmission loss across octave or one-third-octave bands. Laboratory ratings represent ideal conditions and do not account for installation variability.

Field testing (ASTM E336)

Field sound transmission class (FSTC) measurements account for real-world factors such as workmanship, flanking paths, and site-specific conditions. FSTC values are typically 3 to 5 points lower than laboratory STC ratings. Acoustic consultants use testing data to diagnose weaknesses and recommend remediation strategies.

Flanking and ancillary paths

Sound can bypass a partition through ceilings, floors, ductwork, or structural connections. Designers mitigate flanking by sealing penetrations, using acoustical caulk, incorporating resilient mounts, and coordinating with mechanical engineers. The facade noise reduction planner and sound level addition calculator assist with predicting combined effects of direct and flanking transmission.

Design Strategies

Mass, damping, and decoupling

Increasing mass raises transmission loss by resisting airborne vibrations, while damping layers reduce resonances. Decoupling—using resilient channels, double studs, or sound isolation clips—interrupts vibration paths. Designers balance these techniques with structural and cost constraints, leveraging calculators like the transmission loss calculator to evaluate options.

Integration with mechanical systems

Ventilation openings, ducts, and piping can compromise STC performance. Strategies include lined ductwork, acoustical louvers, and vestibules that satisfy airflow requirements from the ACH article while maintaining sound isolation. Coordinated design prevents conflicts between mechanical and acoustical objectives.

Balancing aesthetics and performance

Transparent partitions, operable walls, and historic building retrofits require careful detailing to achieve target STC ratings without compromising aesthetics. Laminated glazing, acoustical seals, and secondary glazing systems offer solutions that preserve visual quality while enhancing isolation.

Importance and Emerging Trends

Health and well-being

Adequate acoustic isolation supports sleep quality, cognitive performance, and privacy. By aligning STC targets with Leq guidelines, designers create environments conducive to health and productivity.

Adaptive reuse and modular construction

Renovation and modular building projects leverage lightweight assemblies that must still achieve high STC ratings. Innovations include prefabricated wall cassettes with integrated damping layers and demountable acoustic seals. Data-driven commissioning verifies performance through field testing.