What is Sound Transmission Class (STC)? | ASLONG Guide

What is Sound Transmission Class (STC)?

Sound Transmission Class (STC) is a standardized rating used to measure the effectiveness of building materials in reducing airborne sound transmission. An important factor in sound insulation, the STC rating quantifies how well materials such as walls, floors, windows, and doors can block or reduce the passage of sound. The higher the STC rating, the better the material is at soundproofing and limiting noise from passing through.

How is STC Measured?

STC is determined through laboratory tests in compliance with ASTM E90 and ASTM E413 standards. During the test, a sample partition is exposed to various frequencies of sound in a controlled setting. The amount of sound that passes through is measured, and translation into a single-number STC rating is calculated. This rating primarily addresses commonly encountered frequencies from 125Hz to 4000Hz, focused on typical speech and general building noises.

Why is STC Important for Sound Insulation Materials?

STC helps professionals objectively compare the noise-reducing performance of different building materials. This is crucial for spaces where privacy and sound quality matter—such as offices, apartments, hotels, and recording studios. Choosing materials with higher STC values ensures quieter and more comfortable environments, protecting privacy and reducing sound distractions in sensitive areas.

What is a Good STC Rating for Different Applications?

- Residential Walls: STC 35–40 blocks normal speech but not loud sounds; STC 50+ is recommended for excellent privacy.

- Floors/Ceilings: Aim for STC 50+ in multifamily complexes to control conversational noise.

- Windows/Doors: Standard windows rate around STC 25; specialized solutions can achieve STC 40+, ideal for urban or noisy environments.

How Can I Improve the STC Rating of a Structure?

Increasing the STC can be achieved by:

- Adding Mass: Using denser materials or additional layers.

- Decoupling Assemblies: Double-stud walls, resilient channels, and sound clips.

- Damping: Utilizing materials that convert sound energy into heat, such as specific sound damping compounds or membranes.

- Sealing Gaps: Even small air leaks can degrade the STC; thorough sealing is essential.

What are Common Misconceptions about STC?

- STC Does Not Cover All Noises: It mainly measures airborne sound transmission. Low-frequency noises like bass or vibration (impact sound) are less accurately reflected in STC ratings—other indices like IIC (Impact Insulation Class) may be more relevant in those cases.

- Higher STC Is Not Always Needed: Not every space requires maximum sound attenuation. Balance cost and functional needs.

- Installation Matters: The best materials only perform well with proper installation and sealing.

For a comprehensive range of high STC performing insulation and guidance on soundproofing solutions, trust ASLONG to provide expert recommendations based on your project’s needs.