The Sound Transmission Class (STC) is a numerical rating system that quantifies a physical structure’s effectiveness in blocking noise. It is widely used to assess the sound-blocking capabilities of walls, ceilings, doors, and windows.

It indicates the extent to which a structure muffles or reduces sound transmission to the other side.

This article explains STC rating, discusses its calculation, identifies the factors involved, and compares it with other standards.

What makes a good STC rating?

Judging soundproofing requires more than the human ear. Use the objective STC scale instead – it runs 20-80, with higher being better noise blocking.

STC RatingObserved Result
25Clear speech can be easily heard from the other side
30Loud speech is intelligible. Normal speech can be heard but not understood
35Loud speech is audible but not clear
40Good level of privacy. Loud speech can be heard as muffled sounds
45Loud speech is barely audible
50Thunderous speech can be faintly heard
Standard STC rating range and the observed results

Small transmission variations substantially shift scores.

A conference room needs far sturdier sound barriers than a living room!

First, assess your space’s needs. Consider consulting acoustic specialists to dial in the right STC rating. Don’t rely on gut feelings.

High vs. Low STC Rating – Which is Better?

When it comes to blocking sound, a higher STC is better. Ratings range from 25 (poor) to 45 (excellent) and indicate how many decibels are stopped.

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But don’t assume top ratings are always the best. The ideal number for your space depends on your needs.

Noisy nightclub? Pump that STC up! Quiet home library? A moderate score suffices.

Also, higher ratings raise costs – balance your budget with noise expectations. Don’t overspend to flaunt giant STC numbers, either!

Consult an acoustic engineer to optimize your sound barriers.

They tailor STC recommendations based on noise sources, room uses, and more – blocking just enough noise without going overboard.

How to find the STC Rating of an assembly

There are two methods to accurately determine an assembly’s Sound Transmission Class (STC) rating: lab testing of its design or field testing of the actual installed assembly.

Field testing is more reliable since it measures the assembly’s real-world noise isolation performance.

This involves generating a range of noise frequencies in one room and recording sound levels in both rooms to plot a curve and calculate the assembly’s sound absorption.

Airborne sound insulation data from NTi Audio based on ASTM E336-20, ASTM E413, etc.
Airborne sound insulation data from NTi Audio based on ASTM E336-20, ASTM E413, etc.

The measured curve is then compared to a set of standard benchmark STC curves. The standard curve it most closely resembles indicates the STC rating.

For example, if the measured curve best matches the basic STC 35 standard curve, then the tested assembly has an STC rating of 35.

By comparing to standardized benchmark curves, the field test accurately assigns an STC rating.

Techniques to Improve STC Rating

You can use several techniques to improve a room’s STC (Sound Transmission Class) rating.

Increasing the mass of the assembly

Increasing assembly mass (thicker drywall, concrete) blocks more noise by absorbing and dissipating sound energy, creating a more impenetrable barrier.

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Installing soundproofing materials

Soundproofing materials like acoustic insulation, sound-deadening paints/sprays/sealants simply improve STC ratings without complex room modifications.

Decoupling

Decoupling in soundproofing
Decoupling in soundproofing (Source: Everkem)

Decoupling walls/floors from building structures using isolation clips, resilient channels, or rubber mounts prevents noise transmission at contact points by isolating surfaces.

Adding an air space

Creating an air space between double drywall layers acts as a decoupling layer. The spaced layers absorb and dissipate transmitted vibrations, reducing sound transfer.

A double drywall installation
A double drywall installation (Source: MicronXD via StackExchange)

Adding a resilient channel

Resilient channels between drywall layers work similarly, designed specifically for soundproofing.

Installing a resilient channel between the floor
Installing a resilient channel between the floor (Source: Simpson Properties – YouTube)

Their flexible material absorbs vibrations before reaching the second drywall. They also provide a stable and impact-resistant drywall mount.

Using acoustic doors and windows

Special doors and windows designed for sound control can block sound from entering or leaving a room.

Acoustic doors and windows effectively reduce the transmission of sound through them, but they may not eliminate all noise.

They are designed to reduce sound transmission, not absorb/dampen.

If you are interested, I have written an article on the best soundproofing windows.

To fully soundproof a room, you will need to use a combination of methods, such as adding insulation, sealing gaps and cracks, and using sound-absorbing materials on walls, ceilings, windows, and floors.

If you work with a professional acoustical engineer, they can help you better evaluate the specific sound transmission issues and design a solution to your situation.

STC vs. SRI vs. STI ratings

STC (US), SRI (Europe), and STI measure sound insulation but vary by region.

STC aligns with ASTM standards in the US; Canada uses STC, too but allows different test methods.

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SRI follows the EN ISO 717-1 standard. STI focuses on speech blocking.

Sound ratings differ internationally – check your country/region’s specific regulations and testing standards when selecting noise barrier products to ensure localized accuracy.

Final Thoughts – Is STC effective?

STC only measures sound transmission through walls, floors, windows, and other similar installations.

It doesn’t measure sound transmission through airborne paths, so it is not a complete measurement of the sound insulation of a building.

Overall, STC is an essential tool that can be used to ensure that buildings are constructed in a way that promotes acoustic comfort and privacy.

Frequently Asked Questions

What is Drywall’s STC Rating?

Drywall typically has an STC rating between 35-40.

What is a good STC rating for walls and floors?

Good STC ratings depend on room use – 55+ for studios/home theaters; under 45 for less critical rooms like laundries.

What does STC 40 mean?

An STC 40 blocks sound transmission by 40 decibels (dB) – an average insulation level for garages or storage.

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