STL Rating 101: Understanding Sound Transmission Loss

What Is STL Rating?

• STL Defined: Sound Transmission Loss (STL) measures how much sound energy is reduced as it passes through a barrier, expressed in decibels (dB).
• Higher = Better: A higher STL means the material blocks more sound.
• Used In: Walls, doors, windows, partitions, and building envelopes.

Sound Transmission Loss (STL) quantifies how effectively a partition stops airborne noise at specific frequencies. Unlike the STC rating, which averages results into a single number, STL provides detailed performance data across the frequency spectrum — usually from 125 Hz to 4000 Hz. Engineers and acoustical consultants use STL curves to identify how a material performs at low, mid, and high frequencies to optimize design performance.

How Is STL Measured?

• Standards: ASTM E90 and ASTM E413
• Method: Measure sound intensity in a source room and a receiving room separated by the test specimen.
• Result: The difference (in dB) across each frequency band is plotted to form the STL curve.

The STL test isolates the partition and measures how much sound is lost as it passes through. These tests are performed in controlled lab conditions using precision microphones and speakers. The curve shows attenuation at low, mid, and high frequencies — data that architects and consultants use to design assemblies that meet specific acoustic goals.

Typical STL Values by Material Type

Material or Assembly	Approx. STL (dB)	Comments
Single ½″ Drywall (stud wall)	20–25	Minimal sound isolation
Double ½″ Drywall on Studs	30–35	Basic residential wall
Double Stud Wall with Insulation	50–60	Strong isolation for offices or apartments
Laminated Acoustic Glass (½″)	35–40	Used in high-performance windows
Solid Core Door	25–30	Average; sealing improves performance
Concrete Wall (8″)	55–65	Excellent low-frequency isolation

These examples show how construction mass, air gaps, and insulation directly affect STL values. Heavy and decoupled assemblies yield higher sound loss across most frequencies.

STL vs STC: What’s the Difference?

While STL measures sound reduction at multiple frequencies, STC compresses those results into a single-number rating for comparison.

• STL: Detailed lab data by frequency (engineering metric)
• STC: Simplified, single value (design and code metric)

In other words, STC gives you a quick summary, while STL tells you the whole story — particularly useful for identifying low-frequency problem areas like bass, HVAC rumble, or mechanical vibration.

What Do Different STL Ratings Mean?

• STL 20–30 dB: Low isolation — normal speech easily heard through.
• STL 35–45 dB: Moderate isolation — normal speech muffled.
• STL 50–60 dB: Strong isolation — raised voices barely audible.
• STL 65 dB+: Excellent isolation — suitable for studios or high-security rooms.

STL ratings are not limited by code but are essential for specifying materials that meet acoustic targets in commercial, residential, or industrial settings.

How to Improve STL Ratings

• Add Mass: Thicker drywall, denser materials, or laminated panels improve attenuation.
• Use Air Gaps or Decoupling: Double studs, resilient channels, or clips reduce structural transmission.
• Seal Openings: Door gaskets, acoustic sealants, and proper perimeter sealing stop sound leaks.
• Add Insulation: Mineral wool or fiberglass in cavities absorbs mid- to high-frequency sound.
• Upgrade Glass: Laminated or multi-pane glazing increases STL for exterior façades.

Each layer or improvement contributes to higher Sound Transmission Loss, reducing noise transfer between spaces and improving comfort and privacy.

Conclusion: STL Provides the Full Acoustic Picture

While STC ratings summarize acoustic performance, STL curves reveal how materials behave across different frequencies. Understanding Sound Transmission Loss helps designers target weak points, select the right assemblies, and achieve precise sound isolation goals. Whether you’re engineering a recording studio, hotel, or apartment complex, STL data is the backbone of reliable noise-control design.

At Commercial Acoustics, we help architects and contractors choose the correct assemblies and hit their acoustic targets efficiently. From preconstruction consulting to turnkey installation, our team is here to guide you through the science of sound.

FAQs: STL Ratings & Acoustics