Transmission loss (TL) is the reduction in sound energy that occurs when a sound wave passes through a building element — a wall, floor, ceiling, door, or window. Measured in decibels at each frequency, TL tells you how effectively a partition blocks sound from travelling between two spaces. A wall with a TL of 45 dB at 500 Hz reduces the sound pressure level on the receiving side by 45 dB compared to the incident level.
Transmission loss is the foundation of sound insulation design. Whether you are separating a bedroom from a highway, a conference room from an open office, or a music studio from a residential unit, the TL of the intervening construction determines whether the occupants can hear what is happening on the other side.
Real-World Analogy
Imagine standing behind a waterfall and trying to see through it. A light mist barely reduces visibility — like a thin partition with low transmission loss. A thundering curtain of water blocks almost everything — like a massive concrete wall with high transmission loss. The waterfall does not eliminate the light; it reduces the amount that passes through. The thicker and denser the curtain, the less light (or sound) gets through.
Or think of winter gloves. A thin cotton glove lets cold pass through easily (low thermal "transmission loss"). A thick insulated ski glove blocks nearly all the cold (high thermal "transmission loss"). Sound insulation works on the same principle — more mass, more layers, more isolation mean more transmission loss.
Technical Definition
Transmission loss is defined as:
TL = 10 x log10(1 / tau)
Where tau (the transmission coefficient) is the ratio of transmitted sound power to incident sound power. If a wall transmits 1% of incident sound power (tau = 0.01), TL = 10 x log10(100) = 20 dB.
TL is measured in laboratory conditions per ISO 10140:2010 (formerly ISO 140), using two reverberant rooms separated by the test partition. The sound pressure level difference between the rooms, corrected for the area of the test specimen and the absorption in the receiving room, gives the frequency-dependent TL.
Mass Law
The simplest predictor of TL is the mass law, which states that TL increases by approximately 6 dB for each doubling of the surface mass density (kg/m^2) or frequency:
TL approximately equals 20 x log10(m x f) - 47 (dB, for normal incidence)
Where m is surface mass density in kg/m^2 and f is frequency in Hz. For random incidence (field conditions), subtract about 5 dB.
A 100 kg/m^2 concrete wall provides roughly 6 dB more TL than a 50 kg/m^2 wall at the same frequency. This is why mass is the first line of defence in sound insulation — heavier walls block more sound.
The Coincidence Dip
Every rigid panel has a coincidence frequency where the bending wavelength in the panel matches the acoustic wavelength in air. At and above this frequency, the panel vibrates in sympathy with the incident sound wave, and TL drops sharply — sometimes by 10 to 15 dB below the mass law prediction. For standard 12.5 mm plasterboard, coincidence occurs around 2500 Hz.
Single-Number Ratings
Because TL varies with frequency, single-number ratings are used for specification:
STC (Sound Transmission Class), per ASTM E413, is the North American standard. The TL curve is compared to a reference contour, and STC is the value of the contour at 500 Hz when fitted according to the standard's rules.
Rw (Weighted Sound Reduction Index), per ISO 717-1:2020, is the international equivalent. Like STC, Rw fits the measured TL data to a reference curve and produces a single number — but the fitting procedure differs slightly from STC.
A typical single-leaf plasterboard wall on steel studs has STC/Rw around 35 to 40. A double-stud wall with cavity insulation and resilient channels can achieve STC/Rw 55 to 65. A 200 mm concrete wall reaches STC/Rw 50 to 55.
Why It Matters for Design
Transmission loss directly determines the noise environment in adjacent spaces:
Residential separation. Building codes worldwide specify minimum STC or Rw values for walls and floors between dwelling units — typically STC 50 (IBC 2021 Section 1207) or Rw 52 (many European codes). Below these values, normal speech and music from neighbours become audible and intrusive.
Office privacy. Conference rooms need STC 45 to 50 walls to keep conversations confidential. Open office partitions at STC 30 to 35 reduce distraction but do not provide privacy. The combination of TL and background noise determines the actual privacy achieved.
Healthcare. Patient examination rooms, therapy offices, and consultation rooms require speech privacy to meet HIPAA requirements in the US and similar regulations elsewhere. Typical targets are STC 45 to 55 depending on the sensitivity of the space.
Studios and entertainment. Recording studios require walls with STC 60 or higher to isolate from external noise. Concert halls, cinemas, and theatres need similar isolation to prevent sound bleed between auditoriums.
The weakest link principle. Sound insulation is only as good as the weakest element. A wall with STC 55 performs no better than STC 25 if it has an unsealed gap under the door. Acoustic design must address every path — walls, floors, ceilings, doors, windows, ducts, and penetrations — because sound will exploit the path of least resistance.
How AcousPlan Uses This
AcousPlan's Sound Insulation Calculator evaluates STC and Rw for common wall and floor assemblies. The database includes over 50 standard assemblies with laboratory-measured TL values at each octave band, allowing you to compare constructions and find options that meet your project's requirements.
The calculator also evaluates flanking transmission paths — sound that travels around, over, or through the structure rather than directly through the rated partition. This is critical because in-situ performance (measured as DnT,w or ASTC) is almost always lower than laboratory TL due to flanking.
The noise criteria module uses TL in combination with source noise levels to predict the resulting background noise in the receiving room, giving you a complete picture from source to receiver.
Related Concepts
- What is a Decibel (dB)? — The unit TL is measured in
- What is Sound Pressure? — The quantity that TL reduces
- What is Flanking Transmission? — Sound that bypasses the direct TL path
- What is Acoustic Impedance? — The property that governs how sound enters a material
- What is Acoustic Damping? — Internal loss that improves TL at coincidence
Calculate Now
Compare wall and floor constructions with the AcousPlan Room Calculator. See STC/Rw ratings, frequency-by-frequency transmission loss, and compliance with building code requirements for your project.