Wall transmission loss testing measures the airborne sound insulation performance of wall assemblies, quantifying how effectively a partition blocks sound energy between adjacent spaces. Performed in accordance with international standards such as ISO 10140 and ASTM E90, this testing produces frequency-dependent transmission loss spectra and single-number ratings such as STC and Rw. Accredited third-party laboratories use coupled reverberation rooms to generate the reliable acoustic data required for product certification, building code compliance, and green building evaluation.
Technical Overview
Wall transmission loss (TL), also known as the sound reduction index (R), is the fundamental quantity used to characterize the airborne sound insulation of a wall or partition. It expresses, in decibels, the ratio between the sound power incident on a test element and the sound power transmitted through it. Because transmission loss varies strongly with frequency, measurements are performed in one-third octave bands, typically spanning 100 Hz to 3150 Hz, with extended ranges of 50 Hz to 5000 Hz available for low-frequency or facade applications.
The measured quantity is derived from the difference in average sound pressure levels between a source room and a receiving room, corrected for the size of the test specimen and the acoustic absorption of the receiving room. The governing relationship is defined in ISO 10140-2 and ASTM E90 as follows:
- R = L1 − L2 + 10 lg(S/A), where L1 is the energy-average sound pressure level in the source room
- L2 is the energy-average sound pressure level in the receiving room, including background noise corrections where applicable
- S is the area of the free test opening filled by the specimen
- A is the equivalent sound absorption area in the receiving room, determined from the reverberation time as A = 0.16 × V/T
To simplify specification and comparison, the measured spectrum is converted into single-number ratings. The most widely used are the Sound Transmission Class (STC) defined in ASTM E413, the weighted sound reduction index (Rw) defined in ISO 717-1, and the Outdoor-Indoor Transmission Class (OITC) defined in ASTM E1332 for transportation noise sources. Spectrum adaptation terms C and Ctr, calculated per ISO 717-1, further characterize performance against A-weighted pink noise and urban traffic noise spectra respectively.
Testing Samples
Masonry and Concrete Walls
Solid brick, hollow concrete block, autoclaved aerated concrete (AAC), and cast-in-place concrete walls represent heavy, homogeneous constructions whose insulation follows the mass law, with transmission loss increasing by approximately 6 dB per doubling of surface mass. These specimens are constructed directly in the laboratory test opening and conditioned until moisture content stabilizes, since residual construction water can depress measured low-frequency absorption and alter results.
Lightweight Framed Partitions
Steel-stud or timber-stud partitions faced with gypsum board, cement board, or calcium silicate board, with or without cavity insulation such as mineral wool or glass wool. The acoustic performance of these assemblies depends on stud gauge and spacing, board thickness and layer count, cavity depth, and the presence of damping or viscoelastic interlayers, making laboratory verification particularly valuable for these systems.
Double-Leaf and Decoupled Constructions
Mass-spring-mass systems including double-leaf masonry cavity walls, staggered-stud partitions, resilient-channel assemblies, and independently framed walls. These specimens exhibit resonance behavior that produces strong frequency dependence, so full one-third octave band spectra are essential for interpretation.
Composite Wall Elements
Wall assemblies incorporating doors, operable partitions, glazing, louvers, or transfer grills. Because the weakest element often dominates overall performance, composite elements are tested as complete systems rather than being predicted from component data.
Prefabricated Panels and Modular Systems
Prefabricated wall panels, cleanroom partitions, demountable office walls, and acoustic board products. Small building elements such as vents, air transfer devices, and small doors may be tested per ISO 10140-5 and rated using the element-normalized level difference Dn,e.
Standard laboratory test openings are typically on the order of 10 square meters, commonly 4 m wide by 2.5 m high, in accordance with ISO 10140-1 requirements. Specimens must be built, sealed, and cured under conditions representative of intended field installation to ensure valid results.
Testing Items
Wall transmission loss testing programs typically report the following quantities:
- Sound transmission loss spectrum in one-third octave bands, 100 Hz to 3150 Hz (standard range) or 50 Hz to 5000 Hz (extended range)
- Weighted sound reduction index Rw with spectrum adaptation terms C and Ctr, calculated per ISO 717-1 or GB/T 50121
- Sound Transmission Class (STC) rated per ASTM E413
- Outdoor-Indoor Transmission Class (OITC) rated per ASTM E1332
- Apparent sound reduction index R' and standardized level difference DnT for field measurements per ISO 16283-1 or ASTM E336
- Element-normalized level difference Dn,e for small building elements per ISO 10140-5
- Reverberation time of the receiving room and equivalent absorption area supporting the calculation
Testing Methods
Laboratory Measurement per ISO 10140-2 and ASTM E90
The reference laboratory method uses two adjacent reverberation rooms acoustically isolated from one another and connected only through a filler wall containing the test opening. The specimen is installed in this opening with perimeter sealing, and any remaining filler wall area is designed to provide transmission loss substantially higher than that of the specimen so that flanking paths do not contaminate the result. A broadband pink noise signal is generated in the source room using loudspeakers positioned to produce a diffuse field, and spatial average sound pressure levels in both rooms are measured using rotating microphone booms or fixed microphone arrays over positions defined by the standards.
Receiving Room Absorption Measurement
The equivalent sound absorption area of the receiving room is determined from reverberation time measurements using the interrupted noise or integrated impulse response methods described in ISO 354. At least six decay measurements across multiple source-receiver combinations are averaged in each frequency band, with corrections applied for climatic conditions and air absorption at high frequencies.
Single-Number Rating Determination
For STC per ASTM E413, a standard reference contour is shifted against the measured transmission loss curve until the sum of deficiencies does not exceed 32 dB and no individual band deficiency exceeds 8 dB. For Rw per ISO 717-1, a similar contour-fitting procedure is applied with corresponding limits on total and individual unfavourable deviations. Spectrum adaptation terms C and Ctr are computed from the shifted contour position and standardized spectral terms.
Field Measurement per ISO 16283-1 and ASTM E336
In-situ measurements evaluate the apparent sound insulation of installed walls, including the contribution of flanking transmission through connected floors and walls. Standardized or normalized level differences such as DnT,w and apparent ratings such as R'w,45 are reported, together with a low-frequency measurement procedure for rooms below 25 cubic meters, which is important because small rooms exhibit non-diffuse low-frequency behavior.
Instrumentation and Quality Assurance
Measurements employ Class 1 sound level meters and microphones conforming to IEC 61672, calibrated before and after testing with acoustic calibrators traceable to national standards. Accredited third-party laboratories operating under ISO/IEC 17025 maintain documented procedures, participate in inter-laboratory comparison programs, and verify test facility performance against reference specimens to ensure repeatability and reproducibility.
Applications
Wall transmission loss test data supports a wide range of engineering, regulatory, and commercial purposes:
- Declaration and certification of acoustic performance for wall products and building systems
- Compliance demonstration with building code minimums for airborne sound insulation between adjoining dwelling units, hotel rooms, and healthcare spaces
- Acoustic credit documentation for green building rating schemes such as LEED, WELL, and BREEAM
- Research and development of drywall systems, masonry units, resilient assemblies, and damped composite boards
- Verification of architectural specifications and tender submittals during construction procurement
- Post-construction diagnostics, dispute resolution, and remediation design for noise complaints
- Input data for building acoustic prediction software and facade noise mitigation design near roads, railways, and airports
FAQ
What is the difference between STC and Rw?
Both are single-number airborne sound insulation ratings, but STC is defined by ASTM E413 and evaluated over 125 Hz to 4000 Hz, while Rw is defined by ISO 717-1 over 100 Hz to 3150 Hz. The rating contours also differ slightly, so an STC value and an Rw value for the same wall are usually close but not identical, and the two ratings should not be treated as interchangeable.
What STC rating is considered good for a wall?
Typical interior partitions achieve STC 35 to 45, while walls separating dwellings or hotel guest rooms are commonly required to meet STC 50 or higher. Luxury residential, healthcare, and performance venues frequently specify STC 55 to 65 or above, often requiring double-leaf or decoupled constructions to reach these levels.
Why are field results usually lower than laboratory results?
Laboratory measurements eliminate flanking transmission and control installation quality, whereas field walls are affected by structure-borne paths through connected floors and walls, imperfect seals at perimeters and penetrations, back-to-back electrical boxes, and workmanship variability. A field rating 5 dB or more below the laboratory rating of the same assembly is common, which is why field performance classes and apparent ratings exist.
What sample size is required for laboratory testing?
ISO 10140-1 requires the test opening to be at least 10 square meters, typically about 4 m by 2.5 m, so masonry specimens must be constructed at full scale. Small elements such as vents, grilles, and small doors may be tested in reduced openings and rated using Dn,e per ISO 10140-5.
How long does a wall transmission loss test take?
The acoustic measurement itself, including level and reverberation time measurements in all bands, typically takes several hours. However, masonry and concrete specimens require curing and drying periods that can extend from several days to several weeks, and laboratories generally issue final reports within five to ten working days after testing.
Which standards govern wall transmission loss testing?
The principal standards are ISO 10140 (laboratory measurement), ISO 717-1 (rating), ISO 16283-1 (field measurement), ASTM E90, ASTM E413, ASTM E1332, ASTM E336, and the corresponding European and Chinese national standards EN ISO 10140, EN ISO 16283, GB/T 19889, and GB/T 50121.
