Sound Pressure Level Transmission Loss Testing

Sound pressure level transmission loss testing measures how effectively a material, panel, or partition blocks airborne sound across the audible frequency spectrum, expressed in decibels of transmission loss (TL). Performed by accredited third-party laboratories using paired reverberation rooms or impedance tubes in accordance with ISO 10140 and ASTM E90, this testing generates single-number ratings such as STC and Rw that support product certification, building code compliance, and acoustic design optimization.

Technical Overview

Sound pressure level transmission loss testing is a laboratory or field measurement technique used to quantify the airborne sound insulation performance of building elements, panels, and engineered materials. Transmission loss (TL), also expressed as the sound reduction index (R), is defined as the ratio between the sound power incident on a test specimen and the sound power transmitted through it, calculated in decibels as TL = 10 log10 (Wi / Wt), where Wi is the incident sound power and Wt is the transmitted sound power. A higher transmission loss value indicates greater sound-blocking capability.

Because transmission loss is strongly frequency-dependent, measurements are performed in one-third octave bands, typically covering the range from 100 Hz to 5000 Hz. The resulting spectrum is then converted into single-number ratings for specification and regulatory purposes, including 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 low-frequency-dominated outdoor noise sources such as traffic and aircraft.

Transmission loss behavior generally follows the mass law at mid frequencies, where each doubling of surface mass increases transmission loss by approximately 6 dB. Deviations occur at the coincidence frequency, above which bending wave matching reduces performance, and in multi-layer or decoupled constructions, which can substantially outperform homogeneous materials of equal mass. Accurate laboratory measurement therefore requires controlled acoustic environments, calibrated instrumentation, and standardized mounting conditions to isolate the intrinsic performance of the test specimen.

Testing Samples

Building Partitions and Architectural Elements

Full-scale wall assemblies, floor and ceiling assemblies, partitions, operable walls, doorsets, windows, glazing units, and curtain wall sections constitute the largest category of test specimens. These elements are installed in a filler wall between two reverberation chambers with perimeter sealing designed to eliminate flanking leakage, ensuring that all measured transmission occurs through the specimen itself.

Material Panels and Composite Structures

Flat coupons and panel samples such as gypsum board, cement board, plywood, OSB, oriented fiberboard, mass-loaded vinyl barriers, metal sheet, honeycomb sandwich panels, and constrained-layer composites are tested to characterize their acoustic transmission properties for use in product data sheets and simulation models.

Automotive and Transportation Materials

Vehicle noise control components including dash insulators, floor barriers, headliners, door trim water deflectors, wheel house liners, and transmission covers are evaluated for insertion loss and transmission loss performance, frequently following the SAE J1400 laboratory procedure adapted to transportation industry requirements.

Industrial and Mechanical Components

HVAC duct sections, acoustic silencers and mufflers, equipment enclosures, acoustic louvers, machinery guards, and pipeline lagging systems are tested to verify their noise reduction performance in industrial noise control applications.

Testing Items

• Sound transmission loss (TL) in one-third octave bands from 100 Hz to 5000 Hz (or extended ranges where required)
• Sound Transmission Class (STC) rating in accordance with ASTM E413
• Weighted sound reduction index Rw, Rw + C, and Rw + Ctr in accordance with ISO 717-1
• Outdoor-Indoor Transmission Class (OITC) in accordance with ASTM E1332
• Field apparent sound transmission class (ASTC) and apparent transmission loss for installed conditions
• Normal incidence transmission loss of small material samples using impedance tube methods
• Insertion loss of silencers, enclosures, and barrier systems
• Reverberation time and equivalent absorption area of the receiving room as supporting measurements

Testing Methods

Two-Reverberation-Room Method (ISO 10140-2, ASTM E90)

The reference method for airborne sound insulation employs two adjacent reverberation rooms acoustically isolated from one another and structurally decoupled. A broadband sound source generates a diffuse sound field in the source room, and spatially averaged sound pressure levels are measured simultaneously in both rooms using rotating microphone booms or fixed microphone arrays. The transmission loss is calculated as TL = L1 − L2 + 10 log10 (S / A), where L1 and L2 are the average sound pressure levels in the source and receiving rooms, S is the area of the test specimen, and A is the equivalent sound absorption area of the receiving room determined from its measured reverberation time.

Impedance Tube Transfer Function Method (ASTM E1050, ISO 10534-2)

For small, uniform material samples, a four-microphone impedance tube configuration measures the normal incidence transmission coefficient using the transfer function technique. This method requires only a small circular or square specimen, provides rapid results, and is well suited to material research, quality control, and comparative screening of barriers and porous layers, although results are limited to normal incidence conditions.

Sound Intensity Method (ISO 15186)

Sound intensity measurement uses a two-microphone intensity probe scanned over the radiating surface on the receiver side to directly determine the radiated sound power. This approach suppresses the influence of flanking transmission and background noise, making it valuable for large elements, in-situ verification, and detailed diagnostic mapping of weak areas in panels and assemblies.

Field Measurement Method (ASTM E336, ISO 16283-1)

Field procedures evaluate the apparent transmission loss of installed partitions in actual buildings, where flanking paths, site workmanship, and room acoustics influence the result. Field ratings are typically lower than laboratory values and are used for compliance verification, acceptance testing, and post-construction diagnostics.

Applications

• Building code compliance and green building certification demonstrating minimum airborne sound insulation of walls, floors, doors, and windows
• Product development and benchmarking for window, curtain wall, door, and acoustic panel manufacturers
• Architectural and acoustic consulting input for performance specifications of schools, hospitals, hotels, and residential buildings
• Automotive, rail, and aerospace NVH development of barrier materials and interior trim components
• Industrial noise control verification for equipment enclosures, plant rooms, and silencer systems
• Regulatory submission and third-party certification documentation supporting market access and tender qualification

FAQ

What is the difference between STC and Rw?

Both are single-number airborne sound insulation ratings derived from the same one-third octave band transmission loss spectrum. STC, defined in ASTM E413, references a contour in the 125 Hz to 4000 Hz range and reflects North American practice, while Rw, defined in ISO 717-1, covers 100 Hz to 3150 Hz and is used internationally. Spectrum adaptation terms C and Ctr can be added to Rw to account for specific noise sources such as traffic.

How does transmission loss differ from sound absorption?

Transmission loss quantifies the sound energy blocked from passing through a partition, whereas absorption quantifies the energy dissipated within a material and not reflected back into the source space. Porous absorbers such as foam and fiberglass are typically poor barriers, while dense massive barriers are typically poor absorbers; effective acoustic designs often combine both.

What sample sizes are required for testing?

Reverberation room methods require full-scale elements, commonly approximately 10 m² for walls and 10 to 20 m² for floor assemblies, mounted in a standardized filler wall. Impedance tube methods require small circular samples, typically 30 mm to 100 mm in diameter depending on the frequency range of interest.

Why do field results differ from laboratory results?

Laboratory measurements eliminate flanking transmission and control all boundary conditions, whereas field measurements include flanking paths through connected structure, on-site sealing quality, and room geometry effects. Consequently, field apparent transmission loss values are typically 3 to 5 dB, and sometimes more, below the corresponding laboratory ratings.

Which standards govern sound pressure level transmission loss testing?

The principal standards include ISO 10140 series for laboratory measurement, ISO 717-1 for rating, ASTM E90 and ASTM E413 for laboratory measurement and classification, ASTM E336 and ISO 16283-1 for field evaluation, ASTM E1050 and ISO 10534-2 for impedance tube measurement, and the corresponding GB/T 19889 national standard series adopted in China.