Sound Transmission Loss Testing of Acoustic and Sound-Absorbing Materials

Sound transmission loss (TL) testing is the core laboratory procedure used to quantify how effectively acoustic and sound-absorbing materials block airborne sound passing through them. Performed by accredited third-party laboratories in accordance with ISO 10140, ASTM E90, ASTM E2611, and related standards, this testing generates the frequency-dependent transmission loss data and single-number ratings such as STC and Rw that underpin product certification, building code compliance, and noise control engineering. This article explains the technical principles, sample requirements, testing items, measurement methods, applications, and frequently asked questions for sound transmission loss testing of acoustic materials.

Technical Overview

Sound transmission loss (TL), known as the sound reduction index (R) in ISO terminology, is a frequency-dependent measure, expressed in decibels, of the sound-insulating capability of a material, panel, or assembly. It is defined as ten times the common logarithm of the ratio of the sound power incident on the specimen (Wi) to the sound power transmitted through it (Wt): TL = 10 × log10(Wi / Wt). A higher transmission loss value indicates that less acoustic energy passes through the material, meaning better sound insulation performance.

It is important to distinguish transmission loss from sound absorption. Sound absorption characterizes the fraction of incident energy that is dissipated rather than reflected by a porous or fibrous material, whereas transmission loss characterizes the reduction of energy that propagates through to the opposite side. Many open-cell sound-absorbing materials, such as flexible foams and glass wool, exhibit high absorption coefficients but relatively low standalone transmission loss, which is why practical acoustic treatments often combine absorptive layers with dense barrier layers to form multilayer composite structures.

The transmission loss of homogeneous single-layer materials broadly follows the mass law, increasing by approximately 6 dB for each doubling of surface mass or frequency, and it is also influenced by stiffness, damping, coincidence effects, and panel resonance. Because of this complex frequency behavior, laboratory measurement across one-third-octave bands is essential. Principal frameworks include the ISO 10140 series and ASTM E90 for laboratory airborne sound insulation, GB/T 19889 (adopting ISO 140/ISO 10140) for the Chinese market, ASTM E2611 for normal-incidence transmission loss using the transfer matrix method, and the single-number rating procedures of ASTM E413 (STC), ASTM E1332 (OITC), and ISO 717-1 (Rw with C and Ctr spectrum adaptation terms).

Testing Samples

Transmission loss testing is applicable to a wide range of specimen types, provided the samples are representative of production materials and are prepared, conditioned, and mounted in strict accordance with the governing standard.

Building Partition Assemblies

Full-scale wall, floor, and ceiling assemblies are tested between coupled reverberation rooms using a standardized test opening of approximately 10 square meters, with smaller defined openings used for doors, windows, and glazing. Typical specimens include gypsum board partitions, mass-loaded vinyl barriers, wood-based panels, cement boards, sandwich panels, composite curtain wall elements, acoustic doors, and insulated glazing units.

Porous and Fibrous Absorptive Materials

Flexible polyurethane foams, melamine foams, mineral wool, glass fiber boards, polyester fiber mats, and recycled fiber materials are tested as small circular specimens, typically 30 mm and 100 mm in diameter, in impedance tube configurations. These materials may be evaluated alone, with facings or skins, or as built-up multilayer stacks to characterize the combined absorption-plus-barrier behavior of composite acoustic treatments.

Automotive and Transportation Trim Materials

Dash insulators, floor damping systems, headliners, door trim water barriers, side panel treatments, and barrier-decibel composite laminates are commonly evaluated as flat plaques or trimmed coupons. Normal-incidence tube data and laboratory panel data support NVH development targets, supplier qualification, and lightweight acoustic package optimization for vehicles, railway rolling stock, aircraft cabins, and marine interiors.

Industrial and Equipment Components

Machine enclosure panels, acoustic cabinet covers, generator set housings, HVAC duct lagging, pipe wraps, cleanroom partitions, and equipment room doors are tested to verify that noise control products meet contractual and regulatory insertion loss or transmission class requirements before installation.

Testing Items

Depending on the selected method and the intended application, a comprehensive transmission loss test program typically reports the following items:

• Sound transmission loss (TL) or sound reduction index (R) in one-third-octave bands, typically from 100 Hz to 5000 Hz in reverberation room measurements and from approximately 50 Hz to over 6000 Hz in impedance tube measurements, depending on tube diameter.
• Sound Transmission Class (STC) single-number rating calculated in accordance with ASTM E413.
• Weighted sound reduction index Rw, together with the spectrum adaptation terms C and Ctr, calculated in accordance with ISO 717-1.
• Outdoor-Indoor Transmission Class (OITC) in accordance with ASTM E1332 for facade elements exposed to transportation and aircraft noise.
• Normal-incidence transmission loss determined by the four-microphone transfer matrix method in accordance with ASTM E2611, including the sound absorption coefficient measured in parallel per ISO 10534-2 or ASTM E1050 for complete material characterization.
• Reverberation room sound absorption coefficient per ISO 354 or ASTM C423 for absorptive layers, and the airborne sound insulation of full assemblies per ISO 10140-2 / ASTM E90.
• Field verification quantities such as apparent sound reduction index (R') and standardized level difference (DnT) per ISO 16283-1 or ASTM E336 when as-built performance confirmation is required.

Testing Methods

Selection of the measurement method depends on specimen size, frequency range of interest, incidence condition, and whether laboratory or field data are required. Accredited third-party laboratories normally guide clients to the appropriate procedure during project scoping.

Two-Chamber Reverberation Room Method

The reference laboratory procedure of ISO 10140-2, ASTM E90, and GB/T 19889.3 uses two acoustically coupled reverberation rooms separated by the test opening. An omnidirectional loudspeaker system generates a diffuse sound field in the source room, and the spatial average sound pressure levels L1 and L2 are measured in the source and receiving rooms using rotating microphone booms or microphone arrays. The equivalent sound absorption area A of the receiving room is determined from reverberation time measurements. Transmission loss is then calculated as TL = L1 − L2 + 10 × log10(S / A), where S is the radiating area of the specimen. Background noise, flanking transmission, and specimen edge sealing are controlled and corrected according to the standard, making this method suitable for regulatory STC and Rw certification.

Four-Microphone Impedance Tube Method

ASTM E2611 specifies the measurement of normal-incidence sound transmission of acoustical materials using an impedance tube equipped with two microphone pairs positioned upstream and downstream of the specimen and an anechoic termination. From the measured transfer functions, the acoustic transfer matrix of the specimen is derived, yielding normal-incidence transmission loss, reflection coefficient, and absorption coefficient. The method requires only small specimens, delivers fast results, and is ideal for research and development screening, multilayer stack optimization, and input data generation for statistical energy analysis and transfer path simulation models.

Sound Intensity Method

ISO 15186 describes the determination of the sound reduction index by measuring sound intensity radiated from the specimen surface using a two-microphone intensity probe scanned over prescribed area segments on the receiving side. This approach is valuable for laboratory measurements, element-by-element diagnosis of complex partitions, identification of weak spots and flanking paths, and low-frequency investigation, and it complements the classical two-room method.

Field Verification Methods

ISO 16283-1, ASTM E336, and ASTM E966 provide procedures for in-situ measurement of airborne sound insulation of interior partitions and building facades. Field results reflect actual installation quality, sealant workmanship, and flanking conditions, and they are commonly used for handover acceptance, green building certification, and diagnostic evaluation of noise complaints.

For all methods, specimens should be conditioned in a controlled environment of stable temperature and relative humidity prior to testing, and the mounting, perimeter sealing, and support conditions must be documented in the test report because they significantly influence the measured transmission loss, particularly at low frequencies.

Applications

Transmission loss test data for sound-absorbing and sound-insulating materials are used throughout the product life cycle across many industries:

• Building construction: demonstrating compliance of partitions, floors, doors, and facades with building code acoustic requirements and green building rating schemes using STC, Rw, and OITC ratings.
• Architectural product certification: qualification of acoustic doors, operable partitions, glazing systems, and curtain wall elements for manufacturers and specifiers.
• Automotive, rail, aerospace, and marine NVH engineering: benchmarking, supplier comparison, lightweighting studies, and validation of interior acoustic packages.
• Industrial noise control: performance verification of machine enclosures, generator housings, acoustic louvers, duct lagging, and pipe insulation against guaranteed noise reduction values.
• Material research and development: screening foam, fiber, membrane, and barrier formulations, optimizing multilayer composites, and calibrating simulation models with measured transfer matrix parameters.
• Consumer and professional products: evaluating appliance panels, headphone and earcup components, office pods, and privacy enclosures.

FAQ

What is the difference between sound absorption and sound transmission loss?

Sound absorption measures the portion of incident sound energy that a material dissipates rather than reflects, which controls reverberation inside a room. Transmission loss measures the reduction of sound energy that passes through the material to the other side, which controls sound insulation between spaces. Many absorptive materials absorb well but transmit easily, so effective partitions usually combine absorptive layers with barrier layers.

Which test standard should be selected for my product?

For full-scale building elements requiring regulatory STC or Rw ratings, the two-room method per ISO 10140 / ASTM E90 / GB/T 19889 is appropriate. For small material coupons, R&D comparison, and multilayer stack optimization, the impedance tube method per ASTM E2611 is faster and more economical. For as-built verification, field methods per ISO 16283 or ASTM E336/E966 apply. Accredited third-party laboratories can recommend the correct scope based on the end use.

What sample sizes are required for transmission loss testing?

Reverberation room testing generally requires a full-scale specimen filling a test opening of approximately 10 square meters for walls, with standardized smaller openings for doors and windows. Impedance tube testing per ASTM E2611 uses circular specimens, commonly 30 mm and 100 mm in diameter, cut to the tube bore and conditioned before measurement.

What frequency range and ratings are reported?

Laboratory two-room results are typically reported in one-third-octave bands from 100 Hz to 5000 Hz, from which single-number ratings such as STC (ASTM E413), OITC (ASTM E1332), and Rw with C and Ctr terms (ISO 717-1) are derived. Impedance tube results are reported as normal-incidence transmission loss from roughly 50 Hz to above 6000 Hz, depending on tube diameter and microphone spacing.

How can transmission loss results be improved for absorptive materials?

Because standalone porous absorbers follow the mass law poorly, transmission loss is usually improved by adding a dense barrier layer such as mass-loaded vinyl or a sealed impermeable facing, by decoupling layers to exploit mass-spring-mass resonance behavior, by sealing all air leaks at joints and perimeters, and by optimizing layer thicknesses using transfer matrix simulation validated by laboratory data.

Are test reports from accredited third-party laboratories recognized internationally?

Yes. Reports issued by accredited third-party laboratories operating under ISO/IEC 17025 quality systems, referencing internationally accepted methods such as ISO 10140, ISO 717-1, ASTM E90, ASTM E413, and ASTM E2611, are widely accepted by regulators, certifiers, architects, and global OEM customers.