Sound Transmission Loss Measurement Methods and Standards

Sound transmission loss (STL) or sound reduction index (R) quantifies the acoustic insulation performance of a partition, wall, floor, window, or composite assembly. It is expressed as a function of frequency and summarised as a single number weighted rating (Rw to ISO 717-1, or STC to ASTM E413) for specification and comparison purposes. Measurement can be performed in a laboratory transmission suite (ISO 10140), in the field (ISO 16283), or on small samples using a two-microphone impedance tube method.

Laboratory Measurement: ISO 10140 Series

• ISO 10140-2: Measurement of airborne sound insulation — requires transmission suite with source and receive rooms
• ISO 10140-3: Measurement of impact sound insulation of floors
• ISO 10140-4: Measurement conditions and procedures for specific test configurations
• Results expressed as R (sound reduction index) in dB, as a function of 1/3 octave band frequency
• Minimum room volume and reverberation requirements specified; test element area defined

Field Measurement: ISO 16283 Series

• ISO 16283-1: Field measurement of airborne sound insulation — results expressed as DnT,w
• ISO 16283-2: Field measurement of impact sound insulation — results expressed as L'nT,w
• Field measurements typically give worse results than laboratory due to flanking transmission
• Requires multi-channel measurement microphone setup and omnidirectional sound source

Impedance Tube Method for Small Samples

For small sample transmission loss measurement of materials and thin panels, the three-microphone two-load impedance tube method (ISO 10534-2 extended) or ASTM E2611 provides transmission loss data from small specimens. This method is suitable for material screening, research, and product development where a full transmission suite test is not warranted. The PLACID impedance tube can be configured for transmission loss measurement with an extended tube and third microphone position.

• Single number ratings: Rw (ISO 717-1) for European CE marking and specification; STC (ASTM E413) for North American use
• Rw and STC are not directly interchangeable — values may differ by several dB for the same partition
• Spectrum adaptation terms C and Ctr (ISO 717-1) adjust Rw for traffic and pink noise spectra

For both laboratory and field transmission loss measurement, calibrated Class 1 microphones are required, and the measurement report must state the calibration status of each microphone used. In laboratory testing to ISO 10140, the microphones are used in both source and receiving rooms; sensitivity matching across rooms is important for accurate spatial averaging. PLACID measurement microphone sets supplied with matched individual calibration certificates simplify this requirement. The impedance tube method provides a compact and practical alternative for material screening where a full-scale transmission suite test is not warranted.