Transmission Loss

Transmission loss (TL), also called sound reduction index (R) in international standards, is the ratio in decibels of the sound power incident on a building element to the sound power transmitted through it. Measured in laboratory conditions per ISO 10140 (or ASTM E90), TL quantifies the airborne sound insulation performance of walls, floors, windows, and doors across third-octave bands from 100 Hz to 5000 Hz. Higher TL values indicate better sound blocking. TL is frequency-dependent, generally increasing with frequency following the mass law at a rate of approximately 6 dB per octave, with deviations at the coincidence frequency and structural resonances. Single-number ratings STC (ASTM E413) and Rw (ISO 717-1) are derived from the TL curve. Field measurements yield apparent TL values lower than laboratory values due to flanking transmission. TL is the fundamental performance parameter for selecting and specifying building partitions, facades, and enclosures for noise control.