All of the single-number indices currently used to assess the sound insulation of walls use one-third octave band sound transmission loss data in the frequency range 125 to 4,000 Hz. Forty-two walls were measured over the ra nge 50 to 5,000 Hz. None of the existing indices correlaled well with the calculated 50-to 4,000· Hz loudness reduction using the International Organization for Standardization method. A new proposed rating, the outdoor-indoor transmission class (OITC), which is based on A-weighted sound reduction in the range 80 to 5,000 Hz, shows significant improvement over other methods. Typically, both the loudness reduction and OITC give lower numbers than sound transmission class for wall constructions.

Single-number sound insulation ratings have been used for many years to determine if the acoustical performance of interior walls between dwellings, offices, and rooms in general was adequate to provide speech privacy and control of radio and television sounds. The sound transmission class (STC)(1) and the International Organization for Standardization (ISO) weighted sound reduction index (R,.)(2) were designed for these purposes, but they were never intended for use in describing sound insulation performance against outdoor traffic and other sounds with strong low-frequency content. Despite these limitations, these rating methods have been used many times to select and compare the performance of exterior walls, windows, and doors, with resultant failure to achieve satisfactory results. The limitations of STC and similar ratings when comparing the loudness reduction of a series of lightweight design walls in the range STC 30 to 69, which were measured for sound transmission loss (TL) at 50 to 5,000 Hz, are demonstrated, and an alternative rating method based on A-weighted sound reduction is offered (3). There were no data available below 80 Hz for exterior wall or window constructions; however, the range of constructions used is believed to be adequately wide.