A brief review is made of previous single-number rating development with emphasis on the sound transmission class (STC). A single-number metric, called Exterior Wall Noise Rating (EWNR), has been developed in a similar, but independent, manner such that the A-weighted interior noise level, due to highway noise sources, can be estimated directly from the value of EWNR and the A-weighted exterior noise level. The development is defined first in terms of the basic theory for noise reduction from outdoors to indoors in a single frequency band. The overall effective noise reduction is a summation of these values over many frequency bands. The EWNR single-number rating replaces this complex summation and provides a greatly simplified calculation procedure. In addition, the EWNR can be used for direct comparisons of the noise insulation properties of exterior building elements. The EWNR metric is applicable to other exterior noise sources if the proper adjustment factors are included. Results are shown from recently conducted field tests which indicate good accuracy for predicting interior noise levels due to aircraft noise sources.