The N number, a speech privacy metric for rating walls, revisited

In 1962, Cavanaugh, et al., published the results of research which lead to a single number rating system for speech privacy of walls referred to as the N number [‘‘Speech Privacy in Buildings,’’ J. Acoust. Soc. Am. 34, 475–492 (1962)]. The N number is derived from measured one‐third octave band transmission loss data weighted using factors signifying the relative contribution to the articulation index. When the N number is summed with factors for source room floor area, source room speech use, measured adjacent room background noise rating, and the privacy requirement, the N number correlated subjective reactions observed in case studies better than average wall transmission loss alone. Owens Corning, the sponsor of this research, published the N number along with sound transmission class (STC) for several years. With the emergence of regulation of speech privacy in health care facilities, the Health Insurance Portability and Accountability Act (HIPAA), it seemed reasonable to revisit the N number. This paper will review the N number methodology and compare some building partition STCs and N numbers, with special attention on the impact of cavity insulation.