The articulation index used in telephone communication, the listening equation for sonar, and the acoustical‐privacy calculation of architectural acoustics are all founded on a computation of an excess of signal level over noise level just sufficient to permit some stated detectability. From this viewpoint, data compiled for “Speech Privacy in Buildings” by Cavanaugh, Farrell, Hirtle, and Watters [J. Acoust. Soc. Am.34, 475–492 (1962)] have been reviewed for possible simplications in their procedure for estimating acoustical privacy. Interrelations are demonstrated among various current methods for rating noise and sound insulation, and a general equation for acoustical privacy is derived. As an example, for the case of “confidential” privacy, 200‐ft2floor area, and raised voice, satisfactory acoustical privacy is to be expected ifD+NA= 85 dB, whereDis the sound isolation between the rooms (the reduction of sound from one room to the next) as rated by a procedure like that for the sound‐transmission class, andNAis the background‐noise level in the receiving room measured with a sound‐level meter on A‐weighting.