The exact description of sound transmission through an enclosure depends, in general, on the details of the source radiation characteristics, coupling of the source to the enclosure acoustic space, and coupling of the acoustic space to the resonant and nonresonant motion of the enclosure walls. Moreover, for an extended source, and a relatively small and absorptive enclosed space, an exact description is beyond the present state of the art. In this paper, a simple design procedure based on an energy flow analysis is described, which provides good lower bound estimates of enclosure performance. The performance parameter that is calculated is the lower bound of a space average, or radiated power insertion loss. When formulated in this way, the performance is reciprocal in the sense that it is independent of whether transmission is from outside to inside or inside to outside. Above the first enclosure volume resonance, the result is PIL ⩾ TL + 10 log A/S where PIL is the power insertion loss, TL is the field incidence transmission loss of the enclosure walls, A is the sabine absorption of the enclosure interior, andSis the radiating area of the walls. A result that is applicable below the first enclosure resonance is also presented. A number of experimental results are presented which confirm the analysis procedures. [Work supported in part by General Electric Co., Aircraft Engine Group, Cincinnati, Ohio, and NASA‐Langley Research Center.]